rs-monkey-lang

Monkey Programming Language written in Rust.

What's Monkey?

The official Monkey logo

Monkey is a programming language designed to learn interpreters.
It's came from Writing An Interpreter In Go.

Try Monkey!

with REPL

$ git clone https://github.com/tsuyoshiwada/rs-monkey-lang.git
$ make repl

with Online Playground

Working with Wasm!! Awesome 🐶
https://tsuyoshiwada.github.io/rs-monkey-lang/

Documentation

I created the document with reference to Writing An Interpreter In Go.

⚠️ Please note that there may be some mistakes.

Table of Contents

• Summary
• Syntax overview
  • If
  • Operators
  • Return
• Variable bindings
• Literals
  • Integer
  • Boolean
  • String
  • Array
  • Hashes
  • Function
• Built-in Functions
  • puts(<arg1>, <arg2>, ...): void
  • len(<arg>): Intger
  • first(<arg>): any
  • last(<arg>): any
  • rest(<arg>): Array
  • push(<arg1>, <arg2>): Array

Summary

• C-like syntax
• variable bindings
• integers and booleans
• a string data structure
• an array data structure
• a hash data structure
• arithmetic expressions
• built-in functions
• first-class and higher-order functions • closures

Syntax overview

An example of Fibonacci function.

let fibonacci = fn(x) {
  if (x == 0) {
    0;
  } else {
    if (x == 1) {
      1;
    } else {
      fibonacci(x - 1) + fibonacci(x - 2);
    }
  }
};

fibonacci(10);

If

It supports the general if. else exists, but else if does not exist.

if (true) {
  10;
} else {
  5;
}

Operators

It supports the general operations.

1 + 2 + (3 * 4) - (10 / 5);
!true;
!false;
+10;
-5;
"Hello" + " " + "World";

Return

It returns the value immediately. No further processing will be executed.

if (true) {
  return;
}

let identity = fn(x) {
  return x;
};

identity("Monkey");

Variable bindings

Variable bindings, such as those supported by many programming languages, are implemented. Variables can be defined using the let keyword.

Format:

let <identifier> = <expression>;

Example:

let x = 0;
let y = 10;
let foobar = add(5, 5);
let alias = foobar;
let identity = fn(x) { x };

Literals

Five types of literals are implemented.

Integer

Integer represents an integer value. Floating point numbers can not be handled.

Format:

[-+]?[1-9][0-9]*;

Example:

10;
1234;

Boolean

Boolean represents a general boolean types.

Format:

true | false;

Example:

true;
false;

let truthy = !false;
let falsy = !true;

String

String represents a string. Only double quotes can be used.

Format:

"<value>";

Example:

"Monkey Programming Language";
"Hello" + " " + "World";

Array

Array represents an ordered contiguous element. Each element can contain different data types.

Format:

[<expression>, <expression>, ...];

Example:

[1, 2, 3 + 3, fn(x) { x }, add(2, 2), true];

let arr = [1, true, fn(x) { x }];

arr[0];
arr[1];
arr[2](10);
arr[1 + 1](10);

Hashes

Hash expresses data associating keys with values.

Format:

{ <expression>: <expression>, <expression>: <expression>, ... };

Example:

let hash = {
  "name": "Jimmy",
  "age": 72,
  true: "a boolean",
  99: "an integer"
};

hash["name"];
hash["a" + "ge"];
hash[true];
hash[99];
hash[100 - 1];

Function

Function supports functions like those supported by other programming languages.

Format:

fn (<parameter one>, <parameter two>, ...) { <block statement> };

Example:

let add = fn(x, y) {
  return x + y;
};

add(10, 20);

let add = fn(x, y) {
  x + y;
};

add(10, 20);

If return does not exist, it returns the result of the last evaluated expression.

let addThree = fn(x) { x + 3 };
let callTwoTimes = fn(x, f) { f(f(x)) };

callTwoTimes(3, addThree);

Passing around functions, higher-order functions and closures will also work.

Built-in Functions

You can use 6 built-in functions 🚀

puts(<arg1>, <arg2>, ...): void

It outputs the specified value to stdout. In the case of Playground, it is output to console.

puts("Hello");
puts("World!");

len(<arg>): Intger

For String, it returns the number of characters. If it's Array, it returns the number of elements.

len("Monkey");
len([0, 1, 2]);

first(<arg>): any

Returns the element at the beginning of Array.

first([0, 1, 2]);

last(<arg>): any

Returns the element at the last of Array.

last([0, 1, 2]);

rest(<arg>): Array

Returns a new Array with the first element removed.

rest([0, 1, 2]);

push(<arg1>, <arg2>): Array

Returns a new Array with the element specified at the end added.

push([0, 1], 2);

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Enjoy Monkey 🐵 !

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License

MIT © tsuyoshiwada