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{-# LANGUAGE UndecidableInstances, PatternSynonyms, GADTs #-}
module T11728 where
import Data.Kind
import Data.Int
import GHC.TypeLits
import Text.Show.Functions
data Ty ty where
I :: Ty Int
A :: Ty a -> Ty [a]
class GetTy ty where getTy :: Ty ty
instance GetTy Int where getTy = I
instance GetTy ty => GetTy [ty] where
getTy = A getTy
data E a where
UnOp :: Unary a b -> (E a -> E b)
pattern LEN :: () => (GetTy a) => E [a] -> E Int
pattern LEN xs <- UnOp (Un OpLen _) xs where
LEN xs = UnOp (Un OpLen length) xs
data Unary a b where
Un :: (GetTy a, GetTy b) => UnOp a b -> (a -> b) -> Unary a b
data UnOp a b where
OpLen :: GetTy a => UnOp [a] Int
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