Add test for stringstore

- Fix return propagation inside loops

Cargo.toml

@@ -4,3 +4,4 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+thiserror = "1.0.30"

README.md

@@ -1,7 +1,440 @@
 # NEK-Lang
+## Table of contents
+- [NEK-Lang](#nek-lang)
+  - [Table of contents](#table-of-contents)
+  - [Variables](#variables)
+    - [Declaration](#declaration)
+    - [Assignment](#assignment)
+  - [Datatypes](#datatypes)
+    - [I64](#i64)
+    - [String](#string)
+    - [Array](#array)
+  - [Expressions](#expressions)
+    - [General](#general)
+    - [Mathematical Operators](#mathematical-operators)
+    - [Bitwise Operators](#bitwise-operators)
+    - [Logical Operators](#logical-operators)
+    - [Equality & Relational Operators](#equality--relational-operators)
+  - [Control-Flow](#control-flow)
+    - [Loop](#loop)
+    - [If / Else](#if--else)
+    - [Block Scopes](#block-scopes)
+  - [Functions](#functions)
+    - [Function definition](#function-definition)
+    - [Function calls](#function-calls)
+  - [IO](#io)
+    - [Print](#print)
+  - [Comments](#comments)
+    - [Line comments](#line-comments)
+- [Feature Tracker](#feature-tracker)
+  - [High level Components](#high-level-components)
+  - [Language features](#language-features)
+- [Parsing Grammar](#parsing-grammar)
+  - [Expressions](#expressions-1)
+  - [Statements](#statements)
+- [Examples](#examples)
+- [Extras](#extras)
+  - [Visual Studio Code Language Support](#visual-studio-code-language-support)
+
+## Variables
+The variables are all contained in scopes. Variables defined in an outer scope can be accessed in 
+inner scoped. All variables defined in a scope that has ended do no longer exist and can't be 
+accessed.
+
+### Declaration
+- Declare and initialize a new variable
+- Declaring a previously declared variable again will shadow the previous variable
+- Declaration is needed before assignment or other usage
+- The variable name is on the left side of the `<-` operator
+- The assigned value is on the right side and can be any expression
+```
+a <- 123;
+```
+Create a new variable named `a` and assign the value `123` to it.
+
+### Assignment
+- Assigning a value to a previously declared variable
+- The variable name is on the left side of the `=` operator
+- The assigned value is on the right side and can be any expression
+```
+a = 123;
+```
+The value `123` is assigned to the variable named `a`. `a` needs to be declared before this.
+
+## Datatypes
+The available variable datatypes are `i64` (64-bit signed integer), `string` (`"this is a string"`) and `array` (`[10]`)
+
+### I64
+- The normal default datatype is `i64` which is a 64-bit signed integer
+- Can be created by just writing an integer literal like `546`
+- Inside the number literal `_` can be inserted for visual separation `100_000`
+- The i64 values can be used as expected in calculations, conditions and so on
+```
+my_i64 <- 123_456;
+```
+
+### String
+- Strings mainly exist for formatting the text output of a program
+- Strings can be created by using doublequotes like in other languages `"Hello world"`
+- There is no way to access or change the characters of the string
+- Unicode characters are supported `"Hello 🌎"`
+- Escape characters `\n`, `\r`, `\t`, `\"`, `\\` are supported
+- String can be assigned to variables, just like i64
+```
+world <- "🌎";
+
+print "Hello ";
+print world;
+print "\n";
+```
+
+### Array
+- Arrays can contain any other datatypes and don't need to have the same type in all cells
+- Arrays can be created by using brackets with the size in between `[size]`
+- Arrays must be assigned to a variable in order to be used
+- All cells will be initialized with i64 0 values
+- The size can be any expression that results in a positive i64 value
+- The array size can't be changed after creation
+- The arrays data is always allocated on the heap
+- The array cells can be accessed by using the variable name and specifying the index in brackets 
+  `my_arr[index]`
+- The index can be any expression that results in a positive i64 value in the range of the arrays 
+  indices
+- The indices start with 0
+- When an array is passed to a function, it is passed by reference
+```
+width <- 5;
+heigt <- 5;
+
+// Initialize array of size 25, initialized with 25x 0
+my_array = [width * height];
+
+// Modify first value
+my_array[0] = 5;
+
+// Print first value
+// Outputs `5`
+print my_array[0];
+```
+
+## Expressions
+The operator precedence is the same order as in `C` for all implemented operators. 
+Refer to the 
+[C Operator Precedence Table](https://en.cppreference.com/w/c/language/operator_precedence) 
+to see the different precedences.
+
+### General
+- Parentheses `(` and `)` can be used to modify evaluation oder just like in any other 
+programming language.
+- For example `(a + b) * c` will evaluate the addition before the multiplication, despite the multiplication having higher binding power
+
+### Mathematical Operators
+Supported mathematical operations:
+- Addition `a + b`
+- Subtraction `a - b`
+- Multiplication `a * b`
+- Division `a / b`
+- Modulo `a % b`
+- Negation `-a`
+
+### Bitwise Operators
+- And `a & b`
+- Or `a | b`
+- Xor `a ^ b`
+- Bitshift left (by `b` bits) `a << b`
+- Bitshift right (by `b` bits) `a >> b`
+- "Bit flip" (One's complement) `~a`
+
+### Logical Operators
+The logical operators evaluate the operands as `false` if they are equal to `0` and `true` if they are not equal to `0`.
+Note that logical operators like AND / OR do not support short-circuit evaluation. So Both sides of 
+the logical operation will be evaluated, even if it might not be necessary.
+- And `a && b`
+- Or `a || b`
+- Not `!a` (if `a` is equal to `0`, the result is `1`, otherwise the result is `0`)
+
+### Equality & Relational Operators
+The equality and relational operations result in `1` if the condition is evaluated as `true` and in `0` if the condition is evaluated as `false`.
+- Equality `a == b`
+- Inequality `a != b`
+- Greater than `a > b`
+- Greater or equal than `a >= b`
+- Less than `a < b`
+- Less or equal than `a <= b`
+
+## Control-Flow
+For conditions like in if or loops, every non-zero value is equal to `true`, and `0` is `false`.
+
+### Loop
+- The `loop` keyword can be used as an infinite loop, as a while loop or as a while loop with 
+  advancement (an expression that is executed after each loop)
+- If only `loop` is used, directly followed by the body, it is an infinite loop that needs to be 
+  terminated by using the `break` keyword
+- The `loop` keyword can be followed by the condition (an expression) without needing parentheses
+- *Optional:* If there is a `;` after the condition, there must be another expression which is used as the advancement
+- The loops body is wrapped in braces (`{ }`) just like in C/C++
+- The `continue` keyword can be used to end the current loop iteration early
+- The `break` keyword can be used to fully break out of the current loop
+
+```
+// Print the numbers from 0 to 9
+
+// With endless loop
+i <- 0;
+loop {
+  if i >= 10 {
+    break;
+  }
+  print i;
+  i = i + 1;
+}
+
+// Without advancement
+i <- 0;
+loop i < 10 {
+  print i;
+  i = i + 1;
+}
+
+// With advancement
+k <- 0;
+loop k < 10; k = k + 1 {
+  print k;
+}
+```
+
+### If / Else
+- The language supports `if` and an optional `else`
+- After the `if` keyword must be the deciding condition, parentheses are not needed
+- The blocks are wrapped in braces (`{ }`)
+- *Optional:* If there is an `else` after the *if-block*, there must be a following *if-false*, aka. else block
+- NOTE: Logical operators like AND / OR do not support short-circuit evaluation. So Both sides of 
+  the logical operations will be evaluated, even if it might not be necessary
+```
+a <- 1;
+b <- 2;
+if a == b {
+  // a is equal to b
+  print 1;
+} else {
+  // a is not equal to b
+  print 0;
+}
+```
+
+### Block Scopes
+
+- It is possible to create a limited scope for local variables that will no longer exist once the 
+  scope ends
+- Shadowing variables by redefining a variable in an inner scope is supported
+```
+var_in_outer_scope <- 5;
+{
+  var_in_inner_scope <- 3;
+  
+  // Inner scope can access both vars
+  print var_in_outer_scope;
+  print var_in_inner_scope;
+}
+
+// Outer scope is still valid
+print var_in_outer_scope;
+
+// !!! THIS DOES NOT WORK !!!
+// The inner scope has ended
+print var_in_inner_scope;
+```
+
+## Functions
+
+### Function definition
+- Functions can be defined by using the `fun` keyword, followed by the function name and the 
+  parameters in parentheses. After the parentheses, the body is specified inside a braces block
+- The function parameters are specified by only their names
+- The function body has its own scope
+- Parameters are only accessible inside the body
+- Variables from the outer scope can be accessed and modified if the are defined before the function
+- Variables from the outer scope are shadowed by parameters or local variables with the same name
+- The `return` keyword can be used to return a value from the function and exit it immediately
+- If no return is specified, a special `void` value is returned. That value can't be used in 
+  calculations or comparisons, but can be stored in a variable (even tho it doesn't make sense)
+- Functions can only be defined at the top-level. So defining a function inside of any other scoped 
+  block (like inside another function, if, loop, ...) is invalid
+- Functions can only be used after definition and there is no forward declaration right now
+- However a function can be called recursively inside of itself
+- Functions can't be redefined, so defining a function with an existing name is invalid
+```
+fun add_maybe(a, b) {
+  if a < 100 {
+    return a;
+  } else {
+    return a + b;
+  }
+}
+
+fun println(val) {
+  print val;
+  print "\n";
+}
+```
+
+### Function calls
+- Function calls are primary expressions, so they can be directly used in calculations (if they 
+  return appropriate values)
+- Function calls are performed by writing the function name, followed by the arguments in parentheses
+- The arguments can be any expressions, separated by commas
+```
+b <- 100;
+result <- add_maybe(250, b);
+
+// Prints 350 + new-line
+println(result);
+```
+
+## IO
+
+### Print
+Printing is implemented via the `print` keyword
+- The `print` keyword is followed by an expression, the value of which will be printed to the terminal
+- To add a line break a string print can be used `print "\n";`
+```
+a <- 1;
+// Outputs `1` to the terminal
+print a;
+
+// Outputs a new-line to the terminal
+print "\n";
+```
+
+## Comments
+
+### Line comments
+Line comments can be initiated by using `//`
+- Everything after `//` up to the end of the current line is ignored and not parsed
+```
+// This is a comment
+```
+
+
+# Feature Tracker
 
 ## High level Components
 
-- [ ] Lexer: Transforms text into Tokens
+- [x] Lexer: Transforms text into Tokens
-- [ ] Parser: Transforms Tokens into Abstract Syntax Tree
+- [x] Parser: Transforms Tokens into Abstract Syntax Tree
-- [ ] Interpreter (tree-walk-interpreter): Walks the tree and evaluates the expressions / statements
+- [x] Interpreter (tree-walk-interpreter): Walks the tree and evaluates the expressions / statements
+- [x] Simple optimizer: Apply trivial optimizations to the Ast
+  - [x] Precalculate binary ops / unary ops that have only literal operands
+
+## Language features
+
+- [x] General expressions
+  - [x] Arithmetic operations
+    - [x] Addition `a + b`
+    - [x] Subtraction `a - b`
+    - [x] Multiplication `a * b`
+    - [x] Division `a / b`
+    - [x] Modulo `a % b`
+    - [x] Negate `-a`
+  - [x] Parentheses `(a + b) * c`
+  - [x] Logical boolean operators
+    - [x] Equal `a == b`
+    - [x] Not equal `a != b`
+    - [x] Greater than `a > b`
+    - [x] Less than `a < b`
+    - [x] Greater than or equal `a >= b`
+    - [x] Less than or equal `a <= b`
+  - [x] Logical operators
+    - [x] And `a && b`
+    - [x] Or `a || b`
+    - [x] Not `!a`
+  - [x] Bitwise operators
+    - [x] Bitwise AND `a & b`
+    - [x] Bitwise OR `a | b`
+    - [x] Bitwise XOR `a ^ b`
+    - [x] Bitwise NOT `~a`
+    - [x] Bitwise left shift `a << b`
+    - [x] Bitwise right shift `a >> b`
+- [x] Variables
+  - [x] Declaration
+  - [x] Assignment
+  - [x] Local variables (for example inside loop, if, else, functions)
+  - [x] Scoped block for specific local vars `{ ... }`
+- [x] Statements with semicolon & Multiline programs
+- [x] Control flow
+  - [x] Loops
+    - [x] While-style loop `loop X { ... }`
+    - [x] For-style loop without with `X` as condition and `Y` as advancement `loop X; Y { ... }`
+    - [x] Infinite loop `loop { ... }`
+    - [x] Break `break`
+    - [x] Continue `continue`
+  - [x] If else statement `if X { ... } else { ... }`
+    - [x] If Statement
+    - [x] Else statement
+- [x] Line comments `//`
+- [x] Strings
+- [x] Arrays
+  - [x] Creating array with size `X` as a variable `arr <- [X]`
+  - [x] Accessing arrays by index `arr[X]`
+- [x] IO Intrinsics
+  - [x] Print
+- [x] Functions
+  - [x] Function declaration `fun f(X, Y, Z) { ... }`
+  - [x] Function calls `f(1, 2, 3)`
+  - [x] Function returns `return X`
+  - [x] Local variables
+  - [x] Pass arrays by-reference, i64 by-vale, string is a const ref
+
+# Parsing Grammar
+
+## Expressions
+```
+ARRAY_LITERAL = "[" expr "]"
+ARRAY_ACCESS = IDENT "[" expr "]"
+FUN_CALL = IDENT "(" (expr ",")* expr? ")"
+LITERAL = I64_LITERAL | STR_LITERAL | ARRAY_LITERAL
+expr_primary = LITERAL | IDENT | FUN_CALL | ARRAY_ACCESS | "(" expr ")" | "-" expr_primary 
+             | "~" expr_primary
+expr_mul = expr_primary (("*" | "/" | "%") expr_primary)*
+expr_add = expr_mul (("+" | "-") expr_mul)*
+expr_shift = expr_add ((">>" | "<<") expr_add)*
+expr_rel = expr_shift ((">" | ">=" | "<" | "<=") expr_shift)*
+expr_equ = expr_rel (("==" | "!=") expr_rel)*
+expr_band = expr_equ ("&" expr_equ)*
+expr_bxor = expr_band ("^" expr_band)*
+expr_bor = expr_bxor ("|" expr_bxor)*
+expr_land = expr_bor ("&&" expr_bor)*
+expr_lor = expr_land ("||" expr_land)*
+expr = expr_lor
+```
+
+## Statements
+```
+stmt_return = "return" expr ";"
+stmt_break = "break" ";"
+stmt_continue = "continue" ";"
+stmt_var_decl = IDENT "<-" expr ";"
+stmt_fun_decl = "fun" IDENT "(" (IDENT ",")* IDENT? ")" "{" stmt* "}"
+stmt_expr = expr ";"
+stmt_block = "{" stmt* "}"
+stmt_loop = "loop" (expr (";" expr)?)? "{" stmt* "}"
+stmt_if = "if" expr "{" stmt* "}" ("else" "{" stmt* "}")?
+stmt_print = "print" expr ";"
+stmt = stmt_return | stmt_break | stmt_continue | stmt_var_decl | stmt_fun_decl 
+     | stmt_expr | stmt_block | stmt_loop | stmt_if | stmt_print
+```
+
+# Examples
+There are a bunch of examples in the [examples](examples/) directory. Those include (non-optimal) solutions to the first five project euler problems, as well as a [simple Game of Life implementation](examples/game_of_life.nek).
+
+To run an example via `cargo-run`, use:
+```
+cargo run --release -- examples/[NAME]
+```
+
+# Extras
+## Visual Studio Code Language Support
+A VSCode extension that provides simple syntax highlighing for nek is also available on 
+[gitlab](https://code.fbi.h-da.de/advanced-systems-programming-ws21/x4/nek-lang-vscode). Since this 
+is a very small scale project, the extension was not published and instuctions on how to install it 
+can be found in the mentioned repository.

