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|
-- | A module containing a box datatype representing a compiled item. This
-- item can be of any type, given that a few restrictions hold:
--
-- * we need a 'Typeable' instance to perform type-safe casts;
--
-- * we need a 'Binary' instance so we can serialize these items to the cache;
--
-- * we need a 'Writable' instance so the results can be saved.
--
{-# LANGUAGE ExistentialQuantification, DeriveDataTypeable #-}
module Hakyll.Core.CompiledItem
( CompiledItem (..)
, compiledItem
, unCompiledItem
) where
import Data.Binary (Binary)
import Data.Typeable (Typeable, cast, typeOf)
import Data.Maybe (fromMaybe)
import Hakyll.Core.Writable
-- | Box type for a compiled item
--
data CompiledItem = forall a. (Binary a, Typeable a, Writable a)
=> CompiledItem a
deriving (Typeable)
instance Writable CompiledItem where
write p (CompiledItem x) = write p x
-- | Box a value into a 'CompiledItem'
--
compiledItem :: (Binary a, Typeable a, Writable a)
=> a
-> CompiledItem
compiledItem = CompiledItem
-- | Unbox a value from a 'CompiledItem'
--
unCompiledItem :: (Binary a, Typeable a, Writable a)
=> CompiledItem
-> a
unCompiledItem (CompiledItem x) = fromMaybe error' $ cast x
where
error' = error $ "Hakyll.Core.CompiledItem.unCompiledItem: "
++ "unsupported type (got " ++ show (typeOf x) ++ ")"
|
