MoonScript, a language that compiles to Lua

quasiceo 2014-01-15

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MoonScript is a dynamic scripting language that compiles into Lua. It gives you the power of one of the fastest scripting languages combined with a rich set of features.

class Thing
  name: "unknown"

class Person extends Thing
  say_name: => print "Hello, I am", @name

with Person!
  .name = "MoonScript"
  \say_name!

MoonScript can either be compiled into Lua and run at a later time, or it can be dynamically compiled and run using the moonloader. It’s as simple as require "moonscript" in order to have Lua understand how to load and run any MoonScript file.

Because it compiles right into Lua code, it is completely compatible with alternative Lua implementations like LuaJIT, and it is also compatible with all existing Lua code and libraries.

The command line tools also let you run MoonScript directly from the command line, like any first-class scripting language.

A comprehensive overview of the language can be found in the reference manual, the rest of this page serves as an overview of the language.

Overview

MoonScript provides a clean syntax using significant whitespace that avoids all the keyword noise typically seen in a Lua script. Below is a sample of some constructs found in the language.

export my_func
x = 2323

collection =
  height: 32434
  hats: {"tophat", "bball", "bowler"}

my_func = (a) -> x + a

print my_func 100

It also adds table comprehensions, implicit return on functions, classes, inheritance, scope management statements import & export, and a convenient object creation statement called with.

import concat, insert from table

double_args = (...) ->
  [x * 2 for x in *{...}]

tuples = [{k, v} for k,v in ipairs my_table]

It can be loaded directly from a Lua script without an intermediate compile step. It even knows how to tell you where errors occurred in the original file when they happen.

Installation

Installing with LuaRocks

If you're on Windows, then install the Windows binaries, otherwise the easiest way to install is to use LuaRocks.

LuaRocks can be obtained here or from your package manager.

After it is installed, run the following in a terminal:

$ luarocks install moonscript

This will provide the moon and moonc executables along with the moonscript and moon Lua module.

Windows Binaries

Procompiled Windows binaries are available to avoid the trouble of compiling:
http:///bin/moonscript-0.2.4.zip

Extract the contents into your PATH.

Optional

If you're on Linux and use watch mode (which compiles .moon files to .lua files as they are changed) you can install linotify to use inotify instead of polling.

Source

The source code to the project lives on github:
https://github.com/leafo/moonscript

Issues with the tool can be reported on the issue tracker:
https://github.com/leafo/moonscript/issues

The latest development version can be installed with the dev rockspec:

$ luarocks build http:///rocks/moonscript-dev-1.rockspec

Dependencies

In addition to Lua 5.1 or 5.2, the following Lua modules are required to run the compiler and associated tools:

All of the required ones can be retrieved automatically using the LuaRocks installation.

Learning

Extras & Addons

Editor Support

Vim syntax and indent:
https://github.com/leafo/moonscript-vim

Textmate (and Sublime Text) syntax and indent:
https://github.com/leafo/moonscript-tmbundle

SciTE (with scintillua) syntax:
https://github.com/leafo/moonscript/tree/master/extra/scintillua

Preconfigured and packaged version of SciTE for Windows with MoonScript support:
http:///scite/

Tools

Online Compiler:
http:///compiler/

Overview of Differences & Highlights

A more detailed overview of the syntax can be found in the documentation.

Whitespace sensitive blocks defined by indenting
All variable declarations are local by default
export keyword to declare global variables, import keyword to make local copies of values from a table
Parentheses are optional for function calls, similar to Ruby
Fat arrow, =>, can be used to create a function with a self argument
@ can be prefixed in front of a name to refer to that name in self
! operator can be used to call a function with no arguments
Implicit return on functions based on the type of last statement
: is used to separate key and value in table literals instead of =
Newlines can be used as table literal entry delimiters in addition to ,
\ is used to call a method on an object instead of :
+=, -=, /=, *=, %= operators
!= is an alias for ~=
Table comprehensions, with convenient slicing and iterator syntax
Lines can be decorated with for loops and if statements at the end of the line
If statements can be used as expressions
Class system with inheritance based on metatable’s __index property
Constructor arguments can begin with @ to cause them to automatically be assigned to the object
Magic super function which maps to super class method of same name in a class method
with statement lets you access anonymous object with short syntax