examples/euler1.nek

@@ -0,0 +1,15 @@
+// If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. 
+// The sum of these multiples is 23.
+// Find the sum of all the multiples of 3 or 5 below 1000.
+// 
+// Correct Answer: 233168
+
+sum <- 0;
+i <- 0;
+loop i < 1_000; i = i + 1 {
+    if i % 3 == 0 || i % 5 == 0 {
+        sum = sum + i;
+    }
+}
+
+print sum;

examples/euler2.nek

@@ -0,0 +1,24 @@
+// Each new term in the Fibonacci sequence is generated by adding the previous two terms. 
+// By starting with 1 and 2, the first 10 terms will be:
+//   1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
+// By considering the terms in the Fibonacci sequence whose values do not exceed four million, 
+// find the sum of the even-valued terms.
+// 
+// Correct Answer: 4613732
+
+sum <- 0;
+
+a <- 0;
+b <- 1;
+loop a < 4_000_000 {
+    if a % 2 == 0 {
+        sum = sum + a;
+    }
+
+    tmp <- a;
+    a = b;
+    b = b + tmp;
+}
+
+print sum;
+

examples/euler3.nek

@@ -0,0 +1,29 @@
+// The prime factors of 13195 are 5, 7, 13 and 29.
+// What is the largest prime factor of the number 600851475143 ?
+// 
+// Correct Answer: 6857
+
+number <- 600_851_475_143;
+result <- 0;
+
+div <- 2;
+
+loop number > 1 {
+    loop number % div == 0 {
+        if div > result {
+            result = div;
+        }
+        number = number / div;
+    }
+
+    div = div + 1;
+    if div * div > number {
+        if number > 1 && number > result {
+            result = number;
+        }
+        break;
+    }
+
+}
+
+print result;

examples/euler4.nek

@@ -0,0 +1,31 @@
+// A palindromic number reads the same both ways. The largest palindrome made from the product of 
+// two 2-digit numbers is 9009 = 91 × 99.
+// Find the largest palindrome made from the product of two 3-digit numbers.
+// 
+// Correct Answer: 906609
+
+fun reverse(n) {
+    rev <- 0;
+    loop n {
+        rev = rev * 10 + n % 10;
+        n = n / 10;
+    }
+    return rev;
+}
+
+res <- 0;
+
+i <- 100;
+loop i < 1_000; i = i + 1 {
+    k <- i;
+    loop k < 1_000; k = k + 1 {
+        num <- i * k;
+        num_rev <- reverse(num);
+
+        if num == num_rev && num > res {
+            res = num;
+        }
+    }
+}
+
+print res;

examples/euler4.py

@@ -0,0 +1,24 @@
+# A palindromic number reads the same both ways. The largest palindrome made from the product of 
+# two 2-digit numbers is 9009 = 91 × 99.
+# Find the largest palindrome made from the product of two 3-digit numbers.
+# 
+# Correct Answer: 906609
+
+def reverse(n):
+    rev = 0
+    while n:
+        rev = rev * 10 + n % 10
+        n //= 10
+    return rev
+
+res = 0
+
+for i in range(100, 1_000):
+    for k in range(i, 1_000):
+        num = i * k
+        num_rev = reverse(num)
+        
+        if num == num_rev and num > res:
+            res = num
+
+print(res)

examples/euler5.nek

@@ -0,0 +1,23 @@
+// 2520 is the smallest number that can be divided by each of the numbers from 1 to 10 without any remainder.
+// What is the smallest positive number that is evenly divisible by all of the numbers from 1 to 20?
+// 
+// Correct Answer: 232_792_560
+
+fun gcd(x, y) {
+    loop y {
+        tmp <- x;
+        x = y;
+        y = tmp % y;
+    }
+
+    return x;
+}
+
+result <- 1;
+
+i <- 1;
+loop i <= 20; i = i + 1 {
+    result = result * (i / gcd(i, result));
+}
+
+print result;

examples/euler5.py

@@ -0,0 +1,15 @@
+# 2520 is the smallest number that can be divided by each of the numbers from 1 to 10 without any remainder.
+# What is the smallest positive number that is evenly divisible by all of the numbers from 1 to 20?
+# 
+# Correct Answer: 232_792_560
+
+def gcd(x, y):
+    while y:
+        x, y = y, x % y
+    return x
+
+result = 1
+for i in range(1, 21):
+    result *= i // gcd(i, result)
+
+print(result)

examples/game_of_life.nek

@@ -0,0 +1,134 @@
+fun print_field(field, width, height) {
+    y <- 0;
+    loop y < height; y = y+1 {
+        x <- 0;
+        loop x < width; x = x+1 {
+            if field[y*height + x] {
+                print "# ";
+            } else {
+                print ". ";
+            }
+        }
+        print "\n";
+    }
+    print "\n";
+}
+
+fun count_neighbours(field, x, y, width, height) {
+    neighbours <- 0;
+    if y > 0 {
+        if x > 0 {
+            if field[(y-1)*width + (x-1)] {
+                // Top left
+                neighbours = neighbours + 1;
+            }
+        }
+
+        if field[(y-1)*width + x] {
+        // Top
+            neighbours = neighbours + 1;
+        }
+
+        if x < width-1 {
+            if field[(y-1)*width + (x+1)] {
+            // Top right
+                neighbours = neighbours + 1;
+            }
+        }
+    }
+
+    if x > 0 {
+        if field[y*width + (x-1)] {
+        // Left
+            neighbours = neighbours + 1;
+        }
+    }
+
+    if x < width-1 {
+        if field[y*width + (x+1)] {
+        // Right
+            neighbours = neighbours + 1;
+        }
+    }
+
+    
+    if y < height-1 {
+        if x > 0 {
+            if field[(y+1)*width + (x-1)] {
+            // Bottom left
+                neighbours = neighbours + 1;
+            }
+        }
+
+        if field[(y+1)*width + x] {
+        // Bottom
+            neighbours = neighbours + 1;
+        }
+
+        if x < width-1 {
+            if field[(y+1)*width + (x+1)] {
+            // Bottom right
+                neighbours = neighbours + 1;
+            }
+        }
+    }
+    return neighbours;
+}
+
+fun copy(from, to, len) {
+    i <- 0;
+    loop i < len; i = i + 1 {
+        to[i] = from[i];
+    }
+}
+
+// Set the width and height of the field
+width <- 10;
+height <- 10;
+
+// Create the main and temporary field
+field <- [width*height];
+field2 <- [width*height];
+
+// Preset the main field with a glider
+field[1] = 1;
+field[12] = 1;
+field[20] = 1;
+field[21] = 1;
+field[22] = 1;
+
+fun run_gol(num_rounds) {
+    runs <- 0;
+    loop runs < num_rounds; runs = runs + 1 {
+        // Print the field
+        print_field(field, width, height);
+
+        // Calculate next stage from field and store into field2
+        y <- 0;
+        loop y < height; y = y+1 {
+            x <- 0;
+            loop x < width; x = x+1 {
+
+                // Get the neighbours of the current cell
+                neighbours <- count_neighbours(field, x, y, width, height);
+
+                // Set the new cell according to the neighbour count
+                if neighbours < 2 || neighbours > 3 {
+                    field2[y*width + x] = 0;
+                } else {
+                    if neighbours == 3 {
+                        field2[y*width + x] = 1;
+                    } else {
+                        field2[y*width + x] = field[y*width + x];
+                    }
+                }
+                
+            }
+        }
+
+        // Transfer from field2 to field
+        copy(field2, field, width*height);
+    }
+}
+
+run_gol(32);

examples/recursive_fib.nek

@@ -0,0 +1,9 @@
+fun fib(n) {
+    if n <= 1 {
+        return n;
+    } else {
+        return fib(n-1) + fib(n-2);
+    }
+}
+
+print fib(30);

examples/recursive_fib.py

@@ -0,0 +1,6 @@
+def fib(n):
+    if n <= 1:
+        return n
+    return fib(n-1) + fib(n-2)
+
+print(fib(30))

examples/test_functions.nek

@@ -0,0 +1,31 @@
+fun square(a) {
+    return a * a;
+}
+
+fun add(a, b) {
+    return a + b;
+}
+
+fun mul(a, b) {
+    return a * b;
+}
+
+// Funtion with multiple args & nested calls to different functions
+fun addmul(a, b, c) {
+    return mul(add(a, b), c);
+}
+
+a <- 10;
+b <- 20;
+c <- 3;
+
+result <- addmul(a, b, c) + square(c);
+
+// Access and modify outer variable. Argument `a` must not be used from outer var
+fun sub_from_result(a) {
+    result = result - a;
+}
+
+sub_from_result(30);
+
+print result;

src/ast.rs

@@ -0,0 +1,211 @@
+use std::rc::Rc;
+
+use crate::stringstore::{Sid, StringStore};
+
+/// Types for binary operations
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum BinOpType {
+    /// Addition ("+")
+    Add,
+
+    /// Subtraction ("-")
+    Sub,
+
+    /// Multiplication ("*")
+    Mul,
+
+    /// Division ("/")
+    Div,
+
+    /// Modulo / Remainder ("%")
+    Mod,
+
+    /// Compare Equal ("==")
+    EquEqu,
+
+    /// Compare Not Equal ("!=")
+    NotEqu,
+
+    /// Compare Less than ("<")
+    Less,
+
+    /// Compare Less than or Equal ("<=")
+    LessEqu,
+
+    /// Compare Greater than (">")
+    Greater,
+
+    /// Compare Greater than or Equal (">=")
+    GreaterEqu,
+
+    /// Bitwise Or ("|")
+    BOr,
+
+    /// Bitwise And ("&")
+    BAnd,
+
+    /// Bitwise Xor / Exclusive Or ("^")
+    BXor,
+
+    /// Logical And ("&&")
+    LAnd,
+
+    /// Logical Or ("||")
+    LOr,
+
+    /// Bitwise Shift Left ("<<")
+    Shl,
+
+    /// Bitwise Shift Right (">>")
+    Shr,
+
+    /// Assign value to variable ("=")
+    Assign,
+}
+
+/// Types for unary operations
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum UnOpType {
+    /// Unary Negation ("-")
+    Negate,
+
+    /// Bitwise Not / Bitflip ("~")
+    BNot,
+
+    /// Logical Not ("!")
+    LNot,
+}
+
+/// Ast Node for possible Expression variants
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum Expression {
+    /// Integer literal (64-bit)
+    I64(i64),
+    /// String literal
+    String(Sid),
+
+    /// Array with size as an expression
+    ArrayLiteral(Box<Expression>),
+    /// Array access with name, stackpos and position as expression
+    ArrayAccess(Sid, usize, Box<Expression>),
+
+    /// Function call with name, stackpos and the arguments as a vec of expressions
+    FunCall(Sid, usize, Vec<Expression>),
+
+    /// Variable with name and the stackpos from behind. This means that stackpos 0 refers to the 
+    /// last variable on the stack and not the first
+    Var(Sid, usize),
+    /// Binary operation. Consists of type, left hand side and right hand side
+    BinOp(BinOpType, Box<Expression>, Box<Expression>),
+    /// Unary operation. Consists of type and operand
+    UnOp(UnOpType, Box<Expression>),
+}
+
+/// Ast Node for a loop
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct Loop {
+    /// The condition that determines if the loop should continue
+    pub condition: Option<Expression>,
+    /// This is executed after each loop to advance the condition variables
+    pub advancement: Option<Expression>,
+    /// The loop body that is executed each loop
+    pub body: BlockScope,
+}
+
+/// Ast Node for an if
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct If {
+    /// The condition
+    pub condition: Expression,
+    /// The body that is executed when condition is true
+    pub body_true: BlockScope,
+    /// The if body that is executed when the condition is false
+    pub body_false: BlockScope,
+}
+
+/// Ast Node for a function declaration
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct FunDecl {
+    /// The function name as StringID, stored in the stringstore
+    pub name: Sid,
+    /// The absolute position on the function stack where the function is stored
+    pub fun_stackpos: usize,
+    /// The argument names as StringIDs
+    pub argnames: Vec<Sid>,
+    /// The function body
+    pub body: Rc<BlockScope>,
+}
+
+/// Ast Node for a variable declaration
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct VarDecl {
+    /// The variable name as StringID, stored in the stringstore
+    pub name: Sid,
+    /// The absolute position on the variable stack where the variable is stored
+    pub var_stackpos: usize,
+    /// The right hand side that generates the initial value for the variable
+    pub rhs: Expression,
+}
+
+/// Ast Node for the possible Statement variants
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum Statement {
+    /// Return from a function with the given result value as an expression
+    Return(Expression),
+    /// Break out of the current loop
+    Break,
+    /// End the current loop iteration early and continue with the next loop iteration
+    Continue,
+    /// A variable declaration
+    Declaration(VarDecl),
+    /// A function declaration
+    FunDeclare(FunDecl),
+    /// A simple expression. This could be a function call or an assignment for example
+    Expr(Expression),
+    /// A freestanding block scope
+    Block(BlockScope),
+    /// A loop
+    Loop(Loop),
+    /// An if
+    If(If),
+    /// A print statement that will output the value of the given expression to the terminal
+    Print(Expression),
+}
+
+/// A number of statements that form a block of code together
+pub type BlockScope = Vec<Statement>;
+
+/// A full abstract syntax tree
+#[derive(Clone, Default)]
+pub struct Ast {
+    /// The stringstore contains the actual string values which are replaced with StringIDs in the
+    /// Ast. So this is needed to get the actual strings later
+    pub stringstore: StringStore,
+    /// The main (top-level) code given as a number of statements
+    pub main: BlockScope,
+}
+
+impl BinOpType {
+    /// Get the precedence for a binary operator. Higher value means the OP is stronger binding.
+    /// For example Multiplication is stronger than addition, so Mul has higher precedence than Add.
+    ///
+    /// The operator precedences are derived from the C language operator precedences. While not all
+    /// C operators are included or the exact same, the precedence oder is the same.
+    /// See: https://en.cppreference.com/w/c/language/operator_precedence
+
+    pub fn precedence(&self) -> u8 {
+        match self {
+            BinOpType::Assign => 1,
+            BinOpType::LOr => 2,
+            BinOpType::LAnd => 3,
+            BinOpType::BOr => 4,
+            BinOpType::BXor => 5,
+            BinOpType::BAnd => 6,
+            BinOpType::EquEqu | BinOpType::NotEqu => 7,
+            BinOpType::Less | BinOpType::LessEqu | BinOpType::Greater | BinOpType::GreaterEqu => 8,
+            BinOpType::Shl | BinOpType::Shr => 9,
+            BinOpType::Add | BinOpType::Sub => 10,
+            BinOpType::Mul | BinOpType::Div | BinOpType::Mod => 11,
+        }
+    }
+}

