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MoonScript dev

New parser

The LPeg parser has been replaced. The grammar is defined in moonscript/parse/grammar.moon and compiled by pgen into a native C module, moonscript.parse.native. A pure Lua version of the same grammar is included as moonscript.parse.slow, a drop-in replacement for the native module; it is not selected automatically.

Parsing is around 5x faster:

Interpreter LPeg parser New parser Speedup
Lua 5.1.5 0.947s 0.180s 5.3x
LuaJIT 2.1 0.817s 0.141s 5.8x

(Parse time for a 405 file, 58,296 line corpus, best of 5 passes.)

The parser produces the same syntax trees as the LPeg grammar. Other changes:

  • Parse error messages have a new format: <message> at line <n>, column <m>: followed by the offending line and a ^ marker, replacing Failed to parse: [<n>] >> <line>. Anything matching on the old error text will need updating.
  • Installing the rock now compiles a C module, so a C compiler is required at install time.
  • moonscript.parse no longer exports build_grammar and extract_line, and the moonscript.parse.util, moonscript.parse.literals and moonscript.parse.env modules have been removed.
  • The MOONSCRIPT_PARSER environment variable is no longer used.
  • LPeg is still a dependency: moonscript.errors and bin/moon-tags use it for parsing unrelated to MoonScript source.

MoonScript v0.7.0 (2026-07-26)

This update includes a bunch of syntax bug fixes and cleanups, with a few new features and an enhanced linter. This will be the last release using LPeg, future versions will use the new parser.

Destructuring updates

Function arguments can be destructured

A function argument can now be written as a table literal to destructure the value passed in. The destructuring patterns are the same ones supported by assignment, including nested patterns, numeric keys, and @field targets:

dist = ({x: x1, y: y1}, {x: x2, y: y2}) ->
  math.sqrt (x2 - x1)^2 + (y2 - y1)^2

class Point
  new: ({x: @x, y: @y}) =>

render = (name, {:width, :height}, scale=1, ...) ->
  print name, width, height, scale, ...

The argument becomes an internal name and the fields are unpacked at the top of the function body:

render = function(name, _arg_0, scale, ...)
  local width, height
  width, height = _arg_0.width, _arg_0.height
  if scale == nil then
    scale = 1
  end
  return print(name, width, height, scale, ...)
end

Destructured arguments always declare fresh locals, so a name in the pattern shadows an enclosing local of the same name instead of assigning to it.

Argument initialization now happens in the order the arguments are written, so a default value can reference a name that an earlier argument destructured:

scaled = ({:base}, amount = base * 2) -> base + amount

Because the destructured names are bound at the top of the body, a name in a pattern can not collide with another argument. ({:a}, a) -> and ({:self}) => are now compile errors.

Destructuring in multiple assignment

Destructuring can now be used mixed in with other names in multiple assignment (#367, #304):

num, {:message} = two_values!
head, {:content}, tail = mix!
{value: built}, {:status} = builder\build!

Previously the assignment was split into separate statements, which broke destructuring output.

Better validation of destructuring targets

The names extracted by a destructuring pattern are now checked with the same rule the parser uses for assignment targets. @@class_field, index expressions, and slices are now allowed:

{x: obj.a, y: obj["b"], z: @prop, w: @@cls_prop} = t

Destructuring into something that can never be assigned to is now a compile error instead of generating broken code:

  • {foo!} = thing reports Can't destructure into chain ending in call
  • {...} = thing reports Can't destructure into '...'

Linter updates

Lint stages

The linter's checks are now organized into named stages: global_access, unused, constant_assign, and import_overwrite. Every stage runs by default. moonc --lint-stage limits reporting to the named stage, and --exclude-lint-stage reports everything but the named stage. Both options can be repeated, and they can not be combined.

moonc -l --lint-stage global_access --lint-stage unused .
moonc -l --exclude-lint-stage import_overwrite .

