39 lines
1.1 KiB
Haskell
39 lines
1.1 KiB
Haskell
-- | A module containing a box datatype representing a compiled item. This
|
|
-- item can be of any type, given that a few restrictions hold (e.g. we want
|
|
-- a 'Typeable' instance to perform type-safe casts).
|
|
--
|
|
{-# LANGUAGE ExistentialQuantification #-}
|
|
module Hakyll.Core.CompiledItem
|
|
( CompiledItem (..)
|
|
, compiledItem
|
|
, unCompiledItem
|
|
) where
|
|
|
|
import Data.Binary (Binary)
|
|
import Data.Typeable (Typeable, cast)
|
|
|
|
import Hakyll.Core.Writable
|
|
|
|
-- | Box type for a compiled item
|
|
--
|
|
data CompiledItem = forall a. (Binary a, Typeable a, Writable a)
|
|
=> CompiledItem a
|
|
|
|
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) = case cast x of
|
|
Just x' -> x'
|
|
Nothing -> error "unCompiledItem: Unsupported type"
|