src/astoptimizer.rs

@@ -0,0 +1,116 @@
+use crate::ast::{Ast, BlockScope, Expression, If, Loop, Statement, BinOpType, UnOpType, VarDecl};
+
+/// A trait that allows to optimize an abstract syntax tree
+pub trait AstOptimizer {
+    /// Consume an abstract syntax tree and return an ast that has the same functionality but with
+    /// optional optimizations.
+    fn optimize(ast: Ast) -> Ast;
+}
+
+/// A very simple optimizer that applies trivial optimizations like precalculation expressions that 
+/// have only literals as operands
+pub struct SimpleAstOptimizer;
+
+impl AstOptimizer for SimpleAstOptimizer {
+    fn optimize(mut ast: Ast) -> Ast {
+        Self::optimize_block(&mut ast.main);
+        ast
+    }
+}
+
+impl SimpleAstOptimizer {
+    fn optimize_block(block: &mut BlockScope) {
+        for stmt in block {
+            match stmt {
+                Statement::Expr(expr) => Self::optimize_expr(expr),
+                Statement::Block(block) => Self::optimize_block(block),
+                Statement::Loop(Loop {
+                    condition,
+                    advancement,
+                    body,
+                }) => {
+                    if let Some(condition) = condition {
+                        Self::optimize_expr(condition);
+                    }
+                    if let Some(advancement) = advancement {
+                        Self::optimize_expr(advancement)
+                    }
+                    Self::optimize_block(body);
+                }
+                Statement::If(If {
+                    condition,
+                    body_true,
+                    body_false,
+                }) => {
+                    Self::optimize_expr(condition);
+                    Self::optimize_block(body_true);
+                    Self::optimize_block(body_false);
+                }
+                Statement::Print(expr) => Self::optimize_expr(expr),
+                Statement::Declaration(VarDecl { name: _, var_stackpos: _, rhs}) => Self::optimize_expr(rhs),
+                Statement::FunDeclare(_) => (),
+                Statement::Return(expr) => Self::optimize_expr(expr),
+                Statement::Break | Statement::Continue => (),
+            }
+        }
+    }
+
+    fn optimize_expr(expr: &mut Expression) {
+        match expr {
+            Expression::BinOp(bo, lhs, rhs) => {
+                Self::optimize_expr(lhs);
+                Self::optimize_expr(rhs);
+
+                // Precalculate binary operations that consist of 2 literals. No need to do this at 
+                // runtime, as all parts of the calculation are known at *compiletime* / parsetime.
+                match (lhs.as_mut(), rhs.as_mut()) {
+                    (Expression::I64(lhs), Expression::I64(rhs)) => {
+                        let new_expr = match bo {
+                            BinOpType::Add => Expression::I64(*lhs + *rhs),
+                            BinOpType::Mul => Expression::I64(*lhs * *rhs),
+                            BinOpType::Sub => Expression::I64(*lhs - *rhs),
+                            BinOpType::Div => Expression::I64(*lhs / *rhs),
+                            BinOpType::Mod => Expression::I64(*lhs % *rhs),
+                            BinOpType::BOr => Expression::I64(*lhs | *rhs),
+                            BinOpType::BAnd => Expression::I64(*lhs & *rhs),
+                            BinOpType::BXor => Expression::I64(*lhs ^ *rhs),
+                            BinOpType::LAnd => Expression::I64(if (*lhs != 0) && (*rhs != 0) { 1 } else { 0 }),
+                            BinOpType::LOr => Expression::I64(if (*lhs != 0) || (*rhs != 0) { 1 } else { 0 }),
+                            BinOpType::Shr => Expression::I64(*lhs >> *rhs),
+                            BinOpType::Shl => Expression::I64(*lhs << *rhs),
+                            BinOpType::EquEqu => Expression::I64(if lhs == rhs { 1 } else { 0 }),
+                            BinOpType::NotEqu => Expression::I64(if lhs != rhs { 1 } else { 0 }),
+                            BinOpType::Less => Expression::I64(if lhs < rhs { 1 } else { 0 }),
+                            BinOpType::LessEqu => Expression::I64(if lhs <= rhs { 1 } else { 0 }),
+                            BinOpType::Greater => Expression::I64(if lhs > rhs { 1 } else { 0 }),
+                            BinOpType::GreaterEqu => Expression::I64(if lhs >= rhs { 1 } else { 0 }),
+            
+                            BinOpType::Assign => unreachable!(),
+                        };
+                        *expr = new_expr;
+                    },
+                    _ => ()
+                }
+
+            }
+            Expression::UnOp(uo, operand) => {
+                Self::optimize_expr(operand);
+
+                // Precalculate unary operations just like binary ones
+                match operand.as_mut() {
+                    Expression::I64(val) => {
+                        let new_expr = match uo {
+                            UnOpType::Negate => Expression::I64(-*val),
+                            UnOpType::BNot => Expression::I64(!*val),
+                            UnOpType::LNot => Expression::I64(if *val == 0 { 1 } else { 0 }),
+                        };
+                        *expr = new_expr;
+                    }
+                    _ => (),
+                }
+            }
+            _ => (),
+        }
+
+    }
+}

src/interpreter.rs

@@ -1,70 +1,588 @@
-use crate::parser::{Ast, BinOpType};
+use std::{cell::RefCell, rc::Rc};
+use thiserror::Error;
 
-#[derive(Debug, PartialEq, Eq, Clone)]
+use crate::{
-pub enum Value {
+    ast::{Ast, BinOpType, BlockScope, Expression, FunDecl, If, Statement, UnOpType},
-    I64(i64),
+    astoptimizer::{AstOptimizer, SimpleAstOptimizer},
+    lexer::lex,
+    nice_panic,
+    parser::parse,
+    stringstore::{Sid, StringStore},
+};
+
+/// Runtime errors that can occur during execution
+#[derive(Debug, Error)]
+pub enum RuntimeError {
+    #[error("Invalid array Index: {0:?}")]
+    InvalidArrayIndex(Value),
+
+    #[error("Variable used but not declared: {0}")]
+    VarUsedNotDeclared(String),
+
+    #[error("Can't index into non-array variable: {0}")]
+    TryingToIndexNonArray(String),
+
+    #[error("Invalid value type for unary operation: {0:?}")]
+    UnOpInvalidType(Value),
+
+    #[error("Incompatible binary operations. Operands don't match: {0:?} and {1:?}")]
+    BinOpIncompatibleTypes(Value, Value),
+
+    #[error("Array access out of bounds: Accessed {0}, size is {1}")]
+    ArrayOutOfBounds(usize, usize),
+
+    #[error("Division by zero")]
+    DivideByZero,
+
+    #[error("Invalid number of arguments for function {0}. Expected {1}, got {2}")]
+    InvalidNumberOfArgs(String, usize, usize),
 }
 
+/// Possible variants for the values
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum Value {
+    /// 64-bit integer value
+    I64(i64),
+    /// String value
+    String(Sid),
+    /// Array value
+    Array(Rc<RefCell<Vec<Value>>>),
+    /// Void value
+    Void,
+}
+
+/// The exit type of a block. When a block ends, the exit type specified why the block ended.
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum BlockExit {
+    /// Normal exit when the block just ends normally (no returns / breaks / continues / etc.)
+    Normal,
+    /// The block ended through a break statement. This will be propagated up to the next loop
+    /// and cause it to fully terminate
+    Break,
+    /// The block ended through a continue statement. This will be propagated up to the next loop
+    /// and cause it to start the next iteration
+    Continue,
+    /// The block ended through a return statement. This will propagate up to the next function
+    /// body end
+    Return(Value),
+}
+
+#[derive(Default)]
 pub struct Interpreter {
-    // Runtime storage, for example variables ...
+    /// Run the SimpleAstOptimizer over the Ast before executing
+    pub optimize_ast: bool,
+
+    /// Print the tokens after lexing
+    pub print_tokens: bool,
+    /// Print the ast after parsing
+    pub print_ast: bool,
+
+    /// Capture the output values of print statements instead of printing them to the terminal
+    pub capture_output: bool,
+    /// The stored values that were captured
+    output: Vec<Value>,
+
+    /// Variable table stores the runtime values of variables as a stack
+    vartable: Vec<Value>,
+
+    /// Function table stores the functions during runtime as a stack
+    funtable: Vec<FunDecl>,
+
+    /// The stringstore contains all strings used throughout the program
+    stringstore: StringStore,
 }
 
 impl Interpreter {
+    /// Create a new Interpreter
     pub fn new() -> Self {
-        Self {}
+        Self {
-    }
+            optimize_ast: true,
-
+            ..Self::default()
-    pub fn run(&mut self, prog: Ast) {
-        let result = self.resolve_expr(prog);
-
-        println!("Result = {:?}", result);
-    }
-
-    fn resolve_expr(&mut self, expr: Ast) -> Value {
-        match expr {
-            Ast::I64(val) => Value::I64(val),
-            Ast::BinOp(bo, lhs, rhs) => self.resolve_binop(bo, *lhs, *rhs),
         }
     }
 
-    fn resolve_binop(&mut self, bo: BinOpType, lhs: Ast, rhs: Ast) -> Value {
+    /// Get the captured output
-        let lhs = self.resolve_expr(lhs);
+    pub fn output(&self) -> &[Value] {
-        let rhs = self.resolve_expr(rhs);
+        &self.output
+    }
 
