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{-# LANGUAGE PatternSynonyms, ExistentialQuantification, GADTSyntax, TypeFamilies #-}
module T11010 where
data Expr a where
Fun :: String -> (a -> b) -> (Expr a -> Expr b)
pattern IntFun :: (a ~ Int) => String -> (a -> b) -> (Expr a -> Expr b)
pattern IntFun str f x = Fun str f x
-- Alternative syntax for pattern synonyms:
-- pattern
-- Suc :: () => (a ~ Int) => Expr a -> Expr Int
-- Suc n <- IntFun _ _ n where
-- Suc n = IntFun "suc" (+ 1) n
pattern Suc :: (a ~ Int) => Expr a -> Expr Int
pattern Suc n <- IntFun _ _ n where
Suc n = IntFun "suc" (+ 1) n
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