adampetrovic / monkey

A toy language parser, lexer and interpreter written in Golang

Monkey - A toy programming language

Monkey is a toy programming language used to learn how to write a lexer, parser and interpreter.

The language is introduced in the book Writing an Interpreter in Go by Thorsten Ball (https://interpreterbook.com/)

In this repo is my implementation of the Monkey language using Go primitives.

The Language

// Bind values to names with let-statements
let version = 1;
let name = "Monkey programming language";
let myArray = [1, 2, 3, 4, 5];
let coolBooleanLiteral = true;

// Use expressions to produce values
let awesomeValue = (10 / 2) * 5 + 30;
let arrayWithValues = [1 + 1, 2 * 2, 3];

Monkey also supports function literals and we can use them to bind a function to a name:

// Define a `fibonacci` function
let fibonacci = fn(x) {
    if (x == 0) {
        0                // Monkey supports implicit returning of values
    } else {
        if (x == 1) {
            return 1;      // ... and explicit return statements
        } else {
            fibonacci(x - 1) + fibonacci(x - 2); // Recursion! Yay!
        }
    }
};

The data types we're going to support in this book are booleans, strings, hashes, integers and arrays. We can combine them!

// Here is an array containing two hashes, that use strings as keys and integers
// and strings as values
let people = [{"name": "Anna", "age": 24}, {"name": "Bob", "age": 99}];

// Getting elements out of the data types is also supported.
// Here is how we can access array elements by using index expressions:
fibonacci(myArray[4]);
// => 5

// We can also access hash elements with index expressions:
let getName = fn(person) { person["name"]; };

// And here we access array elements and call a function with the element as
// argument:
getName(people[0]); // => "Anna"
getName(people[1]); // => "Bob"

That's not all though. Monkey has a few tricks up its sleeve. In Monkey functions are first-class citizens, they are treated like any other value. Thus we can use higher-order functions and pass functions around as values:

// Define the higher-order function `map`, that calls the given function `f`
// on each element in `arr` and returns an array of the produced values.
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, []);
};

// Now let's take the `people` array and the `getName` function from above and
// use them with `map`.
map(people, getName); // => ["Anna", "Bob"]