-        match (lhs, rhs) {
+    /// Try to retrieve a variable value from the varstack. The idx is the index from the back of
+    /// the stack. So 0 is the last value, not the first
+    fn get_var(&self, idx: usize) -> Option<Value> {
+        self.vartable.get(self.vartable.len() - idx - 1).cloned()
+    }
+
+    /// Try to retrieve a mutable reference to a variable value from the varstack. The idx is the
+    /// index from the back of the stack. So 0 is the last value, not the first
+    fn get_var_mut(&mut self, idx: usize) -> Option<&mut Value> {
+        let idx = self.vartable.len() - idx - 1;
+        self.vartable.get_mut(idx)
+    }
+
+    /// Lex, parse and then run the given sourecode. This will terminate the program when an error
+    /// occurs and print an appropriate error message.
+    pub fn run_str(&mut self, code: &str) {
+        // Lex the tokens
+        let tokens = match lex(code) {
+            Ok(tokens) => tokens,
+            Err(e) => nice_panic!("Lexing error: {}", e),
+        };
+
+        if self.print_tokens {
+            println!("Tokens: {:?}", tokens);
+        }
+
+        // Parse the ast
+        let ast = match parse(tokens) {
+            Ok(ast) => ast,
+            Err(e) => nice_panic!("Parsing error: {}", e),
+        };
+
+        // Run the ast
+        match self.run_ast(ast) {
+            Ok(_) => (),
+            Err(e) => nice_panic!("Runtime error: {}", e),
+        }
+    }
+
+    /// Execute the given Ast within the interpreter
+    pub fn run_ast(&mut self, mut ast: Ast) -> Result<(), RuntimeError> {
+        // Optimize the ast
+        if self.optimize_ast {
+            ast = SimpleAstOptimizer::optimize(ast);
+        }
+
+        if self.print_ast {
+            println!("{:#?}", ast.main);
+        }
+
+        // Take over the stringstore of the given ast
+        self.stringstore = ast.stringstore;
+
+        // Run the top level block (the main)
+        self.run_block(&ast.main)?;
+        Ok(())
+    }
+
+    /// Run all statements in the given block
+    pub fn run_block(&mut self, prog: &BlockScope) -> Result<BlockExit, RuntimeError> {
+        self.run_block_fp_offset(prog, 0)
+    }
+
+    /// Same as run_block, but with an additional framepointer offset. This allows to free more
+    /// values from the stack than normally and can be used when passing arguments inside a
+    /// function body scope from the outside
+    pub fn run_block_fp_offset(
+        &mut self,
+        prog: &BlockScope,
+        framepointer_offset: usize,
+    ) -> Result<BlockExit, RuntimeError> {
+        let framepointer = self.vartable.len() - framepointer_offset;
+
+        let mut block_exit = BlockExit::Normal;
+
+        'blockloop: for stmt in prog {
+            match stmt {
+                Statement::Break => return Ok(BlockExit::Break),
+                Statement::Continue => return Ok(BlockExit::Continue),
+
+                Statement::Return(expr) => {
+                    let val = self.resolve_expr(expr)?;
+
+                    block_exit = BlockExit::Return(val);
+                    break 'blockloop;
+                }
+
+                Statement::Expr(expr) => {
+                    self.resolve_expr(expr)?;
+                }
+
+                Statement::Declaration(decl) => {
+                    let rhs = self.resolve_expr(&decl.rhs)?;
+                    self.vartable.push(rhs);
+                }
+
+                Statement::Block(block) => match self.run_block(block)? {
+                    // Propagate return, continue and break
+                    be @ (BlockExit::Return(_) | BlockExit::Continue | BlockExit::Break) => {
+                        block_exit = be;
+                        break 'blockloop;
+                    }
+                    _ => (),
+                },
+
+                Statement::Loop(looop) => {
+                    // loop runs as long condition != 0
+                    loop {
+                        // Check the loop condition
+                        if let Some(condition) = &looop.condition {
+                            if matches!(self.resolve_expr(condition)?, Value::I64(0)) {
+                                break;
+                            }
+                        }
+
+                        // Run the body
+                        let be = self.run_block(&looop.body)?;
+                        match be {
+                            // Propagate return
+                            be @ BlockExit::Return(_) => {
+                                block_exit = be;
+                                break 'blockloop;
+                            }
+                            BlockExit::Break => break,
+                            BlockExit::Continue | BlockExit::Normal => (),
+                        }
+
+                        // Run the advancement
+                        if let Some(adv) = &looop.advancement {
+                            self.resolve_expr(&adv)?;
+                        }
+                    }
+                }
+
+                Statement::Print(expr) => {
+                    let result = self.resolve_expr(expr)?;
+
+                    if self.capture_output {
+                        self.output.push(result)
+                    } else {
+                        print!("{}", self.value_to_string(&result));
+                    }
+                }
+
+                Statement::If(If {
+                    condition,
+                    body_true,
+                    body_false,
+                }) => {
+                    // Run the right block depending on the conditions result being 0 or not 
+                    let exit = if matches!(self.resolve_expr(condition)?, Value::I64(0)) {
+                        self.run_block(body_false)?
+                    } else {
+                        self.run_block(body_true)?
+                    };
+
+                    match exit {
+                        // Propagate return, continue and break
+                        be @ (BlockExit::Return(_) | BlockExit::Continue | BlockExit::Break) => {
+                            block_exit = be;
+                            break 'blockloop;
+                        }
+                        _ => (),
+                    }
+                }
+
+                Statement::FunDeclare(fundec) => {
+                    self.funtable.push(fundec.clone());
+                }
+            }
+        }
+
+        self.vartable.truncate(framepointer);
+
+        Ok(block_exit)
+    }
+
+    /// Execute the given expression to retrieve the resulting value
+    fn resolve_expr(&mut self, expr: &Expression) -> Result<Value, RuntimeError> {
+        let val = match expr {
+            Expression::I64(val) => Value::I64(*val),
+            Expression::ArrayLiteral(size) => {
+                let size = match self.resolve_expr(size)? {
+                    Value::I64(size) if !size.is_negative() => size,
+                    val => return Err(RuntimeError::InvalidArrayIndex(val)),
+                };
+                Value::Array(Rc::new(RefCell::new(vec![Value::I64(0); size as usize])))
+            }
+            Expression::String(text) => Value::String(text.clone()),
+            Expression::BinOp(bo, lhs, rhs) => self.resolve_binop(bo, lhs, rhs)?,
+            Expression::UnOp(uo, operand) => self.resolve_unop(uo, operand)?,
+            Expression::Var(name, idx) => self.resolve_var(*name, *idx)?,
+            Expression::ArrayAccess(name, idx, arr_idx) => {
+                self.resolve_array_access(*name, *idx, arr_idx)?
+            }
+
+            Expression::FunCall(fun_name, fun_stackpos, args) => {
+                let args_len = args.len();
+
+                // All of the arg expressions must be resolved before pushing the vars on the stack,
+                // otherwise the stack positions are incorrect while resolving
+                let args = args
+                    .iter()
+                    .map(|arg| self.resolve_expr(arg))
+                    .collect::<Vec<_>>();
+                for arg in args {
+                    self.vartable.push(arg?);
+                }
+
+                // Function existance has been verified in the parser, so unwrap here shouldn't fail
+                let expected_num_args = self.funtable.get(*fun_stackpos).unwrap().argnames.len();
+
+                // Check if the number of provided arguments matches the number of expected arguments
+                if expected_num_args != args_len {
+                    let fun_name = self
+                        .stringstore
+                        .lookup(*fun_name)
+                        .cloned()
+                        .unwrap_or("<unknown>".to_string());
+                    return Err(RuntimeError::InvalidNumberOfArgs(
+                        fun_name,
+                        expected_num_args,
+                        args_len,
+                    ));
+                }
+
+                // Run the function body and return the BlockExit type
+                match self.run_block_fp_offset(
+                    &Rc::clone(&self.funtable.get(*fun_stackpos).unwrap().body),
+                    expected_num_args,
+                )? {
+                    BlockExit::Normal | BlockExit::Continue | BlockExit::Break => Value::Void,
+                    BlockExit::Return(val) => val,
+                }
+            }
+        };
+
+        Ok(val)
+    }
+
+    /// Retrive the value of a given array at the specified index from the varstack. The name is
+    /// given as a StringID and is used to reference the variable name in case of an error. The
+    /// idx is the stackpos where the array variable should be located and the arr_idx is the
+    /// actual array access index, given as an expression.
+    fn resolve_array_access(
+        &mut self,
+        name: Sid,
+        idx: usize,
+        arr_idx: &Expression,
+    ) -> Result<Value, RuntimeError> {
+        // Resolve the array index into a value and check if it is a valid array index
+        let arr_idx = match self.resolve_expr(arr_idx)? {
+            Value::I64(size) if !size.is_negative() => size,
+            val => return Err(RuntimeError::InvalidArrayIndex(val)),
+        };
+
+        // Get the array value
+        let val = match self.get_var(idx) {
+            Some(val) => val,
+            None => {
+                return Err(RuntimeError::VarUsedNotDeclared(
+                    self.stringstore
+                        .lookup(name)
+                        .cloned()
+                        .unwrap_or_else(|| "<unknown>".to_string()),
+                ))
+            }
+        };
+
+        // Make sure it is an array
+        let arr = match val {
+            Value::Array(arr) => arr,
+            _ => {
+                return Err(RuntimeError::TryingToIndexNonArray(
+                    self.stringstore
+                        .lookup(name)
+                        .cloned()
+                        .unwrap_or_else(|| "<unknown>".to_string()),
+                ))
+            }
+        };
+
+        // Get the value of the requested cell inside the array
+        let arr = arr.borrow();
+        arr.get(arr_idx as usize)
+            .cloned()
+            .ok_or(RuntimeError::ArrayOutOfBounds(arr_idx as usize, arr.len()))
+    }
+
+    /// Retrive the value of a given variable from the varstack. The name is given as a StringID
+    /// and is used to reference the variable name in case of an error. The idx is the stackpos
+    /// where the variable should be located
+    fn resolve_var(&mut self, name: Sid, idx: usize) -> Result<Value, RuntimeError> {
+        match self.get_var(idx) {
+            Some(val) => Ok(val),
+            None => {
+                return Err(RuntimeError::VarUsedNotDeclared(
+                    self.stringstore
+                        .lookup(name)
+                        .cloned()
+                        .unwrap_or_else(|| "<unknown>".to_string()),
+                ))
+            }
+        }
+    }
+
+    /// Execute a unary operation and get the resulting value
+    fn resolve_unop(&mut self, uo: &UnOpType, operand: &Expression) -> Result<Value, RuntimeError> {
+        // Recursively resolve the operands expression into an actual value
+        let operand = self.resolve_expr(operand)?;
+
+        // Perform the correct operation, considering the operation and value type
+        Ok(match (operand, uo) {
+            (Value::I64(val), UnOpType::Negate) => Value::I64(-val),
+            (Value::I64(val), UnOpType::BNot) => Value::I64(!val),
+            (Value::I64(val), UnOpType::LNot) => Value::I64(if val == 0 { 1 } else { 0 }),
+            (val, _) => return Err(RuntimeError::UnOpInvalidType(val)),
+        })
+    }
+
+    /// Execute a binary operation and get the resulting value
+    fn resolve_binop(
+        &mut self,
+        bo: &BinOpType,
+        lhs: &Expression,
+        rhs: &Expression,
+    ) -> Result<Value, RuntimeError> {
+        let rhs = self.resolve_expr(rhs)?;
+
+        // Handle assignments separate from the other binary operations
+        match (&bo, &lhs) {
+            // Normal variable assignment
+            (BinOpType::Assign, Expression::Var(name, idx)) => {
+                // Get the variable mutably and assign the right hand side value
+                match self.get_var_mut(*idx) {
+                    Some(val) => *val = rhs.clone(),
+                    None => {
+                        return Err(RuntimeError::VarUsedNotDeclared(
+                            self.stringstore
+                                .lookup(*name)
+                                .cloned()
+                                .unwrap_or_else(|| "<unknown>".to_string()),
+                        ))
+                    }
+                }
+
+                return Ok(rhs);
+            }
+            // Array index assignment
+            (BinOpType::Assign, Expression::ArrayAccess(name, idx, arr_idx)) => {
+                // Calculate the array index
+                let arr_idx = match self.resolve_expr(arr_idx)? {
+                    Value::I64(size) if !size.is_negative() => size,
+                    val => return Err(RuntimeError::InvalidArrayIndex(val)),
+                };
+
+                // Get the mutable ref to the array variable
+                let val = match self.get_var_mut(*idx) {
+                    Some(val) => val,
+                    None => {
+                        return Err(RuntimeError::VarUsedNotDeclared(
+                            self.stringstore
+                                .lookup(*name)
+                                .cloned()
+                                .unwrap_or_else(|| "<unknown>".to_string()),
+                        ))
+                    }
+                };
+
+                // Verify that it actually is an array
+                match val {
+                    // Assign the right hand side value to the array it the given index
+                    Value::Array(arr) => arr.borrow_mut()[arr_idx as usize] = rhs.clone(),
+                    _ => {
+                        return Err(RuntimeError::TryingToIndexNonArray(
+                            self.stringstore
+                                .lookup(*name)
+                                .cloned()
+                                .unwrap_or_else(|| "<unknown>".to_string()),
+                        ))
+                    }
+                }
+
+                return Ok(rhs);
+            }
+            _ => (),
+        }
+
+        // This code is only executed if the binop is not an assignment as the assignments return
+        // early
+
+        // Resolve the left hand side to the value
+        let lhs = self.resolve_expr(lhs)?;
+
+        // Perform the appropriate calculations considering the operation type and datatypes of the
+        // two values
+        let result = match (lhs, rhs) {
             (Value::I64(lhs), Value::I64(rhs)) => match bo {
                 BinOpType::Add => Value::I64(lhs + rhs),
                 BinOpType::Mul => Value::I64(lhs * rhs),
+                BinOpType::Sub => Value::I64(lhs - rhs),
+                BinOpType::Div => {
+                    Value::I64(lhs.checked_div(rhs).ok_or(RuntimeError::DivideByZero)?)
+                }
+                BinOpType::Mod => {
+                    Value::I64(lhs.checked_rem(rhs).ok_or(RuntimeError::DivideByZero)?)
+                }
+                BinOpType::BOr => Value::I64(lhs | rhs),
+                BinOpType::BAnd => Value::I64(lhs & rhs),
+                BinOpType::BXor => Value::I64(lhs ^ rhs),
+                BinOpType::LAnd => Value::I64(if (lhs != 0) && (rhs != 0) { 1 } else { 0 }),
+                BinOpType::LOr => Value::I64(if (lhs != 0) || (rhs != 0) { 1 } else { 0 }),
+                BinOpType::Shr => Value::I64(lhs >> rhs),
+                BinOpType::Shl => Value::I64(lhs << rhs),
+                BinOpType::EquEqu => Value::I64(if lhs == rhs { 1 } else { 0 }),
+                BinOpType::NotEqu => Value::I64(if lhs != rhs { 1 } else { 0 }),
+                BinOpType::Less => Value::I64(if lhs < rhs { 1 } else { 0 }),
+                BinOpType::LessEqu => Value::I64(if lhs <= rhs { 1 } else { 0 }),
+                BinOpType::Greater => Value::I64(if lhs > rhs { 1 } else { 0 }),
+                BinOpType::GreaterEqu => Value::I64(if lhs >= rhs { 1 } else { 0 }),
+
+                BinOpType::Assign => unreachable!(),
             },
-            // _ => panic!("Value types are not compatible"),
+            (lhs, rhs) => return Err(RuntimeError::BinOpIncompatibleTypes(lhs, rhs)),
+        };
+
+        Ok(result)
+    }
+
+    /// Get a string representation of the given value. This uses the interpreters StringStore to
+    /// retrive the text values of Strings
+    fn value_to_string(&self, val: &Value) -> String {
+        match val {
+            Value::I64(val) => format!("{}", val),
+            Value::Array(val) => format!("{:?}", val.borrow()),
+            Value::String(text) => format!(
+                "{}",
+                self.stringstore
+                    .lookup(*text)
+                    .unwrap_or(&"<invalid string>".to_string())
+            ),
+            Value::Void => format!("void"),
         }
     }
 }
 
 #[cfg(test)]
 mod test {
-    use crate::parser::{Ast, BinOpType};
     use super::{Interpreter, Value};
+    use crate::ast::{BinOpType, Expression};
 
+    /// Simple test to check if a simple expression is executed properly.
+    /// Full system tests from lexing to execution can be found in `lib.rs`
     #[test]
     fn test_interpreter_expr() {
         // Expression: 1 + 2 * 3 + 4
         // With precedence: (1 + (2 * 3)) + 4
-        let ast = Ast::BinOp(
+        let ast = Expression::BinOp(
             BinOpType::Add,
-            Ast::BinOp(
+            Expression::BinOp(
                 BinOpType::Add,
-                Ast::I64(1).into(),
+                Expression::I64(1).into(),
-                Ast::BinOp(BinOpType::Mul, Ast::I64(2).into(), Ast::I64(3).into()).into(),
+                Expression::BinOp(
+                    BinOpType::Mul,
+                    Expression::I64(2).into(),
+                    Expression::I64(3).into(),
                 )
                 .into(),
-            Ast::I64(4).into(),
+            )
+            .into(),
+            Expression::I64(4).into(),
         );
 
         let expected = Value::I64(11);
 
         let mut interpreter = Interpreter::new();
-        let actual = interpreter.resolve_expr(ast);
+        let actual = interpreter.resolve_expr(&ast).unwrap();
 
         assert_eq!(expected, actual);
     }

src/lexer.rs

@@ -1,116 +1,313 @@
 use std::{iter::Peekable, str::Chars};
+use thiserror::Error;
 
