Add general test for functions as example

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/lib.rs

@@ -114,5 +114,23 @@ mod tests {
 
         assert_eq!(interpreter.output(), &expected_output);
     }
+
+    #[test]
+    fn test_functions() {
+        
+        let filename = "test_functions.nek";
+        let correct_result = 69;
+
+        let mut interpreter = Interpreter::new();
+        interpreter.capture_output = true;
+
+        let code = read_to_string(format!("examples/{filename}")).unwrap();
+
+        interpreter.run_str(&code);
+
+        let expected_output = [Value::I64(correct_result)];
+
+        assert_eq!(interpreter.output(), &expected_output);
+    }
 }