Bindings to Lua, an embeddable scripting language


Version on this page:2.3.0
LTS Haskell 22.29:2.3.1
Stackage Nightly 2024-07-16:2.3.1
Latest on Hackage:2.3.1

See all snapshots hslua appears in

MIT licensed by Albert Krewinkel, Gracjan Polak, Ömer Sinan Ağacan
Maintained by [email protected]
This version can be pinned in stack with:hslua-2.3.0@sha256:b7e6adcea95ab50ec4fba460a4af1b6fcc4bad2cbc7269a11e454cb1c0066b24,4809

Module documentation for 2.3.0

HsLua – Bindings to Lua, an embeddable scripting language

Build status AppVeyor Status Hackage

HsLua provides bindings, wrappers, types, and helper functions to bridge Haskell and Lua.

This package is part of HsLua, a Haskell framework built around the embeddable scripting language Lua.


Lua is a small, well-designed, embeddable scripting language. It has become the de-facto default to make programs extensible and is widely used everywhere from servers over games and desktop applications up to security software and embedded devices. This package provides Haskell bindings to Lua, enable coders to embed the language into their programs, making them scriptable.

HsLua ships with batteries included and includes Lua 5.4.4. Cabal flags make it easy to compile against a system-wide Lua installation.

Interacting with Lua

HsLua provides the Lua type to define Lua operations. The operations are executed by calling run. A simple “Hello, World” program, using the Lua print function, is given below:

import Foreign.Lua as Lua

main :: IO ()
main = Lua.run prog
    prog :: Lua ()
    prog = do
      Lua.openlibs  -- load Lua libraries so we can use 'print'
      Lua.callFunc "print" "Hello, World!"

The Lua stack

Lua’s API is stack-centered: most operations involve pushing values to the stack or receiving items from the stack. E.g., calling a function is performed by pushing the function onto the stack, followed by the function arguments in the order they should be passed to the function. The API function call then invokes the function with given numbers of arguments, pops the function and parameters off the stack, and pushes the results.

|  arg 3   |
|  arg 2   |
|  arg 1   |
+----------+                  ,----------.
| function |    call 3 1      | result 1 |
+----------+   ===========>   +----------+
|          |                  |          |
|  stack   |                  |  stack   |
|          |                  |          |

Manually pushing and pulling arguments can become tiresome, so HsLua makes function calling simple by providing callFunc. It uses type-magic to allow different numbers of arguments. Think about it as having the signature

callFunc :: String -> a1 -> a2 -> … -> res

where the arguments a1, a2, … must be of a type which can be pushed to the Lua stack, and the result-type res must be constructable from a value on the Lua stack.

Getting values from and to the Lua stack

Conversion between Haskell and Lua values is governed by two type classes:

-- | A value that can be read from the Lua stack.
class Peekable a where
  -- | Check if at index @n@ there is a convertible Lua value and
  --   if so return it.  Throws a @'LuaException'@ otherwise.
  peek :: StackIndex -> Lua a


-- | A value that can be pushed to the Lua stack.
class Pushable a where
  -- | Pushes a value onto Lua stack, casting it into meaningfully
  --   nearest Lua type.
  push :: a -> Lua ()

Many basic data types (except for numeric types, see the FAQ) have instances for these type classes. New instances can be defined for custom types using the functions in Foreign.Lua.Core (also exported in Foreign.Lua).


  • Can I see some examples? Basic examples are available in the hslua-examples repository.

    A big project build with hslua is Pandoc, the universal document converter. It is written in Haskell and includes a Lua interpreter, enabling programmatic modifications of documents via Lua. Furthermore, custom output formats can be defined via Lua scripts.

  • Where are the coroutine related functions? Yielding from a coroutine works via longjmp, which plays very badly with Haskell’s RTS. Tests to get coroutines working with HsLua were unsuccessful. No coroutine related functions are exported from the default module for that reason. Pull requests intended to fix this are very welcome.



hslua uses PVP Versioning.


Released 2023-03-13.

  • Require version 2.3.* of HsLua packages: hslua-core, hslua-marshalling, hslua-objectorientation, hslua-packaging, hslua-aeson, hslua-classes.

  • Include hslua-typing, re-export HsLua.Typing.


Released 2022-06-19.

  • Require hslua-core-2.2.1.

