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{-# LANGUAGE PatternSynonyms, ViewPatterns, ConstraintKinds, TypeFamilies, PolyKinds, KindSignatures #-}
module T10997_1a where
import Data.Kind
type family Showable (a :: k) :: Constraint where
Showable (a :: Type) = (Show a)
Showable a = ()
extractJust :: Maybe a -> (Bool, a)
extractJust (Just a) = (True, a)
extractJust _ = (False, undefined)
pattern Just' :: Showable a => a -> (Maybe a)
pattern Just' a <- (extractJust -> (True, a)) where
Just' a = Just a
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