SwiftMokey

An interpreter for the Monkey programming language written in Swift.

What is monkey language ?

Monkey is a programming language used in a book "Writing An Interpreter In Go" by Thorsten Ball.

Language Features

• C style syntax
• variable bindings
• integers and booleans
• arithmetic expression
• builtin function
• first class and high-order function
• closure
• string , array , hash data structure

Example

variable

let price = 100;
let name = "Ter";
let discountPrice = price - 20;

arithmetic

2 * 3 + 1 // => 7
-2 + 3 // => 1
2 * (5 + 10) // => 30
50 / 2 * 2 + 10 // => 60

boolean

!true // => false
1 < 2 // => true
1 == 2 // => false
1 != 2 // => true
true == true // => true

condition

if ( 1 < 2 ) { 
  return 10 
} else {
  return 20
}

implicit return

if ( 1 < 2 ) { 
  10 // don't need to write return
}

function

let add = fn(x , y) { x + y }
add(2, 3) // => 5

high-order function

let adder = fn(x) { fn(y) { x + y } };
let addTwo = adder(2);
addTwo(3) // => 5

let add = fn(a, b) { a + b };
let sub = fn(a, b) { a - b };
let apply = fn(a, b, func) { func(a,b) };
apply(2, 2, add); // => 4
apply(10, 2, sub); // => 8

string and built-in function

let a = "Hello"
let b = "World!!"
let helloWorld = a + " " + b // => Hello World!!

len("Hello") // => 5
len(a) // => 5

array and built-in function

let numArr = [1 + 1, 2 * 2] // => [2, 4]

let index = 1;
let arr = ["one", "two", "three"]

arr[0] // => one
arr[index] // => two
arr[1 + 1] // => three

first(arr) // => one
last(arr) // => three
rest(arr) // => ["two", "three"]
let newArr = push(arr, "four") // => ["one", "two", "three", "four"]

hash (int ,string ,boolean as key)

let two = "two";
let h = { "one" : 1, 
          two : 1 + 1, 
          "thr" + "ee": 3,
          4 : 4,
          true: 5,
          false: 6
        };
h["one"] // => 1
h["two"] // => 2
h["three"] // => 3
h[4] // => 4
h[true] // => 5
h[false] // => 6

len(h) // => 6

Demo Program

let map = fn(arr, f) {
 let iterator = fn(arr, accumulated) {
   if (len(arr) == 0) {
     accumulated
   } else {
     iterator(rest(arr) , push(accumulated, f(first(arr))));
   }
 };
 iterator(arr, []);
};

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