  • Require hslua-marshalling-

  • Relax upper bound for mtl, allow mtl-2.3.


Released 2022-02-19.

  • Require version 2.2.* of all hslua-* packages.

  • Include hslua-aeson, and re-export HsLua.Aeson from HsLua.


Released 2022-01-29.

  • Update to hslua-objectorientation 2.1.0. This entails changes to deftype' and deftypeGeneric, switching the order of item pusher and list-extractor function in the tuple passed as the last argument.

  • Update to hslua-core 2.1.0, hslua-marshalling 2.1.0, and hslua-classes 2.1.0.


Released 2021-11-04.

  • Updated lower bounds of hslua packages:

    • hslua >=,
    • hslua-marshalling >= 2.0.1, and
    • hslua-objectorientation >= 2.0.1.

    This fixes a number of smaller issues; see the respective package changelogs for details.


Released 2021-10-21.

  • Move module hierarchy from Foreign.Lua to HsLua.

  • Error handling has been reworked completely. The type of exceptions used and handled by HsLua is now exposed to the type system. The type Lua makes use of a default error type. Custom error handling can be implemented by using the LuaE type with an exception type that is an instance of class LuaError.

  • Renamed stack index helpers to nth, nthTop, nthBottom, top. The following have been removed: stackTop, nthFromTop, nthFromBottom.

  • Extracted raw Lua bindings into new package lua. This means that all cabal flags have been moved to package lua as well. Version lua-1.0.0 contained the Foreign.Lua.Raw hierarchy as present in hslua-1.3.0. See that package’s changelog for info on the additional modifications since then.

  • The module Foreign.Lua.Raw.Error was merged into the HsLua.Core.Error module.

  • The functions getglobal and gettable now return the Lua Type of the pushed value.

  • Extracted new packages:

    • hslua-core: the package contains all modules from the Core sub-hierarchy.

    • hslua-classes: typclasses Peekable and Pushable for pushing and pulling, as well as function calling.

    • tasty-hslua: makes testing helpers available for reuse.

  • Moved run functions from Util to Core.Run.

  • Moved module Utf8 from the base level into Core.

  • Refactored code to expose Haskell functions to Lua:

    • Removed functions newCFunction, freeCFunction. Use pushHaskellFunction instead, it takes care of garbage collection.

    • Renamed typeclass ToHaskellFunction to Exposable, function callFunc to invoke. All these have been moved to hslua-classes.

    • The type PreCFunction is now defined in package lua; HaskellFunction is defined in hslua-core.

    • Changed pushHaskellFunction to only accept HaskellFunction arguments, move it to hslua-core.

    • Removed helper functions addfunction and addfield from Module. Use documented functions and fields instead.

  • Added support for a “since” tag on documented functions; allows to mark the library version when a function was introduced in its present form.


Released 2021-02-06.

  • Fixed build with GHC 9.0.1 (Simon Jakobi).

  • Improved test-suite; fixed memory leaks in some tests.

  • Moved CI to GitHub Actions.


Released 2020-10-16.

  • Upgrade included Lua version to new bug-fix release 5.3.6. See the upstream documentation https://www.lua.org/bugs.html#5.3.5 for the bugs which have been fixed.

  • Stop exporting c_loaded_table and c_prelad_table from module Foreign.Lua.Raw.Auxiliary. Both values are defined only if the flag HARDCODE_REG_KEYS is disabled, leading to compilation errors when the flag is enabled.

  • Add new function peekStringy to Peek module. It allows to peek a value of any IsString type from an UTF-8 encoded string.

  • Various improvements to the continuous integration setup, including cleanup of the config files, version bumps to the ghc/cabal versions used for testing, and running the linter in a dedicated GitHub Action.


Released 2020-08-15

  • New module Foreign.Lua.Call: the module offers an alternative method of exposing Haskell functions to Lua. The focus is on maintainability: types and marshaling methods are made explicit; the possibility of adding documentation and parameter names improves error messages and allows for automatic documentation extraction.

    Work on this module is ongoing; the interface is likely to change. Suggestions and feedback are welcome.

  • New types Module, Field, and new functions registerModule, preloadModule, pushModule, and render exported from Foreign.Lua.Module: this builds on the new Call module and allows the creation of documented modules as well as automatic generation of Markdown-formatted module documentation.