New checks

import_overwrite reports an import that clobbers a name that is already bound in scope:

insert = "hello"
f = ->
  import insert from table -- import overwrites existing binding `insert`

constant_assign reports writing to a name that is tagged as constant. In this version, only the names created by import are tagged as constant. For this release, constant names are only checked in the linter. In the future we will make constant violation a compiler error.

import insert from table
insert = 5 -- assigning to constant `insert`

Compact lint output

moonc --lint-format compact writes one line per issue in the familiar file:line:column: message shape, with the stage name appended, for editors and tools that parse compiler output (#465):

$ moonc -l --lint-format compact lint_example.moon
lint_example.moon:7:5: accessing global `my_nmuber` [global_access]

Standard library updates

New class introspection functions

The moon module has four new functions, documented in the standard library reference:

  • is_class(value) tests if a value is a MoonScript class table
  • is_instance(value) tests if a value is an instance of a MoonScript class
  • is_instance_of(value, cls) tests if a value is an instance of cls or any of its parents. It throws an error if value is not an instance
  • is_subclass_of(cls, parent) tests if cls inherits from parent. A class is not a subclass of itself. It throws an error if cls is not a class

Unlike the old is_object, these can tell classes, instances, and __base tables apart.

is_object is now deprecated. It returns truthy for instances, classes, and __base tables, so it can not be used to distinguish them.

BREAKING: moon.type no longer returns the class object for a class

moon.type now returns the string "class" when given a class, and it only returns the class object for actual instances. Previously a class returned itself, and a __base table returned the class it belonged to, which made it impossible to tell an instance from the class it came from.

class MyClass

type MyClass!       -- MyClass (unchanged)
type MyClass        -- "class" (was MyClass)
type MyClass.__base -- "table" (was MyClass)

BREAKING: changes to moonscript.util

These are internal helpers that were never documented, but they were reachable:

  • util.moon.type is now util.mtype, to avoid the confusion with the type function in the moon module
  • util.moon.is_object removed, replaced by util.moon.is_class and util.moon.is_instance
  • util.moon.is_a removed, replaced by is_instance_of in the moon module

Code Generation

Interpolated strings and nested expressions are parenthesized correctly

The compiler now knows Lua's operator precedence when writing out an expression, and it adds parentheses when a nested expression would otherwise be regrouped by the surrounding operators. String interpolation is the most common way to get a nested expression, and it was silently generating the wrong math (#320):

x = 10 / "#{b}.5"

Before:

local x = 10 / tostring(b) .. ".5"

After:

local x = 10 / (tostring(b) .. ".5")

Because .. is right associative in Lua, an interpolated string on the left hand side of a .. is grouped as well. This is observable through the __concat metamethod:

prefix = "id: #{id}, " .. rest

Before:

local prefix = "id: " .. tostring(id) .. ", " .. rest

After:

local prefix = ("id: " .. tostring(id) .. ", ") .. rest

Prefix operators are grouped in update assignments and switches

The check for whether a value needs to be wrapped in parentheses did not account for the prefix operators #, -, not, and ~, which parse greedily. An update assignment or a switch case with one of them on the left would compile to the wrong grouping (#457):

count -= #datum + 1
total *= -x + 1
flag and= not a or b

Before:

local count = count - #datum + 1
local total = total * -x + 1
local flag = flag and not a or b

After:

local count = count - (#datum + 1)
local total = total * (-x + 1)
local flag = flag and (not a or b)

Expression lines no longer use _

When an expression appears where a statement is expected, the compiler assigns it to a throwaway variable. It used to use the name _, which would clobber a variable of that name in the user's code (#309). It now uses an autogenerated _scrap_N name.

_ = 5
tbl.field
print _

Before:

local _ = 5
_ = tbl.field
return print(_)

After:

local _ = 5
local _scrap_0 = tbl.field
return print(_)

local in a class body is hoisted

A local declaration inside a class body was written out near the bottom of the generated do block, after the methods had already been compiled. Methods referencing the name would close over the outer or global name instead of the class local (#459). The declaration is now hoisted to the top of the class body, placed after the reference to the parent class so a body local can shadow the parent's name.