-use crate::parser::BinOpType;
+use crate::{token::Token, T};
 
-#[derive(Debug, PartialEq, Eq)]
+/// Errors that can occur while lexing a given string
-pub enum Token {
+#[derive(Debug, Error)]
-    /// Integer literal (64-bit)
+pub enum LexErr {
-    I64(i64),
+    #[error("Failed to parse '{0}' as i64")]
+    NumericParse(String),
 
-    /// Plus (+)
+    #[error("Invalid escape character '\\{0}'")]
-    Add,
+    InvalidStrEscape(char),
 
-    /// Asterisk (*)
+    #[error("Lexer encountered unexpected char: '{0}'")]
-    Mul,
+    UnexpectedChar(char),
 
-    /// End of file
+    #[error("Missing closing string quote '\"'")]
-    EoF,
+    MissingClosingString,
-}
-
-struct Lexer<'a> {
-    code: Peekable<Chars<'a>>,
-}
-
-impl<'a> Lexer<'a> {
-    fn new(code: &'a str) -> Self {
-        let code = code.chars().peekable();
-        Self { code }
-    }
-
-    fn lex(&mut self) -> Vec<Token> {
-        let mut tokens = Vec::new();
-
-        while let Some(ch) = self.next() {
-            match ch {
-                // Skip whitespace
-                ' ' => (),
-
-                // Lex numbers
-                '0'..='9' => {
-                    let mut sval = String::from(ch);
-
-                    // Do as long as a next char exists and it is a numeric char
-                    while let Some('0'..='9') = self.peek() {
-                        // The next char is verified to be Some, so unwrap is safe
-                        sval.push(self.next().unwrap());
-                    }
-
-                    // TODO: We only added numeric chars to the string, but the conversion could still fail
-                    tokens.push(Token::I64(sval.parse().unwrap()));
-                }
-
-                '+' => tokens.push(Token::Add),
-                '*' => tokens.push(Token::Mul),
-
-                //TODO: Don't panic, keep calm
-                _ => panic!("Lexer encountered unexpected char: '{}'", ch),
-            }
-        }
-
-        tokens
-    }
-
-    /// Advance to next character and return the removed char
-    fn next(&mut self) -> Option<char> {
-        self.code.next()
-    }
-
-    /// Get the next character without removing it
-    fn peek(&mut self) -> Option<char> {
-        self.code.peek().copied()
-    }
 }
 
 /// Lex the provided code into a Token Buffer
-///
+pub fn lex(code: &str) -> Result<Vec<Token>, LexErr> {
-/// TODO: Don't panic and implement error handling using Result
+    let lexer = Lexer::new(code);
-pub fn lex(code: &str) -> Vec<Token> {
-    let mut lexer = Lexer::new(code);
     lexer.lex()
 }
 
-impl Token {
+/// The lexer is created from a reference to a sourcecode string and is consumed to create a token 
-    pub fn try_to_binop(&self) -> Option<BinOpType> {
+/// buffer from that sourcecode.
-        Some(match self {
+struct Lexer<'a> {
-            Token::Add => BinOpType::Add,
+    /// The sourcecode text as a peekable iterator over the chars. Peekable allows for look-ahead 
-            Token::Mul => BinOpType::Mul,
+    /// and the use of the Chars iterator allows to support unicode characters
-            _ => return None,
+    code: Peekable<Chars<'a>>,
-        })
+    /// The lexed tokens
+    tokens: Vec<Token>,
+    /// The sourcecode character that is currently being lexed
+    current_char: char,
+}
+
+impl<'a> Lexer<'a> {
+
+    /// Create a new lexer from the given sourcecode
+    fn new(code: &'a str) -> Self {
+        let code = code.chars().peekable();
+        let tokens = Vec::new();
+        let current_char = '\0';
+        Self {
+            code,
+            tokens,
+            current_char,
+        }
+    }
+
+    /// Consume the lexer and try to lex the contained sourcecode into a token buffer
+    fn lex(mut self) -> Result<Vec<Token>, LexErr> {
+
+        loop {
+            self.current_char = self.next();
+            // Match on the current and next character. This gives a 1-char look-ahead and
+            // can be used to directly match 2-char tokens
+            match (self.current_char, self.peek()) {
+                // Stop lexing at EOF
+                ('\0', _) => break,
+
+                // Skip / ignore whitespace
+                (' ' | '\t' | '\n' | '\r', _) => (),
+
+                // Line comment. Consume every char until linefeed (next line)
+                ('/', '/') => while !matches!(self.next(), '\n' | '\0') {},
+
+                // Double character tokens
+                ('>', '>') => self.push_tok_consume(T![>>]),
+                ('<', '<') => self.push_tok_consume(T![<<]),
+                ('=', '=') => self.push_tok_consume(T![==]),
+                ('!', '=') => self.push_tok_consume(T![!=]),
+                ('<', '=') => self.push_tok_consume(T![<=]),
+                ('>', '=') => self.push_tok_consume(T![>=]),
+                ('<', '-') => self.push_tok_consume(T![<-]),
+                ('&', '&') => self.push_tok_consume(T![&&]),
+                ('|', '|') => self.push_tok_consume(T![||]),
+
+                // Single character tokens
+                (',', _) => self.push_tok(T![,]),
+                (';', _) => self.push_tok(T![;]),
+                ('+', _) => self.push_tok(T![+]),
+                ('-', _) => self.push_tok(T![-]),
+                ('*', _) => self.push_tok(T![*]),
+                ('/', _) => self.push_tok(T![/]),
+                ('%', _) => self.push_tok(T![%]),
+                ('|', _) => self.push_tok(T![|]),
+                ('&', _) => self.push_tok(T![&]),
+                ('^', _) => self.push_tok(T![^]),
+                ('(', _) => self.push_tok(T!['(']),
+                (')', _) => self.push_tok(T![')']),
+                ('~', _) => self.push_tok(T![~]),
+                ('<', _) => self.push_tok(T![<]),
+                ('>', _) => self.push_tok(T![>]),
+                ('=', _) => self.push_tok(T![=]),
+                ('{', _) => self.push_tok(T!['{']),
+                ('}', _) => self.push_tok(T!['}']),
+                ('!', _) => self.push_tok(T![!]),
+                ('[', _) => self.push_tok(T!['[']),
+                (']', _) => self.push_tok(T![']']),
+
+                // Special tokens with variable length
+
+                // Lex multiple characters together as numbers
+                ('0'..='9', _) => self.lex_number()?,
+
+                // Lex multiple characters together as a string
+                ('"', _) => self.lex_str()?,
+
+                // Lex multiple characters together as identifier or keyword
+                ('a'..='z' | 'A'..='Z' | '_', _) => self.lex_identifier()?,
+
+                // Any character that was not handled otherwise is invalid
+                (ch, _) => Err(LexErr::UnexpectedChar(ch))?,
+            }
+        }
+
+        Ok(self.tokens)
+    }
+
+    /// Lex multiple characters as a number until encountering a non numeric digit. The
+    /// successfully lexed i64 literal token is appended to the stored tokens.
+    fn lex_number(&mut self) -> Result<(), LexErr> {
+        // String representation of the integer value
+        let mut sval = String::from(self.current_char);
+
+        // Do as long as a next char exists and it is a numeric char
+        loop {
+            // The next char is verified to be Some, so unwrap is safe
+            match self.peek() {
+                // Underscore is a separator, so remove it but don't add to number
+                '_' => {
+                    self.next();
+                }
+                '0'..='9' => {
+                    sval.push(self.next());
+                }
+                // Next char is not a number, so stop and finish the number token
+                _ => break,
+            }
+        }
+
+        // Try to convert the string representation of the value to i64. The error is mapped to
+        // the appropriate LexErr
+        let i64val = sval.parse().map_err(|_| LexErr::NumericParse(sval))?;
+
+        self.push_tok(T![i64(i64val)]);
+
+        Ok(())
+    }
+
+    /// Lex characters as a string until encountering an unescaped closing doublequoute char '"'.
+    /// The successfully lexed string literal token is appended to the stored tokens.
+    fn lex_str(&mut self) -> Result<(), LexErr> {
+        // The opening " was consumed in match, so a fresh string can be used
+        let mut text = String::new();
+
+        // Read all chars until encountering the closing "
+        loop {
+            match self.peek() {
+                // An unescaped doubleqoute ends the current string
+                '"' => break,
+
+                // If the end of file is reached while still waiting for '"', error out
+                '\0' => Err(LexErr::MissingClosingString)?,
+
+                _ => match self.next() {
+                    // Backslash indicates an escaped character, so consume one more char and
+                    // treat it as the escaped char
+                    '\\' => match self.next() {
+                        'n' => text.push('\n'),
+                        'r' => text.push('\r'),
+                        't' => text.push('\t'),
+                        '\\' => text.push('\\'),
+                        '"' => text.push('"'),
+                        // If the escaped char is not handled, it is unsupported and an error
+                        ch => Err(LexErr::InvalidStrEscape(ch))?,
+                    },
+                    // All other characters are simply appended to the string
+                    ch => text.push(ch),
+                },
+            }
+        }
+
+        // Consume closing "
+        self.next();
+
+        self.push_tok(T![str(text)]);
+
+        Ok(())
+    }
+
+    /// Lex characters from the text as an identifier. The successfully lexed ident or keyword
+    /// token is appended to the stored tokens.
+    fn lex_identifier(&mut self) -> Result<(), LexErr> {
+        let mut ident = String::from(self.current_char);
+
+        // Do as long as a next char exists and it is a valid char for an identifier
+        loop {
+            match self.peek() {
+                // In the middle of an identifier numbers are also allowed
+                'a'..='z' | 'A'..='Z' | '0'..='9' | '_' => {
+                    ident.push(self.next());
+                }
+                // Next char is not valid, so stop and finish the ident token
+                _ => break,
+            }
+        }
+
+        // Check for pre-defined keywords
+        let token = match ident.as_str() {
+            "loop" => T![loop],
+            "print" => T![print],
+            "if" => T![if],
+            "else" => T![else],
+            "fun" => T![fun],
+            "return" => T![return],
+            "break" => T![break],
+            "continue" => T![continue],
+
+            // If it doesn't match a keyword, it is a normal identifier
+            _ => T![ident(ident)],
+        };
+
+        self.push_tok(token);
+
+        Ok(())
+    }
+
+    /// Push the given token into the stored tokens
+    fn push_tok(&mut self, token: Token) {
+        self.tokens.push(token);
+    }
+
+    /// Same as `push_tok` but also consumes the next token, removing it from the code iter. This
+    /// is useful when lexing double char tokens where the second token has only been peeked.
+    fn push_tok_consume(&mut self, token: Token) {
+        self.next();
+        self.tokens.push(token);
+    }
+
+    /// Advance to next character and return the removed char. When the end of the code is reached,
+    /// `'\0'` is returned. This is used instead of an Option::None since it allows for much 
+    /// shorter and cleaner code in the main loop. The `'\0'` character would not be valid anyways
+    fn next(&mut self) -> char {
+        self.code.next().unwrap_or('\0')
+    }
+
+    /// Get the next character without removing it. When the end of the code is reached,
+    /// `'\0'` is returned. This is used instead of an Option::None since it allows for much 
+    /// shorter and cleaner code in the main loop. The `'\0'` character would not be valid anyways
+    fn peek(&mut self) -> char {
+        self.code.peek().copied().unwrap_or('\0')
     }
 }
 
 #[cfg(test)]
 mod tests {
-    use super::{lex, Token};
+    use crate::{lexer::lex, T};
 
+    /// A general test to check if the lexer actually lexes tokens correctly
     #[test]
     fn test_lexer() {
-        let code = "33   +5*2  + 4456467*2334+3";
+        let code = r#"53+1-567_000 * / % | ~ ! < > & ^ ({[]});= <- >= <=
+            == != && || << >> loop if else print my_123var "hello \t world\r\n\"\\""#;
         let expected = vec![
-            Token::I64(33),
+            T![i64(53)],
-            Token::Add,
+            T![+],
-            Token::I64(5),
+            T![i64(1)],
-            Token::Mul,
+            T![-],
-            Token::I64(2),
+            T![i64(567_000)],
-            Token::Add,
+            T![*],
-            Token::I64(4456467),
+            T![/],
-            Token::Mul,
+            T![%],
-            Token::I64(2334),
+            T![|],
-            Token::Add,
+            T![~],
-            Token::I64(3),
+            T![!],
+            T![<],
+            T![>],
+            T![&],
+            T![^],
+            T!['('],
+            T!['{'],
+            T!['['],
+            T![']'],
+            T!['}'],
+            T![')'],
+            T![;],
+            T![=],
+            T![<-],
+            T![>=],
+            T![<=],
+            T![==],
+            T![!=],
+            T![&&],
+            T![||],
+            T![<<],
+            T![>>],
+            T![loop],
+            T![if],
+            T![else],
+            T![print],
+            T![ident("my_123var".to_string())],
+            T![str("hello \t world\r\n\"\\".to_string())],
         ];
 
-        let actual = lex(code);
+        let actual = lex(code).unwrap();
         assert_eq!(expected, actual);
     }
 }

src/lib.rs

@@ -1,3 +1,68 @@
+pub mod ast;
+pub mod interpreter;
 pub mod lexer;
 pub mod parser;
-pub mod interpreter;
+pub mod token;
+pub mod stringstore;
+pub mod astoptimizer;
+pub mod util;
+
+/// A bunch of full program tests using the example code programs as test subjects.
+#[cfg(test)]
+mod tests {
+    use crate::interpreter::{Interpreter, Value};
+    use std::fs::read_to_string;
+
+    /// Run a nek program with the given filename from the examples directory and assert the
+    /// captured output with the expected result. This only works if the program just outputs one
+    /// value as the result
+    fn run_example_check_single_i64_output(filename: &str, correct_result: i64) {
+        let mut interpreter = Interpreter::new();
+        // Enable output capturing. This captures all calls to `print`
+        interpreter.capture_output = true;
+
+        // Load and run the given program
+        let code = read_to_string(format!("examples/{filename}")).unwrap();
+        interpreter.run_str(&code);
+
+        // Compare the captured output with the expected value
+        let expected_output = [Value::I64(correct_result)];
+        assert_eq!(interpreter.output(), &expected_output);
+    }
+
+    #[test]
+    fn test_euler1() {
+        run_example_check_single_i64_output("euler1.nek", 233168);
+    }
+
+    #[test]
+    fn test_euler2() {
+        run_example_check_single_i64_output("euler2.nek", 4613732);
+    }
+
+    #[test]
+    fn test_euler3() {
+        run_example_check_single_i64_output("euler3.nek", 6857);
+    }
+
+    #[test]
+    fn test_euler4() {
+        run_example_check_single_i64_output("euler4.nek", 906609);
+    }
+
+    #[test]
+    fn test_euler5() {
+        run_example_check_single_i64_output("euler5.nek", 232792560);
+    }
+
+    #[test]
+    fn test_recursive_fib() {
+        run_example_check_single_i64_output("recursive_fib.nek", 832040);
+    }
+
+    #[test]
+    fn test_functions() {
+        run_example_check_single_i64_output("test_functions.nek", 69);
+    }
+}
+

src/main.rs

@@ -1,23 +1,59 @@
-use nek_lang::{lexer::lex, parser::parse, interpreter::Interpreter};
+use std::{env::args, fs, process::exit};
 