  • Export new items nth and top from Foreign.Lua.Core and Foreign.Lua. They are short-hands for nthFromTop and stackTop.

  • Performance improvements: Calling of Lua functions and creation of Haskell data wrapping userdata has been sped up by about 10%. This is mostly due to using of previously missed optimization opportunities.

  • All foreign imports have been moved to into the new Foreign.Lua.Raw module. This module will replace the current Foreign.Lua.Core module in the future and will be distributed as a separate package (likely starting with the 2.0 release); the remaining parts of the current Core module will be promoted one level in the module hierarchy.

    The Raw module can be used whenever the full power of HsLua is not needed.

  • Error-signaling of API wrapper functions has been changed: instead of returning special integer values, functions now take an additional pointer argument, which is set to the status result of the computation.

    The Failable type in Core.Error is no longer needed and has been removed.

  • CI builds now include GHC 8.8 and GHC 8.10, ensuring that all GHC 8.* versions are supported.


Released 2020-06-27

  • Revert signature of function pushList to it’s proper 1.1 value. This fixes a mistake which caused the 1.1.1 release to be in violation of the PVP versioning policy.

  • Module Foreign.Lua.Peek: add function pushKeyValuePairs (Alex Loomis).


Released 2020-06-02

WARNING: This version does not conform to the PVP versioning policy, due to a unintended signature change of function pushList. It is recommended not to use this version.

  • New module Foreign.Lua.Push: provides functions which marshal and push Haskell values onto Lua’s stack.

    Most functions in Foreign.Lua.Types.Pushable are now defined using functions from this module.

  • New module Foreign.Lua.Peek: provides functions which unmarshal and retrieve Haskell values from Lua’s stack. Contrary to peek from Foreign.Lua.Types.Peekable, the peeker functions in this module will never throw errors, but use an Either type to signal retrieval failure.

    The error type PeekError should not be considered final and will likely be subject to change in later versions.

  • Module Foreign.Lua.Utf8: never throw errors when decoding UTF-8 strings. Invalid UTF-8 input bytes no longer cause exceptions, but are replaced with the Unicode replacement character U+FFFD.

  • Fixed missing and faulty Haddock documentation.

  • Fixed a bug which caused unnecessary use of strings to represent floating point numbers under certain configurations.


Released 2020-03-25.

WARNING: The changes in this release are experimental. It is recommended to skip this release unless the newly introduced features are required.

  • Allow custom error handling: conversion of Lua errors to Haskell exceptions and back is made configurable. Users can define their own exception/error handling strategies, even opening up the option to pass arbitrary exceptions through Lua.

    • New types exported from Foreign.Lua.Types:

      • ErrorConversion: defines the ways in which exceptions and errors are handled and converted.
      • LuaEnvironment: environment in which Lua computations are evaluated. Contains the Lua interpreter state and the error conversion strategy.
    • The environment of the Lua type is changed from a plain Lua State to the above mentioned LuaEnvironment.

    • New functions run' is exported from Foreign.Lua.Util and Foreign.Lua: it is analogous to run, but allows to run computations with a custom error conversion strategy.

    • New function runWithConverter exported from Foreign.Lua.Core.Types and Foreign.Lua.Core; like run', but takes a custom state.

    • New function unsafeRunWith exported from Foreign.Lua.Core.Types and Foreign.Lua.Core; runs a computation without proper error handling.

    • New function errorConversion exported from Foreign.Lua.Core.Types and Foreign.Lua.Core: extract the error conversion strategy from the Lua type.

    • New function throwErrorAsException exported from Foreign.Lua.Core.Error and Foreign.Lua.Core: throws a Lua error as Haskell exception, using the current error conversion strategy.

  • Function runWith is moved from module Foreign.Lua.Core to Foreign.Lua.Util.

  • The module Foreign.Lua.Utf8 is now exported.


Released 2019-08-21.

  • Added flag to use hardcoded values for registry keys: The names of the registry keys used to store package information are available as CPP values from file lauxlib.h since Lua 5.3.4; compiling HsLua against older Lua versions was not possible, as those values were expected to exist.

    The respective values are now hardcoded into HsLua, and a new flag hardcode-reg-key is introduced, which will cause the use of these hardcoded values instead of those defined in lauxlib.h. Using this flag makes it possible to compile hslua against all Lua 5.3.* versions.

