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{-# LANGUAGE GADTs #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE TypeOperators #-}
module Codec.Picture.Metadata where
import Control.DeepSeq( NFData( .. ) )
import Data.Typeable( (:~:)( Refl ) )
import Data.Word( Word16 )
data ExifTag
= TagPhotometricInterpretation
| TagUnknown !Word16
deriving Eq
data ExifData = ExifNone
data Keys a where
Exif :: !ExifTag -> Keys ExifData
Unknown :: !String -> Keys Value
data Value
data Elem k =
forall a. (Show a, NFData a) => !(k a) :=> a
keyEq :: Keys a -> Keys b -> Maybe (a :~: b)
keyEq a b = case (a, b) of
(Unknown v1, Unknown v2) | v1 == v2 -> Just Refl
(Exif t1, Exif t2) | t1 == t2 -> Just Refl
_ -> Nothing
newtype Metadatas = Metadatas { getMetadatas :: [Elem Keys] }
lookup :: Keys a -> Metadatas -> Maybe a
lookup k = go . getMetadatas where
go [] = Nothing
go ((k2 :=> v) : rest) = case keyEq k k2 of
Nothing -> go rest
Just Refl -> Just v
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