Monkey+

The Monkey programming language implemented by Ramen.

Overview

The Monkey programming language is from the book series Writing An Interpreter In Go and Writing A Compiler In Go, the author is Thorsten Ball(@mrnugget).

I must say this series is the best books for the beginner of whom want to start their journey of implementing a programming language. If you are also interested in, go and grab your own copy now!

The plus sign means that I have done some additional works to support more basic feature for Monkey. For detailed information, just scroll down and see the Feature section.

IMPORTANT: This language is neither ready to use nor with reliable performance. It's an educational purpose language. Use it at your own risk.

Features

Integer arithmetic

Monkey supports basic integer arithmetic operations:

>> 5 + 5
10
>> 7 - 6
1
>> 5 * 9
45
>> 7 / 3
2
>> 1 / 0
ERROR: the right operand of / is 0

Monkey+ also supports % operator.

>> 149 % 22
17
>> 1 % 0
ERROR: the right operand of % is 0

Boolean operation

Monkey support integer, string and boolean compare operations:

>> 5 > 1
true
>> 4 == 5
false
>> "a" == "b"
false
>> true != false
true

Monkey+ also supports basic logic arithmetic and string compare operations:

>> "aa" > "b"
false
>> "apple" < "appletart"
true
>> true && false
false
>> true && false || true
true

You may noticed, the strings are compared by their lexicographical order.

Variable binding

Monkey support let keyword to bind a value to a variable

>> let a = 5;
>> puts(a);
5

Comments

Monkey+ supports single line comments starts with #.

# This is a single comment
puts(5); # This is an end of line comment

Control Flow

Monkey supports if-else control flow.

let test = fn(x) {
  if (x > 10) {
    return "Large than 10";
  } else {
    return "Less than 10";
  }
};

puts(test(11), test(5));

Function and Closure

Monkey supports functions:

>> let add = fn(a, b) { a + b; };
>> add(5, 6);
11

And high order function and closures!

>> let twice = fn(f, x) { return f(f(x)); };
>> twice(fn(x) { x * x; }, 4);
256
>> let adder = fn(x) { return fn(y) { return x + y; } };
>> let add_five = adder(5);
>> add_five(6);
11

Built-in functions

• len(x): return the length of x. x should be a string, an array or a hash.
• puts(a, b, ...): prints each variable in lines.
• eval(c): eval a code snippet c, the environment will not be exported to current env.
• load(f): load a file f into the global environment.
• type(x): report x's type.

Built-in Data Structures

String

The string is totally re-designed to support escaped characters.

In Monkey+ you can also using [] to access the character in the string.

>> let a = "hello";
>> let b = "world!";
>> a[1]; # It's 0-indexed
e
>> b[10]; # If not in the correct range...
null
>> let c = a + "\n" + b; # Using + to do concatenation!
>> c; # Escaped charater is support too!
hello
world!
>> len(c); # The length is correct too!
12

Array

Monkey support array literal, [] random access. The item in it can be different, like Python's list.

>> let a = [15, 20, 25];
>> a[0];
15
>> a[2];
25
>> a[-1]; # If out of bound...
null
>> len(a);
3

These built-in functions will help you:

• first(a): returns the first item in array a, null if the array is empty.
• last(a): returns the last item in array a, null if the array is empty.
• rest(a): returns the array that except the first element of a, null if the array has no more than one item.
• push(a, el): append el to the end of a and produce a new array. a will stay unmodified after calling that.

>> let a = [15, 20, 25];
>> first(a);
15
>> last(a);
25
>> rest(a);
[20, 25]
>> push(a, 30);
[15, 20, 25, 30]
>> a;
[15, 20, 25]

Hash

Hash is hash map or dictionary in other languages. Like Array, Monkey also supports Hash literal and [] random access.

The key of Hash can be integer, boolean and string and the value can be any valid type.

>> let h = {"a": "b", 1: fn(x) { x + x; }, false: [123]};
>> h["a"];
b
>> h[1](2);
4
>> h[false][0];
123
>> h["b"]; # If the key is not found...
null
>> len(h);
3

Monkey+ extended the built-in functions for Hash so you can do more operations:

• set(h, k, v): set k to v in h. If k exists the value will be replaced to v, otherwise create. It will return a new Hash instead modify the original one.
• contains(h, k): test if k is in h.
• delete(h, k): delete the entry which key is k. It will also return a new Hash instead modify the original one.

>> let h = {"a": "b", 1: 2, false: [123]};
>> set(h, "new", "year");
{1: 2, false: [123], a: b, new: year}
>> set(h, "a", "c");
{a: c, 1: 2, false: [123]}
>> contains(h, "a");
true
>> delete(h, "a");
{1: 2, false: [123]}
>> delete(h, 456);
{a: c, 1: 2, false: [123]}
>> h; # Remain unmodified
{a: c, 1: 2, false: [123]}

Example

# A simple any function
let any = fn(arr, f) {
  let iter = fn(arr, accumulated) {
    if (len(arr) == 0) {
      accumulated;
    } else {
      iter(rest(arr), accumulated || f(first(arr)));
    }
  };

  iter(arr, false);
};

let a = [1, 3, 5, 7, 9];
let b = push(a, 10);

puts(any(a, fn(x) { x % 2 == 0; })); # false
puts(any(b, fn(x) { x % 2 == 0; })); # true

Usage

Currently, only interpreter is supported. The compiler support will be added when I finish the next book :).

Be sure you have installed go. My version is go version go1.13.5 darwin/amd64, but I'm not using any fancy feature of go, so it should works for go 1.7 and later.

Then just clone this repo:

$ git clone https://github.com/lxdlam/monkey-plus
$ cd monkey-plus

There are different running mode:

$ go run main.go # You're entering the REPL
$ go run main.go foo.mp # Running file foo.mp
$ go run main.go -f foo.mp # Same as above
$ go run main.go -c "let a = 5; puts(a)" # Running a code snippet

The tests are also be extended for the new feature, so you can try:

$ go test ./...

Nothing should fail.

License

The original works are licensed under MIT License. Thanks Thorsten Ball!

My works are also licensed under MIT License.