+use nek_lang::{interpreter::Interpreter, nice_panic};
+
+/// Cli configuration flags and arguments. This could be done with `clap`, but since only so few
+/// arguments are supported this seems kind of overkill.
+#[derive(Debug, Default)]
+struct CliConfig {
+    print_tokens: bool,
+    print_ast: bool,
+    no_optimizations: bool,
+    file: Option<String>,
+}
 
 fn main() {
+    let mut conf = CliConfig::default();
 
-    let mut code = String::new();
+    // Go through all commandline arguments except the first (filename)
-
+    for arg in args().skip(1) {
-    std::io::stdin().read_line(&mut code).unwrap();
+        match arg.as_str() {
-    let code = code.trim();
+            "--token" | "-t" => conf.print_tokens = true,
-
+            "--ast" | "-a" => conf.print_ast = true,
-    let tokens = lex(&code);
+            "--no-opt" | "-n" => conf.no_optimizations = true,
-
+            "--help" | "-h" => print_help(),
-    println!("Tokens: {:?}\n", tokens);
+            file if !arg.starts_with("-") && conf.file.is_none() => {
-
+                conf.file = Some(file.to_string())
-    let ast = parse(tokens);
+            }
-
+            _ => nice_panic!("Error: Invalid argument '{}'", arg),
-    println!("Ast: {:#?}\n", ast);
+        }
+    }
 
     let mut interpreter = Interpreter::new();
 
-    interpreter.run(ast);
+    interpreter.print_tokens = conf.print_tokens;
+    interpreter.print_ast = conf.print_ast;
+    interpreter.optimize_ast = !conf.no_optimizations;
 
+    if let Some(file) = &conf.file {
+        let code = match fs::read_to_string(file) {
+            Ok(code) => code,
+            Err(_) => nice_panic!("Error: Could not read file '{}'", file),
+        };
+        // Lex, parse and run the program
+        interpreter.run_str(&code);
+    } else {
+        println!("Error: No file given\n");
+        print_help();
+    }
+}
+
+fn print_help() {
+    println!("Usage nek-lang [FLAGS] [FILE]");
+    println!("FLAGS: ");
+    println!("-t, --token        Print the lexed tokens");
+    println!("-a, --ast          Print the abstract syntax tree");
+    println!("-n, --no-opt       Disable the AST optimizations");
+    println!("-h, --help         Show this help screen");
+    exit(0);
 }

src/parser.rs

@@ -1,48 +1,378 @@
-use std::iter::Peekable;
+use thiserror::Error;
 
-use crate::lexer::Token;
+use crate::{
+    ast::{Ast, BlockScope, Expression, FunDecl, If, Loop, Statement, VarDecl},
+    stringstore::{Sid, StringStore},
+    token::Token,
+    util::{PutBackIter, PutBackableExt},
+    T,
+};
 
-/// Types for binary operators
+/// Errors that can occur while parsing
-#[derive(Debug, PartialEq, Eq, Clone)]
+#[derive(Debug, Error)]
-pub enum BinOpType {
+pub enum ParseErr {
-    /// Addition
+    #[error("Unexpected Token \"{0:?}\", expected \"{1}\"")]
-    Add,
+    UnexpectedToken(Token, String),
 
-    /// Multiplication
+    #[error("Left hand side of declaration is not a variable")]
-    Mul,
+    DeclarationOfNonVar,
+
+    #[error("Use of undefined variable \"{0}\"")]
+    UseOfUndeclaredVar(String),
+
+    #[error("Use of undefined function \"{0}\"")]
+    UseOfUndeclaredFun(String),
+
+    #[error("Redeclation of function \"{0}\"")]
+    RedeclarationFun(String),
+
+    #[error("Function not declared at top level \"{0}\"")]
+    FunctionOnNonTopLevel(String),
 }
 
-#[derive(Debug, PartialEq, Eq, Clone)]
+/// A result that can either be Ok, or a ParseErr
-pub enum Ast {
+type ResPE<T> = Result<T, ParseErr>;
-    /// Integer literal (64-bit)
+
-    I64(i64),
+/// This macro can be used to quickly and easily assert if the next token is matching the expected
-    /// Binary operation. Consists of type, left hand side and right hand side
+/// token and return an appropriate error if not. Since this is intended to be used inside the 
-    BinOp(BinOpType, Box<Ast>, Box<Ast>),
+/// parser, the first argument should always be `self`.
+macro_rules! validate_next {
+    ($self:ident, $expected_tok:pat, $expected_str:expr) => {
+        match $self.next() {
+            $expected_tok => (),
+            tok => return Err(ParseErr::UnexpectedToken(tok, format!("{}", $expected_str))),
+        }
+    };
 }
 
+/// Parse the given tokens into an abstract syntax tree
+pub fn parse<T: Iterator<Item = Token>, A: IntoIterator<IntoIter = T>>(tokens: A) -> ResPE<Ast> {
+    let parser = Parser::new(tokens);
+    parser.parse()
+}
+
+/// A parser that takes in a Token Stream and can create a full abstract syntax tree from it.
 struct Parser<T: Iterator<Item = Token>> {
-    tokens: Peekable<T>,
+    tokens: PutBackIter<T>,
+    string_store: StringStore,
+    var_stack: Vec<Sid>,
+    fun_stack: Vec<Sid>,
+    nesting_level: usize,
 }
 
 impl<T: Iterator<Item = Token>> Parser<T> {
     /// Create a new parser to parse the given Token Stream
-    fn new<A: IntoIterator<IntoIter = T>>(tokens: A) -> Self {
+    pub fn new<A: IntoIterator<IntoIter = T>>(tokens: A) -> Self {
-        let tokens = tokens.into_iter().peekable();
+        let tokens = tokens.into_iter().putbackable();
-        Self { tokens }
+        let string_store = StringStore::new();
+        let var_stack = Vec::new();
+        let fun_stack = Vec::new();
+        Self {
+            tokens,
+            string_store,
+            var_stack,
+            fun_stack,
+            nesting_level: 0,
+        }
     }
 
-    fn parse(&mut self) -> Ast {
+    /// Consume the parser and try to create the abstract syntax tree from the token stream
-        self.parse_expr()
+    pub fn parse(mut self) -> ResPE<Ast> {
+        let main = self.parse_scoped_block()?;
+        Ok(Ast {
+            main,
+            stringstore: self.string_store,
+        })
     }
 
-    fn parse_expr(&mut self) -> Ast {
+    /// Parse a series of statements together as a BlockScope. This will continuously parse 
-        let lhs = self.parse_primary();
+    /// statements until encountering end-of-file or a block end '}' .
+    fn parse_scoped_block(&mut self) -> ResPE<BlockScope> {
+        self.parse_scoped_block_fp_offset(0)
+    }
+
+    /// Same as parse_scoped_block, but an offset to the framepointer can be specified to allow
+    /// for easily passing variables into scopes from the outside. This is used when parsing 
+    /// function calls
+    fn parse_scoped_block_fp_offset(&mut self, framepointer_offset: usize) -> ResPE<BlockScope> {
+        self.nesting_level += 1;
+        let framepointer = self.var_stack.len() - framepointer_offset;
+        let mut prog = Vec::new();
+
+        loop {
+            match self.peek() {
+                // Just a semicolon is an empty statement. So just consume it
+                T![;] => {
+                    self.next();
+                }
+
+                // '}' end the current block and EoF ends everything, as the end of the tokenstream 
+                // is reached
+                T![EoF] | T!['}'] => break,
+
+                // Create a new scoped block
+                T!['{'] => {
+                    self.next();
+                    prog.push(Statement::Block(self.parse_scoped_block()?));
+
+                    validate_next!(self, T!['}'], "}");
+                }
+
+                // By default try to lex statements
+                _ => prog.push(self.parse_stmt()?),
+            }
+        }
+
+        // Reset the stack to where it was before entering the scope
+        self.var_stack.truncate(framepointer);
+        self.nesting_level -= 1;
+
+        Ok(prog)
+    }
+
+    /// Parse a single statement from the tokens
+    fn parse_stmt(&mut self) -> ResPE<Statement> {
+        let stmt = match self.peek() {
+            // Break statement
+            T![break] => {
+                self.next();
+
+                // After the statement, there must be a semicolon
+                validate_next!(self, T![;], ";");
+
+                Statement::Break
+            }
+
+            // Continue statement
+            T![continue] => {
+                self.next();
+
+                // After the statement, there must be a semicolon
+                validate_next!(self, T![;], ";");
+
+                Statement::Continue
+            }
+
+            // Loop statement
+            T![loop] => Statement::Loop(self.parse_loop()?),
+
+            // Print statement
+            T![print] => {
+                self.next();
+
+                let expr = self.parse_expr()?;
+
+                // After the statement, there must be a semicolon
+                validate_next!(self, T![;], ";");
+
+                Statement::Print(expr)
+            }
+
+            // Return statement
+            T![return] => {
+                self.next();
+                let stmt = Statement::Return(self.parse_expr()?);
+
+                // After a statement, there must be a semicolon
+                validate_next!(self, T![;], ";");
+
+                stmt
+            }
+
+            // If statement
+            T![if] => Statement::If(self.parse_if()?),
+
+            // Function definition statement
+            T![fun] => {
+
+                self.next();
+
+                // Expect an identifier as the function name
+                let fun_name = match self.next() {
+                    T![ident(fun_name)] => fun_name,
+                    tok => return Err(ParseErr::UnexpectedToken(tok, "<ident>".to_string())),
+                };
+                
+                // Only allow function definitions on the top level
+                if self.nesting_level > 1 {
+                    return Err(ParseErr::FunctionOnNonTopLevel(fun_name));
+                }
+
+                // Intern the function name
+                let fun_name = self.string_store.intern_or_lookup(&fun_name);
+
+                // Check if the function name already exists
+                if self.fun_stack.contains(&fun_name) {
+                    return Err(ParseErr::RedeclarationFun(
+                        self.string_store
+                            .lookup(fun_name)
+                            .cloned()
+                            .unwrap_or("<unknown>".to_string()),
+                    ));
+                }
+
+                // Put the function name on the fucntion stack for precalculating the stack 
+                // positions
+                let fun_stackpos = self.fun_stack.len();
+                self.fun_stack.push(fun_name);
+
+
+                let mut arg_names = Vec::new();
+
+                validate_next!(self, T!['('], "(");
+
+                // Parse the optional arguments inside the parentheses
+                while matches!(self.peek(), T![ident(_)]) {
+                    let var_name = match self.next() {
+                        T![ident(var_name)] => var_name,
+                        _ => unreachable!(),
+                    };
+
+                    // Intern argument names 
+                    let var_name = self.string_store.intern_or_lookup(&var_name);
+                    arg_names.push(var_name);
+
+                    // Push the variable onto the varstack
+                    self.var_stack.push(var_name);
+
+                    // If there are more args skip the comma so that the loop will read the argname
+                    if self.peek() == &T![,] {
+                        self.next();
+                    }
+                }
+
+                validate_next!(self, T![')'], ")");
+
+                validate_next!(self, T!['{'], "{");
+
+                // Create the scoped block with a stack offset. This will pop the args that are
+                // added to the stack while parsing args
+                let body = self.parse_scoped_block_fp_offset(arg_names.len())?;
+
+                validate_next!(self, T!['}'], "}");
+
+                Statement::FunDeclare(FunDecl {
+                    name: fun_name,
+                    fun_stackpos,
+                    argnames: arg_names,
+                    body: body.into(),
+                })
+            }
+
+            // Either a variable declaration statement or an expression statement
+            _ => {
+                // To decide if it is a declaration or an expression, a lookahead is needed 
+                let first = self.next();
+
+                let stmt = match (first, self.peek()) {
+                    // Identifier and "<-" is a declaration
+                    (T![ident(name)], T![<-]) => {
+                        self.next();
+
+                        let rhs = self.parse_expr()?;
+
+                        let sid = self.string_store.intern_or_lookup(&name);
+                        let sp = self.var_stack.len();
+                        self.var_stack.push(sid);
+
+                        Statement::Declaration(VarDecl {
+                            name: sid,
+                            var_stackpos: sp,
+                            rhs,
+                        })
+                    }
+                    // Anything else must be an expression
+                    (first, _) => {
+                        // Put the first token back in order for the parse_expr to see it 
+                        self.putback(first);
+                        Statement::Expr(self.parse_expr()?)
+                    }
+                };
+
+                // After a statement, there must be a semicolon
+                validate_next!(self, T![;], ";");
+
+                stmt
+            }
+        };
+        Ok(stmt)
+    }
+
+    /// Parse an if statement from the tokens
+    fn parse_if(&mut self) -> ResPE<If> {
+        validate_next!(self, T![if], "if");
+
+        let condition = self.parse_expr()?;
+
+        validate_next!(self, T!['{'], "{");
+
+        let body_true = self.parse_scoped_block()?;
+
+        validate_next!(self, T!['}'], "}");
+
+        let mut body_false = BlockScope::default();
+
+        // Optionally parse the else part
+        if self.peek() == &T![else] {
+            self.next();
+
+            validate_next!(self, T!['{'], "{");
+
+            body_false = self.parse_scoped_block()?;
+
+            validate_next!(self, T!['}'], "}");
+        }
+
+        Ok(If {
+            condition,
+            body_true,
+            body_false,
+        })
+    }
+
+    /// Parse a loop statement from the tokens
+    fn parse_loop(&mut self) -> ResPE<Loop> {
+        validate_next!(self, T![loop], "loop");
+
+        let mut condition = None;
+        let mut advancement = None;
+
+        // Check if the optional condition is present
+        if !matches!(self.peek(), T!['{']) {
+            condition = Some(self.parse_expr()?);
+
+            // Check if the optional advancement is present
+            if matches!(self.peek(), T![;]) {
+                self.next();
+                advancement = Some(self.parse_expr()?);
+            }
+        }
+
+        validate_next!(self, T!['{'], "{");
+
+        let body = self.parse_scoped_block()?;
+
+        validate_next!(self, T!['}'], "}");
+
+        Ok(Loop {
+            condition,
+            advancement,
+            body,
+        })
+    }
+
+    /// Parse a single expression from the tokens
+    fn parse_expr(&mut self) -> ResPE<Expression> {
+        let lhs = self.parse_primary()?;
         self.parse_expr_precedence(lhs, 0)
     }
 