class Counter
  local count
  count = 0

  inc: => count += 1

Bug Fixes

  • Updated all compiler thrown errors to ensure they include positional information so the error messages are more informative
  • Trailing continue in a loop body generated invalid Lua (break was followed by more statements)
  • a, b = x\some_method and other assignments of a single complex value to multiple names silently dropped every name after the first
  • A \stub without a base crashed the compiler with an internal error. Inside a with block it now uses the with value, and outside of one it reports Short-colon syntax must be called within a with block (#428)
  • continue outside of a loop is now a compile error with a line number instead of an internal error with a traceback
  • Destructuring against the result of an if, switch, for, while, do, with, or comprehension declared the names in the wrong scope (#449, #411, #391, #451)
  • Destructuring against a receiver that can not be indexed directly, like ..., nil, not thing, #thing, or a table literal, generated invalid Lua
  • An expression inside a string interpolation that can't be parsed now throws a parse error instead of putting the malformed code as raw characters in the string
  • Fixed broken moonc --version which reported missing argument instead of printing version (#471)
  • Errors raised without a position now report the position of the statement being compiled, instead of no position at all
  • An internal compiler error no longer throws a second error while trying to report the first one

Binaries

  • The static binaries now report build information with --version:

    $ moon --version
MoonScript version 0.7.0 (static build)
Runtime: LuaJIT 2.1.1748459687
Commit: v0.7.0
Built: 2026-07-24T09:07:45-07:00

  • LuaJIT builds are now included alongside the Lua 5.1 builds

  • Linux arm64 builds are now included

New Parser in Testing

The next version of MoonScript will likely include the new C parser generated by pgen, replacing LPeg. Early testing shows the new parser is be about 5x faster.

Setting the MOONSCRIPT_PARSER=pgen environment variable will make the compiler parse with the moonscript_parser module instead of building the LPeg grammar at runtime. This is opt in and experimental, the LPeg grammar is still the default

MoonScript v0.6.0 (2026-01-10)

New Features

Command Line Improvements

  • Switched from alt_getopt to argparse - Both moon and moonc now use argparse for argument parsing, providing better help messages via --help and more robust option handling
  • Added -e/--execute flag to moon - Execute MoonScript code directly from the command line: 
    moon -e "print 'Hello World'"
  • New --transform option for moonc - Allows custom AST transformations before compilation by specifying a module that receives and returns the syntax tree
  • Improved - (stdin) handling - Now properly enforces that - must be the only argument

moonc Option Renames

  • -w is now also available as --watch
  • -l is now also available as --lint
  • -t is now also available as --output-to

New Tools

  • moon-tags - New script for generating ctags-compatible tag files for MoonScript, with support for:
    • Class definitions
    • Class methods
    • Top-level function definitions (exported via {:func} pattern)
    • Lapis route detection (--lapis flag)
    • Optional line numbers (--include-line flag)
    • Optionally skip header (--no-header flag)

Utility Improvements

  • moon.p() now prints multiple arguments - Pass multiple values and each will be dumped
  • util.dump shows class names - When dumping objects, the class name is displayed (e.g., <MyClass>{...})

Compiler Enhancements

  • Lua keyword property access on self - Properties with Lua keyword names (like @then or @@then) now compile correctly using bracket notation instead of invalid self.then

Bug Fixes

  • Fixed ambiguous Lua generation after import - Semicolons are now correctly inserted when the next line starts with (, preventing parsing ambiguity
  • Fixed update operators with complex chain indexes - Expressions like a[func()].x += 1 now correctly lift the index expression to avoid double evaluation
  • Exit with proper error code - moon now exits with code 1 when the executed script fails
  • Fixed moonc - - Reading from stdin now works correctly
  • Removed accidental debug print - Removed stray print file, time in watcher code
  • dump.tree returns string - Now returns the string instead of printing directly
  • Removed noisy "Built" message - Single file compilation no longer prints "Built" to stderr

Internal Changes

  • Migrated CI from Travis to GitHub Actions
  • Added comprehensive compiler and transform specs
  • Improved binary building workflow for Windows and Linux
  • Better error messages for invalid destructure assignments
  • Updated splat.moon to use argparse, added --strip-prefix option

MoonScript v0.5.0 (2016-9-25)

Syntax updates

Function calls

Function calls with parentheses can now have free whitespace around the arguments. Additionally, a line break may be used in place of a comma:

my_func(
  "first arg"
  =>
    print "some func"

