monkey

Implementation of the Monkey language from Writing an interpreter in go and Writing a compiler in go. Monkey is a programming language created by Thorsten Ball.

Tests

go test $(go list ./... | grep -v /docs) - run all tests except docs

Build

go build -o monkey

Build for Playground

This also contains a web Playground. To build for the Playground run: GOARCH=wasm GOOS=js go build -o ./docs/monkey.wasm ./docs/main.go

Running the playground

Use any static file-server. Example: cd docs && python -m http.server and then navigate to http://localhost:8000

Running as REPL

go run main.go -repl or go build -o monkey && ./monkey -repl

Running monkey programs

To execute a monkey program saved in a file

go run main.go ./examples/conditionals.monkey or ./monkey ./examples/conditionals.monkey

Language Features

Supported data types

Monkey supports following datatypes natively:

• integers
• strings
• booleans
• arrays
• dictionary or hash
• functions

Variable bindings

let x = 10;
let str = "hello, world!";
let bool = x == 10;

Arithmetic operators

Monkey supports following arithmetic operators which are similar to other programming languages:

• + (addition)
• - (subtraction)
• * (multiplication)
• / (division)

Comparision operators

Monkey supports following comparison operators

• == (equals)
• != (not equals)
• > (greater than)
• < (less than)

Creating string literals

let s = "hello world";

Creating array literals

Monkey arrays can contains objects of any type supported by monkey

let add = fn(a, b) { a + b};
let arr = [1, "two", "3", true, false, add]

We can access arrays by using positive indices from 0 to n - 1 (n being the length of the array)

let first_v = arr[0]
let last_v = arr[len(arr) - 1]

Creating monkey dictionaries

We can use integers, strings and boolean types as keys in dictionaries in monkey

let d = {"foo": 1, "bar": 2, true: 3, false: 4, 1: 5};

Values can be accessed from dictionaries as follows:

let one = d["foo"];
let two = d["bar"];
let three = d[true];
let five = d[1];

Functions

let factorial = fn(n) {
    if (n == 0) {
        return 1;
    }
    return n * factorial(n - 1);
}

Calling functions

>> factorial(3)
6

Higher order functions

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, []);
};

>> let a = [1, 2, 3, 4];
>> let double = fn(x) { x * 2 };
>> map(a, double);
[2, 4, 6, 8]

if expressions

if statements can produce values in monkey.

let x = 10;
let y = 0;
if (x == 10) {
    x
} else {
    y
}

Builtin functions

len len returns the length of the object, it is supported for strings, arrays and dictionaries

>> let s = "hello";
>> 
5

>> let arr = [1, 2, 3];
>> len(arr)
3

first

first returns the first element from an array, it's not supported for other types

>> let arr = [1, 2, 3]
>> first(arr)
1

last

last returns the last element from an array, it's not supported for other types

>> let arr = [1, 2, 3]
>> last(arr)
3

rest

rest returns a new array which is a copy of the given array except its first element

>> let arr = [1, 2, 3, 4]
>> rest(arr)
[2, 3, 4]

>> rest(arr(arr))
[3, 4]

push

push returns a new array by copying the old array and adds new element at its end

>> let arr = [1, 2, 3]
>> push(arr, 4)
[1, 2, 3, 4]

puts

puts prints the value of a monkey object on stdout

>> let s = "hello";
>> puts(s)
hello

>> let arr = [1, 2, 3];
>> puts(arr)
[1, 2, 3]