A Lua language interpreter embedding in Haskell

HsLua – Bindings to Lua, an embeddable scripting language

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 (i.e., Lua 5.3.5). 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
  where
    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 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 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

and

-- | 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).

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.

• pkg-config: Use pkg-config to discover library and include paths. Setting this flag implies system-lua.

• 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).

• export-dynamic: Add all symbols to dynamic symbol table; disabling this will make it possible to create fully static binaries, but renders loading of dynamic C libraries impossible.

Example: using a different lua version

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

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

or with stack:

stack install pandoc --flag=hslua:system-lua

Q&A

• 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.