  "third arg", "fourth arg"
)

Function argument definitions

Just like the function all update, function argument definitions have no whitespace restrictions between arguments, and line breaks can be used to separate arguments:

some_func = (
  name
  type
  action="print"
) =>
  print name, type, action

Additions

  • elseif can be used part of an unless block (nymphium)
  • unless conditional expression can contain an assignment like an if statement (#251)
  • Lua 5.3 bitwise operator support (nymphium) (Kawahara Satoru)
  • Makefile is Lua version agnostic (nymphium)
  • Lint flag can be used with moonc watch mode (ChickenNuggers)
  • Lint exits with status 1 if there was a problem detected (ChickenNuggers)
  • Compiler can be used with lulpeg

Bug Fixes

  • Slice boundaries can be full expressions (#233)
  • Destructure works when used as loop variable in comprehension (#236)
  • Proper name local hoisting works for classes again (#287)
  • Quoted table key literals can now be parsed when table declaration is in single line (#286)
  • Fix an issue where else could get attached to wrong if statement (#276)
  • Loop variables will no longer overwrite variables of the same name in the same scope (egonSchiele)
  • A file being deleted will not crash polling watch mode (ChickenNuggers)
  • The compiler will not try to compile a directory ending in .moon (Gskartwii)
  • alt_getopt import works with modern version (Jon Allen)
  • Code coverage not being able to find file from chunk name

MoonScript v0.4.0 (2015-12-06)

Changes to super

super now looks up the parent method via the class reference, instead of a (fixed) closure to the parent class.

Given the following code:

class MyThing extends OtherThing
  the_method: =>
    super!

In the past super would compile to something like this:

_parent_0.the_method(self)

Where _parent_0 was an internal local variable that contains a reference to the parent class. Because the reference to parent is an internal local variable, you could never swap out the parent unless resorting to the debug library.

This version will compile to:

_class_0.__parent.__base.the_method(self)

Where _class_0 is an internal local variable that contains the current class (MyThing).

Another difference is that the instance method is looked up on __base instead of the class. The old variation would trigger the metamethod for looking up on the instance, but a class method of the same name could conflict, take precedence, and be retuned instead. By referencing __base directly we avoid this issue.

Super on class methods

super can now be used on class methods. It works exactly as you would expect.

class MyThing extends OtherThing
  @static_method: =>
    print super!

Calling super will compile to:

_class_0.__parent.static_method(self)

Improved scoping for super

The scoping of super is more intelligent. You can warp your methods in other code and super will still generate correctly. For example, syntax like this will now work as expected:

class Sub extends Base
  value: if debugging
    => super! + 100
  else
    => super! + 10

  other_value: some_decorator {
    the_func: =>
      super!
  }

super will refer to the lexically closest class declaration to find the name of the method it should call on the parent.

Bug Fixes

  • Nested with blocks used incorrect ref (#214 by @geomaster)
  • Lua quote string literals had wrong precedence (#200 by @nonchip)
  • Returning from with block would generate two return statements (#208)
  • Including return or break in a continue wrapped block would generate invalid code (#215 #190 #183)

Other

  • Refactor transformer out into multiple files
  • moon command line script rewritten in MoonScript
  • moonscript.parse.build_grammar function for getting new instance of parser grammar
  • Chain AST updated to be simpler

MoonScript v0.3.2 (2015-6-01)

Bug Fixes

  • package.moonpath geneator does not use paths that don't end in lua

MoonScript v0.3.1 (2015-3-07)

Bug Fixes

  • Fixed a bug where an error from a previous compile would prevent the compiler from running again

MoonScript v0.3.0 (2015-2-28)

New Features

Parsing Updates

Whitespace parsing has been relaxed in a handful of locations:

  • You can put unrestricted whitespace/newlines after operator in a binary operator before writing the right hand side. The following are now valid:
x = really_long_function! +
  2304

big_math = 123 /
  12 -
  43 * 17


bool_exp = nice_shirt and cool_shoes or
  skateboard and shades
  • You can put unrestricted whitespace/newlines immediately after an opening parenthesis, and immediately before closing parenthesis. The following are now valid:
hello = 100 + (
  var * 0.23
) - 15


funcall(
  "height", "age", "weight"
)


takes_two_functions (->
  print "hello"
), ->
  print "world"
  • You can put unrestricted whitespace/newlines immediately after a : when defining a table literal. The following is now valid:
x = {
  hello:
    call_a_function "football", "hut"
}

