Monkey is a programming language designed for Writing An Interpreter In Go.

Test

• Run all
  • $ rake test
• Run each file
  • $ rake test TEST=test/test_xxx.rb
  • e.g. $ rake test TEST=test/test_lexer.rb

REPL

$ ruby lib/repr.rb
> 

Monkey Language

Variable Binding

> let a = 1;
> let b = a + 2;
> b;
3
> let a = -12;
> a;
-12
> b;
3

If-Else Expression

> let a = if (1) { -1 } else { 2 };
> a;
-1

You can use only if block (without else). In that case, NULL object null may appear.

> if (123 < 45) { 6 };
null

The only falsy values are false and null.

> if (0) { 12 } else { 345 };
12

Function

All functions are closure in Monkey language.
Closure is evaluated with specified environment: when the closure is defined.

> let x = 123;
> let double_x = fn() { x * 2; };
> double_x();
246
> let triple_y = fn() { y * 3; };
> let y = -45;
> triple_y();
-135
> let new_adder = fn(x) { return fn(y) { x + y }; };
> let add_two = new_adder(2);
> add_two(3);
5
> let apply_func = fn(a, b, func) { func(a, b); };
> apply_func(1, 23, fn(x, y) { x - y; });
-22

Implicit Return

Each of the following two functions returns same value 345.

• fn() { 345; }
• fn() { 12; return 345; 6; }

String

> let s = "hello,"
> len(s)
6
> s + " world!"
hello, world!

Array

> let a = [1, 3, 5]
> len(a)
3
> a[2]
5
> a[3]
null
> push(a, 7)
[1, 3, 5, 7]
> rest(push(a, 7))
[3, 5, 7]

Hash

> let h = {"k": "v", 123: [4, 5]}
> h["k"]
v
> h[123][0]
4

Each of

• String
• Integer
• Bool

object can be used as hash key.

Struct

> let T = struct{"first", "second", "third"}
> let t = T{1, [2, 3]}
> t.second[0]
2
> t.third
null