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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeFamilyDependencies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE UndecidableInstances #-}
module T16512a where
import Data.Kind
( Type )
-- HOAS representation
data AST (a :: Type) :: Type where
(:$) :: AST ( a -> b ) -> AST a -> AST b
-- Lam :: ( AST a -> AST b ) -> AST ( a -> b )
-- PrimOp :: PrimOp op a => AST a
-- ...
data ASTs (as :: [Type]) :: Type where
NilASTs :: ASTs '[]
ConsAST :: AST a -> ASTs as -> ASTs (a ': as)
type family ListVariadic (as :: [Type]) (b :: Type) = (r :: Type) | r -> as b where
ListVariadic (a ': as) b = a -> ListVariadic as b
-- ListVariadic '[] () = ()
-- ListVariadic '[] Bool = Bool
-- ListVariadic '[] Word = Word
-- ListVariadic '[] Int = Int
-- ListVariadic '[] Float = Float
-- ListVariadic '[] Double = Double
-- ...
data AnApplication b where
AnApplication :: AST (ListVariadic as b) -> ASTs as -> AnApplication b
unapply :: AST b -> AnApplication b
unapply (f :$ a)
= case unapply f of
AnApplication g as ->
AnApplication g (a `ConsAST` as)
-- no other cases with this simplified AST
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