Exposable.hs

{-# LANGUAGE FlexibleInstances     #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings     #-}
{-# LANGUAGE ScopedTypeVariables   #-}
{-|
Module      : HsLua.Class.Exposable
Copyright   : © 2007–2012 Gracjan Polak,
                2012–2016 Ömer Sinan Ağacan,
                2017-2026 Albert Krewinkel
License     : MIT
Maintainer  : Albert Krewinkel <tarleb@hslua.org>

Call Haskell functions from Lua.
-}
module HsLua.Class.Exposable
  ( Exposable (..)
  , toHaskellFunction
  , pushAsHaskellFunction
  , registerHaskellFunction
  ) where

import Data.String (fromString)
import HsLua.Core as Lua
import HsLua.Marshalling (Peek, forcePeek, liftLua, retrieving, withContext)
import HsLua.Class.Peekable (Peekable (safepeek))
import HsLua.Class.Pushable (Pushable (push))

-- | Operations and functions that can be pushed to the Lua stack. This
-- is a helper function not intended to be used directly. Use the
-- @'toHaskellFunction'@ wrapper instead.
class LuaError e => Exposable e a where
  -- | Helper function, called by @'toHaskellFunction'@. Should do a
  -- partial application of the argument at the given index to the
  -- underlying function. Recurses if necessary, causing further partial
  -- applications until the operation is a easily exposable to Lua.
  partialApply :: StackIndex -> a -> Peek e NumResults

instance {-# OVERLAPPING #-} LuaError e =>
         Exposable e (HaskellFunction e) where
  partialApply _ = liftLua

instance (LuaError e, Pushable a) => Exposable e (LuaE e a) where
  partialApply _narg x = 1 <$ liftLua (x >>= push)

instance (LuaError e, Pushable a) => Exposable e (Peek e a) where
  partialApply _narg x = 1 <$ (x >>= liftLua . push)

instance (Peekable a, Exposable e b) => Exposable e (a -> b) where
  partialApply narg f = getArg >>= partialApply (narg + 1) . f
    where
      getArg = retrieving (fromString errorPrefix) (safepeek narg)
      errorPrefix = "argument " ++ show (fromStackIndex narg)

-- | Convert a Haskell function to a function type directly exposable to
-- Lua. Any Haskell function can be converted provided that:
--
--   * all arguments are instances of @'Peekable'@
--   * return type is @LuaE e a@, where @a@ is an instance of
--     @'Pushable'@
--
-- Any exception of type @e@ will be caught.
--
-- /Important/: this does __not__ catch exceptions other than @e@;
-- exception handling must be done by the Haskell function. Failure to
-- do so will cause the program to crash.
--
-- E.g., the following code could be used to handle an Exception
-- of type FooException, if that type is an instance of
-- 'Control.Monad.Catch.MonadCatch' and 'Pushable':
--
-- > toHaskellFunction (myFun `catchM` (\e -> raiseError (e :: FooException)))
toHaskellFunction :: forall e a. Exposable e a => a -> HaskellFunction e
toHaskellFunction a = forcePeek $ do
  withContext "executing function call" $ partialApply 1 a

-- | Pushes the given value as a function to the Lua stack.
--
-- See 'toHaskellFunction' for details.
pushAsHaskellFunction :: forall e a. Exposable e a => a -> LuaE e ()
pushAsHaskellFunction = pushHaskellFunction . toHaskellFunction

-- | Imports a Haskell function and registers it at global name.
registerHaskellFunction :: Exposable e a
                        => Name -> a -> LuaE e ()
registerHaskellFunction n f = do
  pushAsHaskellFunction f
  setglobal n