Code Generation

  • Single value import/destructure compiles directly into single assignment

Bug Fixes

  • Some moonc command line flags were being ignored
  • Linter would not report global reference when inside self assign in table
  • Fixed an issue where parser would crash in Lpeg 0.12 when compiling hundreds of times per process

Misc

  • MoonScript parser now written in MoonScript

MoonScript v0.2.6 (2014-6-18)

Bug Fixes

  • Fixes to posmap generation for multi-line mappings and variable declarations
  • Prefix file name with @ when loading code so stack traces tread it as file
  • Fix bug where moonc couldn't work with absolute paths
  • Improve target file path generation for moonc

MoonScript v0.2.5 (2014-3-5)

New Things

  • New code coverage tool built into moonc
  • New linting tool built into moonc, identifies global variable references that don't pass whitelist
  • Numbers can have LL and ULL suffixes for LuaJIT

Bug Fixes

  • Error messages from moonc are written to standard error
  • Moonloader correctly throws error when moon file can't be parsed, instead of skipping the module
  • Line number rewriting is no longer incorrectly offset due to multiline strings

Code Generation

Bound functions will avoid creating an anonymous function unless necessary.

x = hello\world

Before:

local x = (function()
  local _base_0 = hello
  local _fn_0 = _base_0.world
  return function(...)
    return _fn_0(_base_0, ...)
  end
end)()

After:

local x
do
  local _base_0 = hello
  local _fn_0 = _base_0.world
  x = function(...)
    return _fn_0(_base_0, ...)
  end
end

Explicit return statement now avoids creating anonymous function for statements where return can be cascaded into the body.

->
  if test1
    return [x for x in *y]

  if test2
    return if true
      "yes"
    else
      "no"

  false

Before:

local _
_ = function()
  if test1 then
    return (function()
      local _accum_0 = { }
      local _len_0 = 1
      local _list_0 = y
      for _index_0 = 1, #_list_0 do
        local x = _list_0[_index_0]
        _accum_0[_len_0] = x
        _len_0 = _len_0 + 1
      end
      return _accum_0
    end)()
  end
  if test2 then
    return (function()
      if true then
        return "yes"
      else
        return "no"
      end
    end)()
  end
  return false
end

After:

local _
_ = function()
  if test1 then
    local _accum_0 = { }
    local _len_0 = 1
    local _list_0 = y
    for _index_0 = 1, #_list_0 do
      local x = _list_0[_index_0]
      _accum_0[_len_0] = x
      _len_0 = _len_0 + 1
    end
    return _accum_0
  end
  if test2 then
    if true then
      return "yes"
    else
      return "no"
    end
  end
  return false
end

MoonScript v0.2.4 (2013-07-02)

Changes

  • The way the subtraction operator works has changed. There was always a little confusion as to the rules regarding whitespace around it and it was recommended to always add whitespace around the operator when doing subtraction. Not anymore. Hopefully it now works how you would expect. (a-b compiles to a - b and not a(-b) anymore).
  • The moon library is no longer sets a global variable and instead returns the module. Your code should now be:
moon = require "moon"
  • Generated code will reuse local variables when appropriate. Local variables are guaranteed to not have side effects when being accessed as opposed to expressions and global variables. MoonScript will now take advantage of this and reuse those variable without creating and copying to a temporary name.
  • Reduced the creation of anonymous functions that are called immediately. MoonScript uses this technique to convert a series of statements into a single expression. It's inefficient because it allocates a new function object and has to do a function call. It also obfuscates stack traces. MoonScript will flatten these functions into the current scope in a lot of situations now.
  • Reduced the amount of code generated for classes. Parent class code it left out if there is no parent.