  • Added missing C files for benchmarking to list of extra-source-files.


Released 2019-05-08.

  • Prevent filenames being treated as strings in debug messages. Lua’s loadbuffer takes a source description as an argument, which is used for debug messages. The loadfile function now adds a special prefix (@) to source, thus marking it as a filename.


Released 2019-05-04.

  • New module Foreign.Lua.Module, containing helper functions to define and load modules from Haskell.

  • Improve documentation of open<lib> (many thanks to Christian Charukiewicz.)


Released 2019-01-05.

  • Fixed cross-compilation: placement of C import declarations were fixed, thereby resolving issues with cross-compilation. (Vanessa McHale and Faraz Maleknia)

  • Added .gitattributes file, fixing the wrong language classification of the GitHub repository. (Vanessa McHale)

  • Improved toHaskellFunction documentation. The documentation is now more specific on which Haskell exceptions are caught and which will lead to crashes.


  • Exposed more functions from Lua’s lauxlib library:

    • getmetafield,
    • getmetatable',
    • getsubtable, and
    • traceback.

    The function getsubtable is a reimplementation instead of a wrapper to the C function for simplicity (thereby avoiding additional C wrappers).

  • Fixed tests for GHC 8.6 by no longer depending on failable pattern matching.


New features

  • Error handling at language borders has been vastly improved and is now mostly automatic. Haskell’s Foreign.Lua.Exceptions are transformed into Lua errors and vice versa. Lua-side wrappers are no longer necessary.

  • Haskell functions are no longer pushed as userdata by pushHaskellFunction, but as C functions. This simplifies tasks where Lua expects true function objects object (for example when looking for module loaders).

  • Added stack instance for

    • Data.Set.Set,
    • Integer,
    • Int,
    • Float, and
    • Double.

    Instances for numbers fall back to strings when the representation as a Lua number would cause a loss of precision.

  • Haskell functions pushed with pushHaskellFunction can now be garbage collected by Lua without having to call back into Haskell. The callback into Haskell by the GC had previously caused programs to hang in some situations.

  • Bindings to more Lua C API functions and macros: isinteger, load, loadbuffer, and pushglobaltable.

  • Any Haskell value can be pushed to the Lua stack as userdata via pushAny and retrieved via peekAny. Additional functions are provided to setup the userdata metatable.

  • The C preprocessor constants LUA_LOADED_TABLE and LUA_PRELOAD_TABLE are made available as loadedTableRegistryField and preloadTableRegistryField, respectively.

  • Additional small helper functions:

    • peekRead – read value from a string.
    • popValue – peek value at the top of the Lua stack, then remove it from the stack regardless of whether peeking was successful or not.


  • The Lua prefix was removed from types (State, Integer, Number, Exception) and the respective infix from functions (try, run, runWith, runEither). HsLua should be imported qualified to avoid name collisions.

  • Terminology now consistently uses exception to refer to Haskell exceptions, and error for Lua errors; function names changed accordingly (throwException, catchException, withExceptionMessage).

  • Module Foreign.Lua.Api was renamed to Foreign.Lua.Core.

  • Foreign.Lua.lerror was renamed to Foreign.Lua.error.

  • Typeclass ToLuaStack was renamed to Pushable.

  • Typeclass FromLuaStack was renamed to Peekable.

  • Cabal flag use-pkgconfig was renamed to pkg-config (which is the flag name used by other projects such a zlib).

Type signatures

  • The return value of lua_newuserdata is CSize (was CInt).

  • Table index parameter in rawgeti and rawseti must be of type LuaInteger, but were of type Int.

  • The number of upvalues passed to pushcclosure must be of type NumArgs.

  • Lua.error has type Lua NumResults, simplifying its use in HaskellFunctions.

  • Retrieval functions which can fail, i.e. tocfunction, tointeger, tonumber, tostring, tothread, and touserdata, use the Maybe type to indicate success or failure, avoiding the need to perform additional checks.

Removed Features

  • Support for Lua versions before 5.3 has been dropped.

  • Support for GHC 7.8 has been dropped.

  • wrapHaskellFunction has been made internal and is no longer exported.

