HsLua – Bindings to Lua, an embeddable scripting language
HsLua provides bindings, wrappers, types, and helper functions to bridge Haskell and 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 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
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
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
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
Foreign.Lua.Core (also exported in
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
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
cabal install pandoc --constraint="hslua +system-lua"
or with stack:
stack install pandoc --flag=hslua:system-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.
Exposed more functions from Lua’s
getsubtableis 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.
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
- Float, and
Instances for numbers fall back to strings when the representation as a Lua number would cause a loss of precision.
Haskell functions pushed with
pushHaskellFunctioncan 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:
Any Haskell value can be pushed to the Lua stack as userdata via
pushAnyand retrieved via
peekAny. Additional functions are provided to setup the userdata metatable.
The C preprocessor constants
LUA_PRELOAD_TABLEare made available as
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 (
Exception) and the respective infix from functions (
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 (
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).
The return value of
lua_newuserdatais CSize (was CInt).
Table index parameter in
rawsetimust be of type LuaInteger, but were of type Int.
The number of upvalues passed to
pushcclosuremust be of type NumArgs.
Lua.errorhas type Lua NumResults, simplifying its use in HaskellFunctions.
Retrieval functions which can fail, i.e.
touserdata, use the Maybe type to indicate success or failure, avoiding the need to perform additional checks.
Support for Lua versions before 5.3 has been dropped.
Support for GHC 7.8 has been dropped.
wrapHaskellFunctionhas been made internal and is no longer exported.
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
falseare peeked as
False, all other as
- Many internal improvements and additions such as a benchmarking suite, code cleanups, better tests, etc.
- 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.
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
toListwhich 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
pairsFromTablewhich 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
nthFromTopas well as the constants
stackBottomhave 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
idxinto an equivalent absolute index (that is, one that does not depend on the stack top). The function calls
lua_absindexwhen compiled with Lua 5.2 or later; for Lua 5.1, it is reimplemented in Haskell.
- Functions in
tastywhich have been deprecated have been replaced with non-deprecated alternatives.
- Re-export more FunctionCalling helpers in
Foreign.Lua: The typeclass
ToHaskellFunctionand the helper function
toHaskellFunctionare useful when working with functions. Importing them separately from
Foreign.Lua.FunctionCallingwas an unnecessary burden; they are therefor now re-exported by the main module.
- Export registry-relatd constants
noref: The constants are related to Lua’s registry functions (
- Add helper to convert functions into CFunction: A new helper
wrapHaskellFunctionis 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
lerrorinto Lua errors, i.e. it calls
- 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
toHaskellFunctionhinted 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-gcis 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.
ToLuaStackinstances 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
Intare removed. The correctness of these instances cannot be guaranteed if Lua was compiled with a non-standard integer type.
- The flag
lua_32bitswas 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
- New functions
dofileare provided to load and run strings and files in a single step.
LuaStatuswas renamed to
Status, the Lua prefix was removed from its type constructors.
- The constructor
ErrFilewas added to
Status. It is returned by
loadfileif 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
NumResults, but also the newly introduced newtypes
- Add functions
tonumberxwhich can be used to get and check values from the stack in a single step.
- The signature of
concatwas changed from
Int -> Lua ()to
NumArgs -> Lua ().
- The signature of
loadfilewas changed from
String -> Lua Intto
String -> Lua Status.
- The type
LTYPEwas renamed to
Type, its constructors were renamed to follow the pattern
LuaRelationwas renamed to
RelationalOperator, the Lua prefix was removed from its constructors.
- Add function
tolistto allow getting a generic list from the stack without having to worry about the overlapping instance with
- Supported Lua Versions now include Lua 5.2 and Lua 5.3. LuaJIT and Lua 5.1 remain supported as well.
use-pkgconfigwas 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
Foreign.Lua. The code is now split over multiple sub-modules. Files processed with hsc2hs are restricted to Foreign.Lua.Api.
Luamonad (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
longjmpare now caught with the help of additional C wrapper functions. Those no longer lead to uncontrolled program termination but are converted into a LuaException.
peekno longer returns
Maybe abut just
a. A LuaException is thrown if an error occurs (i.e. in situtations where Nothing would have been returned previously).
StackValuetypeclass has been split into
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_debugand their high-level wrappers (with names
- Remove custom versions of
- Drop support for GHC versions < 7.8, avoid compiler warnings.
- Ensure no symbols are stripped when linking the bundled lua interpreter.
tostringfunction definition. (Sean Proctor)
- Explicitly deprecate
strlen. (Sean Proctor)
- Add links to lua documentation for functions wrapping the official lua C API. (Sean Proctor).
tolistwasn’t popping elements of the list from stack.
StackValue [Char]instance is removed,
StackValue ByteStringis added.
StackValue a => StackValue [a]instance is added. It pushes a Lua array to the stack.
tolistfunctions are added.
- Type errors in Haskell functions now propagated differently. See the
Scripting.Luadocumentation for detailed explanation. This should fix segfaults reported several times.
lua_errorfunction 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-librariesto link with Lua.
luajitflag is added to link hslua with LuaJIT.
- Small bugfix related with GHCi running under Windows.
registerrawhsfunctionfunctions are added.
apicheckflag is added to Cabal package to enable Lua API checking. (useful for debugging)
luaL_unreffunctions are added.