hslua – Lua interpreter interface for Haskell

Hslua provides bindings, wrappers, types, and helper functions to bridge haskell and lua.

Overview

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 the most recent Lua version. However, cabal flags make it easy to swap this out in favor of a Lua version already installed on the host system. It supports the versions 5.1, 5.2, 5.3, and LuaJIT.

Interacting with Lua

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

import Foreign.Lua

main :: IO ()
main = runLua prog
  where
    prog :: Lua ()
    prog = do
      openlibs  -- load lua libraries so we can use 'print'
      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 of 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 FromLuaStack 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

and

-- | A value that can be pushed to the Lua stack.
class ToLuaStack 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.Api (also exported in Foreign.Lua).

Build flags

The following cabal build flags are supported:

• system-lua: Use the locally installed Lua version instead of the version shipped as part of HsLua.

• use-pkgconfig: Use pkg-config to discover library and include paths. This is used only when the system-lua flag is set or implied.

• lua501: Build against Lua 5.1; this implies the flag system-lua as well.

• lua502: Build against Lua 5.2; this implies the flag system-lua as well.

• luajit: Build against LuaJIT; this implies the flag system-lua as well.

• allow-unsafe-gc: Allow optimizations which make Lua's garbage collection potentially unsafe; haskell finalizers must be handled with extreme care. This is enabled per default, as this is rarely a problem in practice.

• apicheck: Compile Lua with its API checks enabled.

• lua_32bits: Compile Lua for a 32-bits system (e.g., i386, PowerPC G4).

Example: using a different lua version

To use a system-wide installed Lua/LuaJIT when linking hslua as a dependency, build/install your package using --constraint="hslua +system-lua" or for LuaJIT: --constraint="hslua +luajit". For example, you can install Pandoc with hslua that uses system-wide LuaJIT like this:

cabal install pandoc --constraint="hslua +system-lua +luajit"

or with stack:

stack install pandoc --flag=hslua:luajit

FAQ

Is anybody using this? Absolutely. E.g., Pandoc, the universal document converter, 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. This has been used in pandoc-scholar (paper) to allow for semantically enriched scholarly articles.

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. However, raw bindings to the C API functions are still provided in Foreign.Lua.RawBindings. If you get coroutines to work, or just believe that there should be wrapper functions for other reasons, we'd love to hear from you.

Why are there no predefined stack instances for default numerical types? HsLua defines instances for the FromLuaStack and ToLuaStack type-classes only if the following law holds: return x == push x *> peek x. Lua can be compiled with customized number types, making it impossible to verify the correctness of the above equation. Furthermore, instances for numerical types can be based on those of LuaInteger and LuaNumber and are easy to write. Therefor hslua doesn't provide any such instances.