New Things

  • You can now put line breaks inside of string literals. It will be replaced with \n in the generated code.
x = "hello
world"
  • Added moonscript.base module. It's a way of including the moonscript module without automatically installing the moonloader.
  • You are free to use any whitespace around the name list in an import statement. It has the same rules as an array table, meaning you can delimit names with line breaks.
import a, b
  c, d from z
  • Added significantly better tests. Previously the testing suite would only verify that code compiled to an expected string. Now there are unit tests that execute the code as well. This will make it easier to change the generated output while still guaranteeing the semantics are the same.

Bug Fixes

  • b is not longer treated as self assign in { a : b }
  • load functions will return nil instead of throwing error, as described in documentation
  • fixed an issue with moon.mixin where it did not work as described

MoonScript v0.2.3-2 (2013-01-29)

Fixed bug with moonloader not loading anything

MoonScript v0.2.3 (2013-01-24)

Changes

  • For loops when used as expressions will no longer discard nil values when accumulating into an array table. This is a backwards incompatible change. Instead you should use the continue keyword to filter out iterations you don't want to keep. Read more here.
  • The moonscript module no longer sets a global value for moonscript and instead returns it. You should update your code:
moonscript = require "moonscript"

New Things

Bug Fixes

  • Numbers that start with a dot, like .03, are correctly parsed
  • Fixed typo in fold library function
  • Fix declaration hoisting inside of class body, works the same as local * now

Other Stuff

MoonScript has made its way into GitHub. .moon files should start to be recognized in the near future.

MoonScript v0.2.2 (2012-11-03)

Changes

New Things

The Language

The API

The Tools

Bug Fixes

  • Significantly improved the line number rewriter. It should now accurately report all line numbers.
  • Generic for loops correctly parse for multiple values as defined in Lua.
  • Update expressions don't fail with certain combinations of precedence.
  • All statements/expressions are allowed in a class body, not just some.
  • x = "hello" if something will extract the declaration of x if it's not in scope yet. Preventing an impossible to access variable from being created.
  • varargs, ..., correctly bubble up through automatically generated anonymous functions.
  • Compiler doesn't crash if you try to assign something that isn't assignable.
  • Numerous other small fixes. See commit log.

MoonScript v0.2.0 (2011-12-12)

Changes

  • , is used instead of : for delimiting table slice parts.
  • Class objects store the metatable of their instances in __base. __base is also used in inheritance when chaining metatables.

New Things

The Language

  • Added key-value table comprehensions.
  • Added a switch statement.
  • The body of a class can contain arbitrary expressions in addition to assigning properties. self in this scope refers to the class itself.
  • Class objects themselves support accessing the properties of the superclass they extend (like instances).
  • Class objects store their name as a string in the __name property.
  • Enhanced the super keyword in instance methods.
  • Bound methods can be created for an object by using object\function_name as a value. Called function stubs.
  • Added export * statement to export all assigned names following the statement.
  • Added export ^ statement to export all assigning names that begin with a capital letter following the statement.
  • export can be used before any assignment or class declaration to export just that assignment (or class declaration).
  • Argument lists can be broken up over several lines with trailing comma.
  • :hello is short hand for hello: hello inside of table literal.
  • Added ..= for string concatenation.
  • table.insert no longer used to build accumlated values in comprehensions.

The API

  • Added loadfile, loadstring, and dofile functions to moonscript module to load/run MoonScript code.
  • Added to_lua function to moonscript module to convert a MoonScript code string to Lua string.

The Tools

Standard Library

I'm now including a small set of useful functions in a single module called moon:

require "moon"

Documentation is available here.

Bug Fixes

  • Windows line endings don't break the parser.
  • Fixed issues when using ... within comprehensions when the compiled code uses an intermediate function in the output.
  • Names whose first characters happen to be a keyword don't break parser.
  • Return statement can have no arguments
  • argument names prefixed with @ in function definitions work outside of classes work with default values.
  • Fixed parse issues with the shorthand values within a with block.
  • Numerous other small fixes. See commit log.

Other Stuff

Since the first release, I've written one other project in MoonScript (other than the compiler). It's a static site generator called sitegen. It's what I now use to generate all of my project pages and this blog.

MoonScript 0.1.0 (2011-08-12)

Initial release