Built-in Functions

Monkey provides several built-in functions that are available globally without any imports. These functions provide essential operations for working with the language's data types.

Array Functions

len(array)

Returns the length of an array or string.

let numbers = [1, 2, 3, 4, 5];
let count = len(numbers); // 5

let text = "Hello";
let textLength = len(text); // 5

Parameters:

• array - An array or string

Returns:

• Integer representing the length

Errors:

• Throws an error if the argument is not an array or string

first(array)

Returns the first element of an array.

let numbers = [10, 20, 30];
let firstNum = first(numbers); // 10

let empty = [];
let firstEmpty = first(empty); // null

Parameters:

• array - An array

Returns:

• The first element of the array, or null if the array is empty

Errors:

• Throws an error if the argument is not an array

last(array)

Returns the last element of an array.

let numbers = [10, 20, 30];
let lastNum = last(numbers); // 30

let empty = [];
let lastEmpty = last(empty); // null

Parameters:

• array - An array

Returns:

• The last element of the array, or null if the array is empty

Errors:

• Throws an error if the argument is not an array

rest(array)

Returns a new array containing all elements except the first one.

let numbers = [1, 2, 3, 4, 5];
let tail = rest(numbers); // [2, 3, 4, 5]

let single = [42];
let restSingle = rest(single); // []

let empty = [];
let restEmpty = rest(empty); // null

Parameters:

• array - An array

Returns:

• A new array with all elements except the first, or null if the array is empty

Errors:

• Throws an error if the argument is not an array

push(array, element)

Returns a new array with the element added to the end. The original array is not modified.

let numbers = [1, 2, 3];
let extended = push(numbers, 4); // [1, 2, 3, 4]
// numbers is still [1, 2, 3]

let mixed = push([1, "hello"], true); // [1, "hello", true]

Parameters:

• array - An array
• element - Any value to add to the array

Returns:

• A new array with the element appended

Errors:

• Throws an error if the first argument is not an array

Output Functions

puts(...args)

Prints the given arguments to standard output, each on a new line.

puts("Hello, World!");
puts(42);
puts(true, "multiple", "arguments");

let name = "Alice";
puts("Hello,", name);

Parameters:

• ...args - Any number of arguments of any type

Returns:

• null

Notes:

• Each argument is printed on a separate line
• Objects are converted to their string representation
• Always returns null

Usage Examples

Here are some practical examples of using built-in functions:

Working with Arrays

// Create and manipulate arrays
let numbers = [1, 2, 3, 4, 5];

puts("Array length:", len(numbers));
puts("First element:", first(numbers));
puts("Last element:", last(numbers));
puts("All but first:", rest(numbers));

// Build arrays incrementally
let empty = [];
let withOne = push(empty, 1);
let withTwo = push(withOne, 2);
puts("Built array:", withTwo);

Implementing Higher-Order Functions

// Map function using built-ins
let map = fn(arr, f) {
  let iter = fn(arr, accumulated) {
    if (len(arr) == 0) {
      accumulated;
    } else {
      iter(rest(arr), push(accumulated, f(first(arr))));
    }
  };
  iter(arr, []);
};

// Filter function using built-ins
let filter = fn(arr, predicate) {
  let iter = fn(arr, accumulated) {
    if (len(arr) == 0) {
      accumulated;
    } else {
      let head = first(arr);
      let tail = rest(arr);
      if (predicate(head)) {
        iter(tail, push(accumulated, head));
      } else {
        iter(tail, accumulated);
      }
    }
  };
  iter(arr, []);
};

// Usage
let numbers = [1, 2, 3, 4, 5];
let doubled = map(numbers, fn(x) { x * 2 });
let evens = filter(numbers, fn(x) { x % 2 == 0 });

puts("Original:", numbers);
puts("Doubled:", doubled);
puts("Evens:", evens);

String Processing

let processText = fn(text) {
  puts("Text:", text);
  puts("Length:", len(text));

  if (len(text) > 10) {
    puts("This is a long text");
  } else {
    puts("This is a short text");
  }
};

processText("Hello");
processText("This is a longer string");