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{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
import Data.Functor.Identity
data HideArg f where
HideArg :: f a -> HideArg f
data family App :: tF -> tF
data instance App f x = App1 (f x)
class WrappedIn s a | s -> a where
unwrap :: s -> a
instance WrappedIn (App f a) (f a) where
unwrap (App1 fa) = fa
pattern Unwrapped :: WrappedIn s a => a -> s
pattern Unwrapped x <- (unwrap -> x)
{-# COMPLETE Unwrapped :: App #-}
boom :: HideArg (App Identity) -> Bool
boom (HideArg (Unwrapped (Identity _))) = True
main :: IO ()
main = print ":("
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