src/Hakyll/Core/CompiledItem.hs · hakyll

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hakyll/src/Hakyll/Core/CompiledItem.hs

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1--------------------------------------------------------------------------------

2-- | A module containing a box datatype representing a compiled item. This

3-- item can be of any type, given that a few restrictions hold:

4--

5-- * we need a 'Typeable' instance to perform type-safe casts;

6--

7-- * we need a 'Binary' instance so we can serialize these items to the cache;

8--

9-- * we need a 'Writable' instance so the results can be saved.

10{-# LANGUAGE DeriveDataTypeable #-}

11{-# LANGUAGE ExistentialQuantification #-}

12module Hakyll.Core.CompiledItem

13 ( CompiledItem (..)

14 , compiledItem

15 , unCompiledItem

16 ) where

19--------------------------------------------------------------------------------

20import Data.Binary (Binary)

21import Data.Maybe (fromMaybe)

22import Data.Typeable (Typeable, cast, typeOf)

25--------------------------------------------------------------------------------

26import Hakyll.Core.Writable

29--------------------------------------------------------------------------------

30-- | Box type for a compiled item

32data CompiledItem = forall a. (Binary a, Typeable a, Writable a)

33 => CompiledItem a

34 deriving (Typeable)

37--------------------------------------------------------------------------------

38instance Writable CompiledItem where

39 write p (CompiledItem x) = write p x

42--------------------------------------------------------------------------------

43-- | Box a value into a 'CompiledItem'

44compiledItem :: (Binary a, Typeable a, Writable a) => a -> CompiledItem

45compiledItem = CompiledItem

48--------------------------------------------------------------------------------

49-- | Unbox a value from a 'CompiledItem'

50unCompiledItem :: (Binary a, Typeable a, Writable a) => CompiledItem -> a

51unCompiledItem (CompiledItem x) = fromMaybe error' $ cast x

52 where

53 error' = error $

54 "Hakyll.Core.CompiledItem.unCompiledItem: " ++

55 "unsupported type (got " ++ show (typeOf x) ++ ")"