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+{- git-annex deferred parse values
+ -
+ - Copyright 2015 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# LANGUAGE FlexibleInstances #-}
+
+module Types.DeferredParse where
+
+import Annex
+import Common
+
+-- Some values cannot be fully parsed without performing an action.
+-- The action may be expensive, so it's best to call finishParse on such a
+-- value before using getParsed repeatedly.
+data DeferredParse a = DeferredParse (Annex a) | ReadyParse a
+
+class DeferredParseClass a where
+	finishParse :: a -> Annex a
+
+getParsed :: DeferredParse a -> Annex a
+getParsed (DeferredParse a) = a
+getParsed (ReadyParse a) = pure a
+
+instance DeferredParseClass (DeferredParse a) where
+	finishParse (DeferredParse a) = ReadyParse <$> a
+	finishParse (ReadyParse a) = pure (ReadyParse a)
+
+instance DeferredParseClass (Maybe (DeferredParse a)) where
+	finishParse Nothing = pure Nothing
+	finishParse (Just v) = Just <$> finishParse v