Bindings to Lua, an embeddable scripting language


Version on this page:2.3.1
LTS Haskell 22.30:2.3.2
Stackage Nightly 2024-07-22:2.3.2
Latest on Hackage:2.3.2

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MIT licensed by Albert Krewinkel, Gracjan Polak, Ömer Sinan Ağacan
Maintained by [email protected]
This version can be pinned in stack with:hslua-core-2.3.1@sha256:07098032df954b0f437ca2ff79ce486b186f75bc910f2e838006d99169ddc0b8,4431


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Basic building blocks to interface Haskell and Lua in a Haskell-idiomatic style.


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 the basic building blocks for coders to embed Lua into their programs.

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

Interacting with Lua

HsLua core 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 HsLua.Core.Lua as Lua

main :: IO ()
main = Lua.run prog
    prog :: Lua ()
    prog = do
      Lua.openlibs  -- load Lua libraries so we can use 'print'
      Lua.getglobal "print"            -- push print function
      Lua.pushstring "Hello, World!"   -- push string argument
        (NumArgs 1)    -- number of arguments passed to the function
        (NumResults 0) -- number of results expected
                       -- as return values

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   |
|          |                  |          |

This package provides all basic building blocks to interact with the Lua stack. If you’d like more comfort, please consider using the hslua-packaging and hslua-classes packages.

Error handling

Errors and exceptions must always be caught and converted when passing language boundaries. The exception type which can be handled is encoded as the type e in the monad LuaE e. Only exceptions of this type may be thrown; throwing different exceptions across language boundaries will lead to a program crash.

Exceptions must support certain operations as defined by the LuaError typeclass. The class ensures that errors can be converted from and to Lua values, and that a new exception can be created from a String message.



hslua-core uses PVP Versioning.


Released 2023-03-17.

  • New module HsLua.Core.Debug: the module provides bindings to a subset of functions of the Lua debug interface. Currently the module only exports getupvalue and setupvalue, both of which are also re-exported from HsLua.Core.


Released 2023-03-13.

  • The functions loadfile, dofile, and dofileTrace now expect the argument to be of type Maybe FilePath. The functions load from stdin when the argument is Nothing.

  • Added setwarnf' for simple warning messgae handling: The built-in method of setting a warn function is flexible but not straight-forward to use. The new setwarnf' function allows to set a Haskell action as a warning hook: the default warning behavior is kept in place, but, in addition to the default action, the hook is called on the concatenated warning messages. This can be used to plug Lua warnings into an application specific Haskell reporting system.

  • Export GCManagedState, newGCManagedState, closeGCManagedState, and withGCManagedState from HsLua.Core.


Released 2022-06-19.

  • Ensure that loadfile works with umlauts in filepath: The OS does not necessarily expect filenames to be UTF-8 encoded, especially Windows. On non-Windows systems, the current file system encoding is now used to convert filenames to C strings. On Windows, the CP_ACP codepage is used, as required by the Windows API.

  • GC managed Lua state: Add new type GCManagedState and functions newGCManagedState, closeGCManagedState, and withGCManagedState. These allow to create and use a Lua state in flexible ways in that it does not require the state to be closed explicitly. The state will be closed when the respective variable is collected.

  • Require lua-2.2.1.

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


Released 2022-02-19.

  • Use lua-2.2.0, which requires Lua 5.4.

  • Rename newuserdata to newuserdatauv and let it take the number of associated uservalues as an additional argument.

    Similarly, newhsuserdata is now newhsuserdatauv.

  • Rename getuservalue and setuservalue to getiuservalue and setiuservalue, respectively. Like both functions now take an extra argument specifying the number of the uservalue that should be retrieved or set.

    It is now possible for setiuservalue to fail, so it returns a boolean to indicate whether the action was successful.

  • The GCControl type has been updated to match the new gc control:

    • The GCStep constructor takes an argument “stepsize”;
    • constructors GCGen and GCInc have been added;
    • constructors GCSetPause and GCSetStepMul have been removed.


Released 29-01-2022.

  • The functions rawget, rawgeti, and getref now return the type of the value that was pushed to the stack.

  • A new function checkstack' is added to HsLua.Core.Auxiliary and exported from the main HsLua.Core module. The function throws an exception if the stack cannot be grown to accommodate a given number of elements; it is similar to luaL_checkstack.

  • Added function requiref, which safely wraps the unsafe luaL_requiref function.

  • New functions pcallTrace, callTrace, dostringTrace, and dofileTrace: behaves like the respective unsuffixed functions, but use a message handler that creates a stack traceback on error.

  • Added function rotate, wrapping lua_rotate.

  • Package helper requirehs signature changed to

    requirehs :: LuaError e
              => Name                 -- ^ modname
              -> (Name -> LuaE e ())  -- ^ openf
              -> LuaE e ()

    The function creating and pushing the module value now takes the module name as an argument. It also behaves more like luaL_requiref in that it reloads the module if the value in the LOADED table is falsy.


Released 2021-11-03.

  • Fixed output of pushTypeMismatchError when there is no value at the given index. Previously the function would report the value as type string and now reports it as no value.


Released 2021-10-29.

  • Fixed bug in pushTypeMismatchError. The function did not use an absolute stack index in one place, which sometimes lead to incorrect actual types being reported.


Released 2021-10-21.

  • 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.

  • Added new module HsLua.Core.Userdata. It contains thin wrappers around the functions available for creating Haskell-value-wrapping userdata objects.

  • Added new module HsLua.Core.Closures, containing functions to expose Haskell functions to Lua.

  • Reverted to using the auxlib luaL_loadfile function to load a Lua file. Previously files were opened and read in Haskell, but some functionality of the auxlib function was missing.


Released 2021-02-27.

Extracted from hslua-1.3.0.