Changed behavior

  • Peekable instances for numbers and strings became more forgiving. Peeking of basic types now follows Lua’s default conversion rules:

    • numbers can be given as strings, and vice versa;
    • any value can be converted into a boolean – only nil and false are peeked as False, all other as True.


  • Many internal improvements and additions such as a benchmarking suite, code cleanups, better tests, etc.


  • Relaxed upper bound on exceptions. Again.


  • Relaxed upper bound on exceptions.


  • Provide Optional as a replacement for OrNil. Exports of the latter have been fixed.
  • Provide utility function raiseError: Its argument will be thrown as an error in Lua.
  • Add modifyLuaError: The function lives in Foreign.Lua.Error and allows to alter error messages. This is most useful for amending errors with additional information.
  • Fixed a bug in toList which left a element on the stack if deserializing that element lead to an error. This also affected the FromLuaStack instance for lists.
  • Fixed a bug in pairsFromTable which left a key-value pair on the stack if either of them could not be read into the expected type. This also affected the FromLuaStack instance for Map.


  • Make Lua an instance of MonadMask: MonadMask from Control.Monad.Catch allows to mask asynchronous exceptions. This allows to define a finalizer for Lua operations.
  • Add functions and constants to refer to stack indices: The functions nthFromBottom, nthFromTop as well as the constants stackTop and stackBottom have been introduced. Numeric constants are less clear, and named constants can aid readability.
  • Add type OrNil: This type can be used when dealing with optional arguments to Lua functions.
  • Add function absindex: it converts the acceptable index idx into an equivalent absolute index (that is, one that does not depend on the stack top). The function calls lua_absindex when compiled with Lua 5.2 or later; for Lua 5.1, it is reimplemented in Haskell.
  • Functions in tasty which have been deprecated have been replaced with non-deprecated alternatives.


  • Re-export more FunctionCalling helpers in Foreign.Lua: The typeclass ToHaskellFunction and the helper function toHaskellFunction are useful when working with functions. Importing them separately from Foreign.Lua.FunctionCalling was an unnecessary burden; they are therefor now re-exported by the main module.
  • Export registry-relatd constants refnil and noref: The constants are related to Lua’s registry functions (ref and unref).
  • Add helper to convert functions into CFunction: A new helper wrapHaskellFunction is provided. It expects a HaskellImportedFunction userdata (as produced by pushHaskellFunction) on top of the stack and replaces it with a C function. The new function converts error values generated with lerror into Lua errors, i.e. it calls lua_error.
  • Add utility function setglobal': It works like setglobal, but works with packages and nested tables (dot-notation only).


  • Add cabal flag ‘export-dynamic’: Default behavior is to include all symbols in the dynamic symbol table, as this enables users to load dynamic lua libraries. However, it is sometimes desirable to disable, e.g., when compiling a fully static binary. See jgm/pandoc#3986.


  • Increase user-friendlyness of error messages: The error message returned by toHaskellFunction hinted at the fact that the failing function is a Haskell function. This is mostly unnecessary information and might have confused users.


  • Added cabal flag to allow fully safe garbage collection: Lua garbage collection can occur in most of the API functions, even in those usually not calling back into haskell and hence marked as optimizable. The effect of this is that finalizers which call Haskell functions will cause the program to hang. A new flag allow-unsafe-gc is introduced and enabled by default. Disabling this flag will mark more C API functions as potentially calling back into Haskell. This has a serious performance impact.
  • FromLuaStack and ToLuaStack instances for lazy ByteStrings are added.
  • None-string error messages are handled properly: Lua allows error messages to be of any type, but the haskell error handlers expected string values. Tables, booleans, and other non-string values are now handled as well and converted to strings.


  • Use newtype definitions instead of type aliases for LuaNumber and LuaInteger. This makes it easier to ensure the correct numeric instances in situations where Lua might have been compiled with 32-bit numbers.
  • Instances of FromLuaStack and ToLuaStack for Int are removed. The correctness of these instances cannot be guaranteed if Lua was compiled with a non-standard integer type.


  • The flag lua_32bits was added to allow users to compile Lua for 32-bit systems.
  • When reading a list, throw an error if the lua value isn’t a table instead of silently returning an empty list.