-    /// Parse binary expressions with a precedence equal to or higher than min_prec
+    /// Parse binary expressions with a precedence equal to or higher than min_prec.
-    fn parse_expr_precedence(&mut self, mut lhs: Ast, min_prec: u8) -> Ast {
+    /// This uses the precedence climbing methode for dealing with the operator precedences:
+    /// https://en.wikipedia.org/wiki/Operator-precedence_parser#Precedence_climbing_method
+    fn parse_expr_precedence(&mut self, mut lhs: Expression, min_prec: u8) -> ResPE<Expression> {
         while let Some(binop) = &self.peek().try_to_binop() {
+            // Stop if the next operator has a lower binding power
             if !(binop.precedence() >= min_prec) {
                 break;
             }
@@ -51,89 +381,211 @@ impl<T: Iterator<Item = Token>> Parser<T> {
             // valid
             let binop = self.next().try_to_binop().unwrap();
 
-            let mut rhs = self.parse_primary();
+            let mut rhs = self.parse_primary()?;
 
             while let Some(binop2) = &self.peek().try_to_binop() {
                 if !(binop2.precedence() > binop.precedence()) {
                     break;
                 }
 
-                rhs = self.parse_expr_precedence(rhs, binop.precedence() + 1);
+                rhs = self.parse_expr_precedence(rhs, binop.precedence() + 1)?;
             }
 
-            lhs = Ast::BinOp(binop, lhs.into(), rhs.into());
+            lhs = Expression::BinOp(binop, lhs.into(), rhs.into());
         }
 
-        lhs
+        Ok(lhs)
     }
 
-    /// Parse a primary expression (for now only number)
+    /// Parse a primary expression. A primary can be a literal value, variable, function call,
-    fn parse_primary(&mut self) -> Ast {
+    /// array indexing, parentheses grouping or a unary operation
-        match self.next() {
+    fn parse_primary(&mut self) -> ResPE<Expression> {
-            Token::I64(val) => Ast::I64(val),
+        let primary = match self.next() {
+            // Literal i64
+            T![i64(val)] => Expression::I64(val),
 
-            tok => panic!("Error parsing primary expr: Unexpected Token '{:?}'", tok),
+            // Literal String
+            T![str(text)] => Expression::String(self.string_store.intern_or_lookup(&text)),
+
+            // Array literal. Square brackets containing the array size as expression
+            T!['['] => {
+                let size = self.parse_expr()?;
+
+                validate_next!(self, T![']'], "]");
+
+                Expression::ArrayLiteral(size.into())
+            }
+
+            // Array sccess, aka indexing. An ident followed by square brackets containing the
+            // index as an expression
+            T![ident(name)] if self.peek() == &T!['['] => {
+                // Get the stack position of the array variable
+                let sid = self.string_store.intern_or_lookup(&name);
+                let stackpos = self.get_stackpos(sid)?;
+
+                self.next();
+
+                let index = self.parse_expr()?;
+
+                validate_next!(self, T![']'], "]");
+
+                Expression::ArrayAccess(sid, stackpos, index.into())
+            }
+
+            // Identifier followed by parenthesis is a function call
+            T![ident(name)] if self.peek() == &T!['('] => {
+                // Skip the opening parenthesis
+                self.next();
+
+                let sid = self.string_store.intern_or_lookup(&name);
+
+                let mut args = Vec::new();
+
+                // Parse the arguments as expressions
+                while !matches!(self.peek(), T![')']) {
+                    let arg = self.parse_expr()?;
+                    args.push(arg);
+
+                    // If there are more args skip the comma so that the loop will read the argname
+                    if self.peek() == &T![,] {
+                        self.next();
                     }
                 }
 
-    /// Get the next Token without removing it
+                validate_next!(self, T![')'], ")");
+
+                // Find the function stack position
+                let fun_stackpos = self.get_fun_stackpos(sid)?;
+
+                Expression::FunCall(sid, fun_stackpos, args)
+            }
+
+            // Just an identifier is a variable
+            T![ident(name)] => {
+                // Find the variable stack position
+                let sid = self.string_store.intern_or_lookup(&name);
+                let stackpos = self.get_stackpos(sid)?;
+
+                Expression::Var(sid, stackpos)
+            }
+
+            // Parentheses grouping
+            T!['('] => {
+                // Contained inbetween the parentheses can be any other expression
+                let inner_expr = self.parse_expr()?;
+
+                // Verify that there is a closing parenthesis
+                validate_next!(self, T![')'], ")");
+
+                inner_expr
+            }
+
+            // Unary operations or invalid token
+            tok => match tok.try_to_unop() {
+                // If the token is a valid unary operation, parse it as such
+                Some(uot) => Expression::UnOp(uot, self.parse_primary()?.into()),
+                
+                // Otherwise it's an unexpected token
+                None => return Err(ParseErr::UnexpectedToken(tok, "primary".to_string())),
+            },
+        };
+
+        Ok(primary)
+    }
+
+    /// Try to get the position of a variable on the variable stack. This is needed to precalculate 
+    /// the stackpositions in order to save time when executing
+    fn get_stackpos(&self, varid: Sid) -> ResPE<usize> {
+        self.var_stack
+            .iter()
+            .rev()
+            .position(|it| *it == varid)
+            .map(|it| it)
+            .ok_or(ParseErr::UseOfUndeclaredVar(
+                self.string_store
+                    .lookup(varid)
+                    .map(String::from)
+                    .unwrap_or("<unknown>".to_string()),
+            ))
+    }
+
+    /// Try to get the position of a function on the function stack. This is needed to precalculate 
+    /// the stackpositions in order to save time when executing
+    fn get_fun_stackpos(&self, varid: Sid) -> ResPE<usize> {
+        self.fun_stack
+            .iter()
+            .rev()
+            .position(|it| *it == varid)
+            .map(|it| self.fun_stack.len() - it - 1)
+            .ok_or(ParseErr::UseOfUndeclaredFun(
+                self.string_store
+                    .lookup(varid)
+                    .map(String::from)
+                    .unwrap_or("<unknown>".to_string()),
+            ))
+    }
+
+    /// Get the next Token without removing it. If there are no more tokens left, the EoF token is 
+    /// returned. This follows the same reasoning as in the Lexer
     fn peek(&mut self) -> &Token {
-        self.tokens.peek().unwrap_or(&Token::EoF)
+        self.tokens.peek().unwrap_or(&T![EoF])
     }
 
-    /// Advance to next Token and return the removed Token
+    /// Put a single token back into the token stream
+    fn putback(&mut self, tok: Token) {
+        self.tokens.putback(tok);
+    }
+
+    /// Advance to next Token and return the removed Token. If there are no more tokens left, the 
+    /// EoF token is returned. This follows the same reasoning as in the Lexer
     fn next(&mut self) -> Token {
-        self.tokens.next().unwrap_or(Token::EoF)
+        self.tokens.next().unwrap_or(T![EoF])
-    }
-}
-
-pub fn parse<T: Iterator<Item = Token>, A: IntoIterator<IntoIter = T>>(tokens: A) -> Ast {
-    let mut parser = Parser::new(tokens);
-    parser.parse()
-}
-
-impl BinOpType {
-    /// Get the precedence for a binary operator. Higher value means the OP is stronger binding.
-    /// For example Multiplication is stronger than addition, so Mul has higher precedence than Add.
-    fn precedence(&self) -> u8 {
-        match self {
-            BinOpType::Add => 0,
-            BinOpType::Mul => 1,
-        }
     }
 }
 
 #[cfg(test)]
 mod tests {
-    use super::{parse, Ast, BinOpType};
+    use crate::{
-    use crate::lexer::Token;
+        ast::{BinOpType, Expression, Statement},
+        parser::parse,
+        T,
+    };
 
+    /// A very simple test to check if the parser correctly parses a simple expression
     #[test]
     fn test_parser() {
-        // Expression: 1 + 2 * 3 + 4
+        // Expression: 1 + 2 * 3 - 4
-        // With precedence: (1 + (2 * 3)) + 4
+        // With precedence: (1 + (2 * 3)) - 4
         let tokens = [
-            Token::I64(1),
+            T![i64(1)],
-            Token::Add,
+            T![+],
-            Token::I64(2),
+            T![i64(2)],
-            Token::Mul,
+            T![*],
-            Token::I64(3),
+            T![i64(3)],
-            Token::Add,
+            T![-],
-            Token::I64(4),
+            T![i64(4)],
+            T![;],
         ];
 
-        let expected = Ast::BinOp(
+        let expected = Statement::Expr(Expression::BinOp(
+            BinOpType::Sub,
+            Expression::BinOp(
                 BinOpType::Add,
-            Ast::BinOp(
+                Expression::I64(1).into(),
-                BinOpType::Add,
+                Expression::BinOp(
-                Ast::I64(1).into(),
+                    BinOpType::Mul,
-                Ast::BinOp(BinOpType::Mul, Ast::I64(2).into(), Ast::I64(3).into()).into(),
+                    Expression::I64(2).into(),
+                    Expression::I64(3).into(),
                 )
                 .into(),
-            Ast::I64(4).into(),
+            )
-        );
+            .into(),
+            Expression::I64(4).into(),
+        ));
 
-        let actual = parse(tokens);
+        let expected = vec![expected];
-        assert_eq!(expected, actual);
+
+        let actual = parse(tokens).unwrap();
+        assert_eq!(expected, actual.main);
     }
 }

src/stringstore.rs

@@ -0,0 +1,104 @@
+use std::collections::HashMap;
+
+/// A StringID that identifies a String inside the stringstore. This is only valid for the 
+/// StringStore that created the ID. These StringIDs can be trivialy and cheaply copied
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Sid(usize);
+
+/// A Datastructure that stores strings, handing out StringIDs that can be used to retrieve the
+/// real strings at a later point. This is called interning.
+#[derive(Clone, Default)]
+pub struct StringStore {
+    /// The actual strings that are stored in the StringStore. The StringIDs match the index of the
+    /// string inside of this strings vector
+    strings: Vec<String>,
+    /// A Hashmap that allows to match already interned Strings to their StringID. This allows for
+    /// deduplication since the same string won't be stored twice
+    sids: HashMap<String, Sid>,
+}
+
+impl StringStore {
+
+    /// Create a new empty StringStore
+    pub fn new() -> Self {
+        Self { strings: Vec::new(), sids: HashMap::new() }
+    }
+
+    /// Put the given string into the StringStore and get a StringID in return. If the string is 
+    /// not yet stored, it will be after this.
+    /// 
+    /// Note: The generated StringIDs are only valid for the StringStore that created them. Using 
+    /// the IDs with another StringStore is undefined behavior. It might return wrong Strings or 
+    /// None.
+    pub fn intern_or_lookup(&mut self, text: &str) -> Sid {
+        self.sids.get(text).copied().unwrap_or_else(|| {
+            let sid = Sid(self.strings.len());
+            self.strings.push(text.to_string());
+            self.sids.insert(text.to_string(), sid);
+            sid
+        })
+    }
+
+    /// Lookup and retrieve a string by the StringID. If the String is not found, None is returned.
+    /// 
+    /// Note: The generated StringIDs are only valid for the StringStore that created them. Using 
+    /// the IDs with another StringStore is undefined behavior. It might return wrong Strings or 
+    /// None.
+    pub fn lookup(&self, sid: Sid) -> Option<&String> {
+        self.strings.get(sid.0)
+    }
+
+}
+
+#[cfg(test)]
+mod tests {
+    use super::StringStore;
+
+    #[test]
+    fn test_stringstore_intern_lookup() {
+        let mut ss = StringStore::new();
+        let s1 = "Hello";
+        let s2 = "World";
+
+        let id1 = ss.intern_or_lookup(s1);
+        assert_eq!(ss.lookup(id1).unwrap().as_str(), s1);
+
+        let id2 = ss.intern_or_lookup(s2);
+        assert_eq!(ss.lookup(id2).unwrap().as_str(), s2);
+        assert_eq!(ss.lookup(id1).unwrap().as_str(), s1);
+    }
+
+    #[test]
+    fn test_stringstore_no_duplicates() {
+        let mut ss = StringStore::new();
+        let s1 = "Hello";
+        let s2 = "World";
+
+        let id1_1 = ss.intern_or_lookup(s1);
+        assert_eq!(ss.lookup(id1_1).unwrap().as_str(), s1);
+
+        let id1_2 = ss.intern_or_lookup(s1);
+        assert_eq!(ss.lookup(id1_2).unwrap().as_str(), s1);
+
+        // Check that the string is the same
+        assert_eq!(id1_1, id1_2);
+
+        // Check that only one string is actually stored
+        assert_eq!(ss.strings.len(), 1);
+        assert_eq!(ss.sids.len(), 1);
+
+
+
+        let id2_1 = ss.intern_or_lookup(s2);
+        assert_eq!(ss.lookup(id2_1).unwrap().as_str(), s2);
+
+        let id2_2 = ss.intern_or_lookup(s2);
+        assert_eq!(ss.lookup(id2_2).unwrap().as_str(), s2);
+
+        // Check that the string is the same
+        assert_eq!(id2_1, id2_2);
+
+        assert_eq!(ss.strings.len(), 2);
+        assert_eq!(ss.sids.len(), 2);
+    }
+}

