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