  • Tuples from pairs to octuples have been made instances of FromLuaStack and ToLuaStack.
  • New functions dostring and dofile are provided to load and run strings and files in a single step.
  • LuaStatus was renamed to Status, the Lua prefix was removed from its type constructors.
  • The constructor ErrFile was added to Status. It is returned by loadfile if the file cannot be read.
  • Remove unused FFI bindings and unused types, including all functions unsafe to use from within Haskell and the library functions added with 0.5.0. Users with special requirements should define their own wrappers and raw bindings.
  • The module Foreign.Lua.Api.SafeBindings was merge into Foreign.Lua.Api.RawBindings.
  • FFI bindings are changed to use newtypes where sensible, most notably StackIndex, NumArgs, and NumResults, but also the newly introduced newtypes StatusCode, TypeCode, and LuaBool.
  • Add functions tointegerx and tonumberx which can be used to get and check values from the stack in a single step.
  • The signature of concat was changed from Int -> Lua () to NumArgs -> Lua ().
  • The signature of loadfile was changed from String -> Lua Int to String -> Lua Status.
  • The type LTYPE was renamed to Type, its constructors were renamed to follow the pattern Type<Typename>. LuaRelation was renamed to RelationalOperator, the Lua prefix was removed from its constructors.
  • Add function tolist to allow getting a generic list from the stack without having to worry about the overlapping instance with [Char].


  • Supported Lua Versions now include Lua 5.2 and Lua 5.3. LuaJIT and Lua 5.1 remain supported as well.
  • Flag use-pkgconfig was added to allow discovery of library and include paths via pkg-config. Setting a specific Lua version flag now implies system-lua. (Sean Proctor)
  • The module was renamed from Scripting.Lua to Foreign.Lua. The code is now split over multiple sub-modules. Files processed with hsc2hs are restricted to Foreign.Lua.Api.
  • A Lua monad (reader monad over LuaState) is introduced. Functions which took a LuaState as their first argument are changed into monadic functions within that monad.
  • Error handling has been redesigned completely. A new LuaException was introduced and is thrown in unexpected situations. Errors in lua which are leading to a longjmp are now caught with the help of additional C wrapper functions. Those no longer lead to uncontrolled program termination but are converted into a LuaException.
  • peek no longer returns Maybe a but just a. A LuaException is thrown if an error occurs (i.e. in situtations where Nothing would have been returned previously).
  • The StackValue typeclass has been split into FromLuaStack and ToLuaStack. Instances not satisfying the law x == push x *> peek (-1) have been dropped.
  • Documentation of API functions was improved. Most docstrings have been copied from the official Lua manual, enriched with proper markup and links, and changed to properly describe hslua specifics when necessary.
  • Example programs have been moved to a separate repository.
  • Unused files were removed. (Sean Proctor)


  • New raw functions for luaopen_base, luaopen_package, luaopen_string, luaopen_table, luaopen_math, luaopen_io, luaopen_os, luaopen_debug and their high-level wrappers (with names openbase, opentable etc.) implemented.
  • Remove custom versions of loadfile and loadstring.
  • Drop support for GHC versions < 7.8, avoid compiler warnings.
  • Ensure no symbols are stripped when linking the bundled lua interpreter.
  • Simplify tostring function definition. (Sean Proctor)
  • Explicitly deprecate strlen. (Sean Proctor)
  • Add links to lua documentation for functions wrapping the official lua C API. (Sean Proctor).


  • Bugfix(#30): tolist wasn’t popping elements of the list from stack.


  • pushstring and tostring now uses ByteString instead of [Char].
  • StackValue [Char] instance is removed, StackValue ByteString is added.
  • StackValue a => StackValue [a] instance is added. It pushes a Lua array to the stack. pushlist, islist and tolist functions are added.
  • Type errors in Haskell functions now propagated differently. See the Scripting.Lua documentation for detailed explanation. This should fix segfaults reported several times.
  • lua_error function is removed, it’s never safe to call in Haskell.

Related issues and pull requests: #12, #26, #24, #23, #18.


  • Pkgconf-based setup removed. Cabal is now using extra-libraries to link with Lua.
  • luajit flag is added to link hslua with LuaJIT.


  • Small bugfix related with GHCi running under Windows.


  • pushrawhsfunction and registerrawhsfunction functions are added.
  • apicheck flag is added to Cabal package to enable Lua API checking. (useful for debugging)


  • luaL_ref and luaL_unref functions are added.