src/token.rs

@@ -0,0 +1,379 @@
+use crate::{
+    ast::{BinOpType, UnOpType},
+    T,
+};
+
+/// Language keywords
+#[derive(Debug, PartialEq, Eq)]
+pub enum Keyword {
+    /// Loop keyword ("loop")
+    Loop,
+    /// Print keyword ("print")
+    Print,
+    /// If keyword ("if")
+    If,
+    /// Else keyword ("else")
+    Else,
+    /// Function declaration keyword ("fun")
+    Fun,
+    /// Return keyword ("return")
+    Return,
+    /// Break keyword ("break")
+    Break,
+    /// Continue keyword ("continue")
+    Continue,
+}
+
+/// Literal values
+#[derive(Debug, PartialEq, Eq)]
+pub enum Literal {
+    /// Integer literal (64-bit)
+    I64(i64),
+    /// String literal
+    String(String),
+}
+
+/// Combined tokens that consist of a combination of characters
+#[derive(Debug, PartialEq, Eq)]
+pub enum Combo {
+    /// Equal Equal ("==")
+    Equal2,
+
+    /// Exclamation mark Equal ("!=")
+    ExclamationMarkEqual,
+
+    /// Ampersand Ampersand ("&&")
+    Ampersand2,
+
+    /// Pipe Pipe ("||")
+    Pipe2,
+
+    /// LessThan LessThan ("<<")
+    LessThan2,
+
+    /// GreaterThan GreaterThan (">>")
+    GreaterThan2,
+
+    /// LessThan Equal ("<=")
+    LessThanEqual,
+
+    /// GreaterThan Equal (">=")
+    GreaterThanEqual,
+
+    /// LessThan Minus ("<-")
+    LessThanMinus,
+}
+
+/// Tokens are a group of one or more sourcecode characters that have a meaning together
+#[derive(Debug, PartialEq, Eq)]
+pub enum Token {
+    /// Literal value token
+    Literal(Literal),
+
+    /// Keyword token
+    Keyword(Keyword),
+
+    /// Identifier token (names for variables, functions, ...)
+    Ident(String),
+
+    /// Combined tokens consisting of multiple characters
+    Combo(Combo),
+
+    /// Comma (",")
+    Comma,
+
+    /// Equal Sign ("=")
+    Equal,
+
+    /// Semicolon (";")
+    Semicolon,
+
+    /// End of file (This is not generated by the lexer, but the parser uses this to find the 
+    /// end of the token stream)
+    EoF,
+
+    /// Left Bracket ("[")
+    LBracket,
+
+    /// Right Bracket ("]")
+    RBracket,
+
+    /// Left Parenthesis ("(")
+    LParen,
+
+    /// Right Parenthesis (")"")
+    RParen,
+
+    /// Left curly braces ("{")
+    LBraces,
+
+    /// Right curly braces ("}")
+    RBraces,
+
+    /// Plus ("+")
+    Plus,
+
+    /// Minus ("-")
+    Minus,
+
+    /// Asterisk ("*")
+    Asterisk,
+
+    /// Slash ("/")
+    Slash,
+
+    /// Percent ("%")
+    Percent,
+
+    /// Pipe ("|")
+    Pipe,
+
+    /// Tilde ("~")
+    Tilde,
+
+    /// Logical not ("!")
+    Exclamationmark,
+
+    /// Left angle bracket ("<")
+    LessThan,
+
+    /// Right angle bracket (">")
+    GreaterThan,
+
+    /// Ampersand ("&")
+    Ampersand,
+
+    /// Circumflex ("^")
+    Circumflex,
+}
+
+impl Token {
+    /// If the Token can be used as a binary operation type, get the matching BinOpType. Otherwise
+    /// return None.
+    pub fn try_to_binop(&self) -> Option<BinOpType> {
+        Some(match self {
+            T![+] => BinOpType::Add,
+            T![-] => BinOpType::Sub,
+
+            T![*] => BinOpType::Mul,
+            T![/] => BinOpType::Div,
+            T![%] => BinOpType::Mod,
+
+            T![&] => BinOpType::BAnd,
+            T![|] => BinOpType::BOr,
+            T![^] => BinOpType::BXor,
+
+            T![&&] => BinOpType::LAnd,
+            T![||] => BinOpType::LOr,
+
+            T![<<] => BinOpType::Shl,
+            T![>>] => BinOpType::Shr,
+
+            T![==] => BinOpType::EquEqu,
+            T![!=] => BinOpType::NotEqu,
+
+            T![<] => BinOpType::Less,
+            T![<=] => BinOpType::LessEqu,
+
+            T![>] => BinOpType::Greater,
+            T![>=] => BinOpType::GreaterEqu,
+
+            T![=] => BinOpType::Assign,
+
+            _ => return None,
+        })
+    }
+
+    /// If the token can be used as a unary operation type, get the matching UnOpType. Otherwise 
+    /// return None
+    pub fn try_to_unop(&self) -> Option<UnOpType> {
+        Some(match self {
+            T![-] => UnOpType::Negate,
+            T![!] => UnOpType::LNot,
+            T![~] => UnOpType::BNot,
+
+            _ => return None,
+        })
+    }
+}
+
+/// Macro to quickly create a token of the specified kind. As this is implemented as a macro, it 
+/// can be used anywhere including in patterns.
+/// 
+/// An implementation should exist for each token, so that there is no need to ever write out the
+/// long token definitions.
+#[macro_export]
+macro_rules! T {
+    // Keywords
+    [loop] => {
+        crate::token::Token::Keyword(crate::token::Keyword::Loop)
+    };
+
+    [print] => {
+        crate::token::Token::Keyword(crate::token::Keyword::Print)
+    };
+
+    [if] => {
+        crate::token::Token::Keyword(crate::token::Keyword::If)
+    };
+
+    [else] => {
+        crate::token::Token::Keyword(crate::token::Keyword::Else)
+    };
+
+    [fun] => {
+        crate::token::Token::Keyword(crate::token::Keyword::Fun)
+    };
+
+    [return] => {
+        crate::token::Token::Keyword(crate::token::Keyword::Return)
+    };
+
+    [break] => {
+        crate::token::Token::Keyword(crate::token::Keyword::Break)
+    };
+
+    [continue] => {
+        crate::token::Token::Keyword(crate::token::Keyword::Continue)
+    };
+
+    // Literals
+    [i64($($val:tt)*)] => {
+        crate::token::Token::Literal(crate::token::Literal::I64($($val)*))
+    };
+
+    [str($($val:tt)*)] => {
+        crate::token::Token::Literal(crate::token::Literal::String($($val)*))
+    };
+
+    // Ident
+    [ident($($val:tt)*)] => {
+        crate::token::Token::Ident($($val)*)
+    };
+
+    // Combo crate::token::Tokens
+    [==] => {
+        crate::token::Token::Combo(crate::token::Combo::Equal2)
+    };
+
+    [!=] => {
+        crate::token::Token::Combo(crate::token::Combo::ExclamationMarkEqual)
+    };
+
+    [&&] => {
+        crate::token::Token::Combo(crate::token::Combo::Ampersand2)
+    };
+
+    [||] => {
+        crate::token::Token::Combo(crate::token::Combo::Pipe2)
+    };
+
+    [<<] => {
+        crate::token::Token::Combo(crate::token::Combo::LessThan2)
+    };
+
+    [>>] => {
+        crate::token::Token::Combo(crate::token::Combo::GreaterThan2)
+    };
+
+    [<=] => {
+        crate::token::Token::Combo(crate::token::Combo::LessThanEqual)
+    };
+
+    [>=] => {
+        crate::token::Token::Combo(crate::token::Combo::GreaterThanEqual)
+    };
+
+    [<-] => {
+        crate::token::Token::Combo(crate::token::Combo::LessThanMinus)
+    };
+
+    // Normal Tokens
+    [,] => {
+        crate::token::Token::Comma
+    };
+
+    [=] => {
+        crate::token::Token::Equal
+    };
+
+    [;] => {
+        crate::token::Token::Semicolon
+    };
+
+    [EoF] => {
+        crate::token::Token::EoF
+    };
+
+    ['['] => {
+        crate::token::Token::LBracket
+    };
+
+    [']'] => {
+        crate::token::Token::RBracket
+    };
+
+    ['('] => {
+        crate::token::Token::LParen
+    };
+
+    [')'] => {
+        crate::token::Token::RParen
+    };
+
+    ['{'] => {
+        crate::token::Token::LBraces
+    };
+
+    ['}'] => {
+        crate::token::Token::RBraces
+    };
+
+    [+] => {
+        crate::token::Token::Plus
+    };
+
+    [-] => {
+        crate::token::Token::Minus
+    };
+
+    [*] => {
+        crate::token::Token::Asterisk
+    };
+
+    [/] => {
+        crate::token::Token::Slash
+    };
+
+    [%] => {
+        crate::token::Token::Percent
+    };
+
+    [|] => {
+        crate::token::Token::Pipe
+    };
+
+    [~] => {
+        crate::token::Token::Tilde
+    };
+
+    [!] => {
+        crate::token::Token::Exclamationmark
+    };
+
+    [<] => {
+        crate::token::Token::LessThan
+    };
+
+    [>] => {
+        crate::token::Token::GreaterThan
+    };
+
+    [&] => {
+        crate::token::Token::Ampersand
+    };
+
+    [^] => {
+        crate::token::Token::Circumflex
+    };
+}

src/util.rs

@@ -0,0 +1,167 @@
+/// Exit the program with error code 1 and format-print the given text on stderr. This pretty much
+/// works like panic, but doesn't show the additional information that panic adds. Those can be 
+/// interesting for debugging, but don't look that great when building a release executable for an
+/// end user.
+/// When running tests or running in debug mode, panic is used to ensure the tests working 
+/// correctly.
+#[macro_export]
+macro_rules! nice_panic {
+    ($fmt:expr) => {
+        {
+            if cfg!(test) || cfg!(debug_assertions) {
+                panic!($fmt);
+            } else {
+                eprintln!($fmt);
+                std::process::exit(1);
+            }
+        }
+    };
+    ($fmt:expr, $($arg:tt)*) => {
+        {
+            if cfg!(test) || cfg!(debug_assertions) {
+                panic!($fmt, $($arg)*);
+            } else {
+                eprintln!($fmt, $($arg)*);
+                std::process::exit(1);
+            }
+        }
+    };
+}
+
+/// The PutBackIter allows for items to be put back back and to be peeked. Putting an item back
+/// will cause it to be the next item returned by `next`. Peeking an item will get a reference to
+/// the next item in the iterator without removing it.
+///
+/// The whole PutBackIter behaves analogous to `std::iter::Peekable` with the addition of the
+/// `putback` function. This is slightly slower than `Peekable`, but allows for an unlimited number
+/// of putbacks and therefore an unlimited look-ahead range.
+pub struct PutBackIter<T: Iterator> {
+    iter: T,
+    putback_stack: Vec<T::Item>,
+}
+
+impl<T> PutBackIter<T>
+where
+    T: Iterator,
+{
+    /// Make the given iterator putbackable, wrapping it in the PutBackIter type. This effectively
+    /// adds the `peek` and `putback` functions.
+    pub fn new(iter: T) -> Self {
+        Self {
+            iter,
+            putback_stack: Vec::new(),
+        }
+    }
+
+    /// Put the given item back into the iterator. This causes the putbacked items to be returned by
+    /// next in last-in-first-out order (aka. stack order). Only after all previously putback items
+    /// have been returned, the actual underlying iterator is used to get items.
+    /// The number of items that can be put back is unlimited.
+    pub fn putback(&mut self, it: T::Item) {
+        self.putback_stack.push(it);
+    }
+
+    /// Peek the next item, getting a reference to it without removing it from the iterator. This
+    /// also includes items that were previsouly put back and not yet removed.
+    pub fn peek(&mut self) -> Option<&T::Item> {
+        if self.putback_stack.is_empty() {
+            let it = self.next()?;
+            self.putback(it);
+        }
+
+        self.putback_stack.last()
+    }
+}
+
+impl<T> Iterator for PutBackIter<T>
+where
+    T: Iterator,
+{
+    type Item = T::Item;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.putback_stack.pop() {
+            Some(it) => Some(it),
+            None => self.iter.next(),
+        }
+    }
+}
+
+pub trait PutBackableExt {
+    /// Make the iterator putbackable, wrapping it in the PutBackIter type. This effectively
+    /// adds the `peek` and `putback` functions.
+    fn putbackable(self) -> PutBackIter<Self>
+    where
+        Self: Iterator + Sized,
+    {
+        PutBackIter::new(self)
+    }
+}
+
+impl<T: Iterator> PutBackableExt for T {}
+
+#[cfg(test)]
+mod tests {
+    use super::PutBackableExt;
+
+    #[test]
+    fn putback_iter_next() {
+        let mut iter = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].into_iter();
+        let mut pb_iter = iter.clone().putbackable();
+
+        // Check if next works
+        for _ in 0..iter.len() {
+            assert_eq!(pb_iter.next(), iter.next());
+        }
+    }
+
+    #[test]
+    fn putback_iter_peek() {
+        let mut iter_orig = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].into_iter();
+        let mut iter = iter_orig.clone();
+        let mut pb_iter = iter.clone().putbackable();
+
+        for _ in 0..iter.len() {
+            // Check if peek gives a preview of the actual next element
+            assert_eq!(pb_iter.peek(), iter.next().as_ref());
+            // Check if next still returns the next (just peeked) element and not the one after
+            assert_eq!(pb_iter.next(), iter_orig.next());
+        }
+    }
+
+    #[test]
+    fn putback_iter_putback() {
+        let mut iter_orig = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].into_iter();
+        let mut iter = iter_orig.clone();
+        let mut pb_iter = iter.clone().putbackable();
+
+        // Get the first 5 items with next and check if they match
+        let it0 = pb_iter.next();
+        assert_eq!(it0, iter.next());
+        let it1 = pb_iter.next();
+        assert_eq!(it1, iter.next());
+        let it2 = pb_iter.next();
+        assert_eq!(it2, iter.next());
+        let it3 = pb_iter.next();
+        assert_eq!(it3, iter.next());
+        let it4 = pb_iter.next();
+        assert_eq!(it4, iter.next());
+
+        // Put one value back and check if `next` works as expected, returning the just put back
+        // item
+        pb_iter.putback(it0.unwrap());
+        assert_eq!(pb_iter.next(), it0);
+
+        // Put all values back
+        pb_iter.putback(it4.unwrap());
+        pb_iter.putback(it3.unwrap());
+        pb_iter.putback(it2.unwrap());
+        pb_iter.putback(it1.unwrap());
+        pb_iter.putback(it0.unwrap());
+
+        // After all values have been put back, the iter should match the original again
+        for _ in 0..iter.len() {
+            assert_eq!(pb_iter.next(), iter_orig.next());
+        }
+    }
+}