leoffx/monkey-lang

the monkey programming language

Monkey is an implementation of a toy programming language for me to learn more about interpreters and compilers.

Here is how we bind values to names in Monkey:

let age = 1;
let name = "Monkey";
let result = 10 * (20 / 2);

Besides integers, booleans and strings, the Monkey interpreter also supports arrays and hashmaps. Here’s what binding an array of integers to a name looks like:

let myArray = [1, 2, 3, 4, 5];

And here is a hashmap, where values are associated with keys:

let leo = {"name": "Leo", "age": 27};

Accessing the elements in arrays and hashmaps is done with index expressions:

myArray[0]  // => 1
leo["name"] // => "Leo"

The let statements can also be used to bind functions to names. Here’s a small function that adds two numbers:

let add = fn(a, b) { return a + b; };

But Monkey not only supports return statements. Implicit return values are also possible, which means we can leave out the return if we want to:

let add = fn(a, b) { a + b; };

And calling a function is as easy as you’d expect:

add(1, 2);

A more complex function, such as a fibonacci function that returns the Nth Fibonacci number, might look like this:

let fibonacci = fn(x) {
  if (x == 0) {
    0
  } else {
    if (x == 1) {
      1
    } else {
      fibonacci(x - 1) + fibonacci(x - 2);
    }
  }
};

Note the recursive calls to fibonacci itself!

Monkey also supports a special type of functions, called higher order functions. These are functions that take other functions as arguments. Here is an example:

let twice = fn(f, x) {
  return f(f(x));
};

let addTwo = fn(x) {
  return x + 2;
};

twice(addTwo, 2); // => 6