Gradual type checking for Lua functions

Copyright (C) 2014-2026 Gary V. Vaughan

A Luaish run-time gradual type checking system, for argument and return types at function boundaries with simple annotations that can be disabled in production code. Its API and type mismatch errors are modelled on the core Lua C-language argcheck () API.

• Luaish: Type check failures show error messages in the same format as Lua itself;
• run time: Without changing any library code, the application can decide at run time whether to enable type checking as it loads the library;
• gradual: Type checks can be introduced to the functions in your code gradually, to as few or as many as seem useful;
• type checking: function argument types and return types are checked against the specification, and raise an error if some don't match

This is a light-weight library for Lua 5.1 (including LuaJIT), 5.2, 5.3, 5.4 and 5.5 written in pure Lua.

Installation

The simplest and best way to install typecheck is with LuaRocks. To install the latest release (recommended):

 luarocks install typecheck

To install current git master (for testing, before submitting a bug report for example):

 luarocks install http://raw.githubusercontent.com/gvvaughan/typecheck/master/typecheck-git-1.rockspec

The best way to install without LuaRocks is to copy the entire lib/typecheck directory into a subdirectory of your package search path, along with the modules listed as dependencies in the included rockspec.

Use

Add expressive type assertions on specific arguments right in the body of a function, for cases where that function can only handle specific types in that argument:

localargcheck=require"typecheck".argchecklocalfunctioncase (with, branches)
argcheck ("std.functional.case", 2, "#table", branches)
...

Or more comprehensively, wrap exported functions to raise an error if the return or argument types do not meet your specification:

return {
len=argscheck ("string.len (string) => int", string.len),
...

Alternatively, argscheck can be used as an annotation, which makes it look nicer when used at declaration time:

localmy_function=argscheck"my_function (int, int) => int" ..function (a, b)
returna+bend

By default, type checks are performed on every call. But, they can be turned off and all of the run-time overhead eliminated in production code, either by calling require 'std._debug' (false) prior to loading typecheck or, more precisely, by setting require 'std._debug'.argcheck = false

Documentation

The latest release is documented with LDoc. Pre-built HTML files are included in the release tarball.

Bug reports and code contributions

Please make bug reports and suggestions as GitHub Issues. Pull requests are especially appreciated.

But first, please check that your issue has not already been reported by someone else, and that it is not already fixed by master in preparation for the next release (see Installation section above for how to temporarily install master with LuaRocks).

There is no strict coding style, but please bear in mind the following points when proposing changes:

0. Follow existing code. There are a lot of useful patterns and avoided traps there.

1. 3-character indentation using SPACES in Lua sources: It makes rogue TABs easier to see, and lines up nicely with 'if' and 'end' keywords.

2. Simple strings are easiest to type using single-quote delimiters, saving double-quotes for where a string contains apostrophes.

3. Save horizontal space by only using SPACEs where the parser requires them.

4. Use vertical space to separate out compound statements to help the coverage reports discover untested lines.

5. Prefer explicit string function calls over object methods, to mitigate issues with monkey-patching in caller environment.