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{-# LANGUAGE DataKinds, FlexibleInstances, GADTs, KindSignatures | |
, MultiParamTypeClasses, PolyKinds, ScopedTypeVariables, TypeFamilies | |
, TypeOperators, UndecidableInstances #-} | |
{-# OPTIONS_GHC -Wall #-} | |
module GaborTEq where | |
import GHC.TypeLits (Symbol, symbolVal, KnownSymbol) | |
import Data.Proxy | |
data Path (names :: [Symbol]) where | |
Empty :: Path '[] | |
Longer :: KnownSymbol name => Proxy name -> Path names -> Path (name ': names) | |
showPath :: Path ns -> String | |
showPath Empty = "" | |
showPath (Longer p Empty) = symbolVal p | |
showPath (Longer p ps) = symbolVal p ++ " -> " ++ showPath ps | |
data Nat = Z | S Nat | |
type family MaybeMap (f :: k -> k') t where | |
MaybeMap f 'Nothing = 'Nothing | |
MaybeMap f ('Just a) = 'Just (f a) | |
type family Index t ts :: Maybe Nat where | |
Index t '[] = 'Nothing | |
Index t (t ': ts) = 'Just 'Z | |
Index t (t' ': ts) = MaybeMap 'S (Index t ts) | |
type family StripOut i path where | |
StripOut 'Nothing ns = ns | |
StripOut ('Just 'Z) (name ': ns) = ns | |
StripOut ('Just ('S i)) (name ': ns) = name ': StripOut ('Just i) ns | |
StripOut i '[] = '[] | |
class StripOutAux (i :: Maybe Nat) ts where | |
stripOutAux :: proxy i -> Path ts -> Path (StripOut i ts) | |
instance StripOutAux 'Nothing ts where | |
stripOutAux _ p = p | |
instance StripOutAux ('Just 'Z) (t ': ts) where | |
stripOutAux _ (Longer _ path) = path | |
instance StripOutAux ('Just i) ts => StripOutAux ('Just ('S i)) (t ': ts) where | |
stripOutAux _ (Longer n' path) = | |
Longer n' (stripOutAux (Proxy::Proxy ('Just i)) path) | |
stripOut :: forall name names i. (i ~ Index name names, StripOutAux i names) | |
=> Proxy name -> Path names -> Path (StripOut i names) | |
stripOut _ = stripOutAux (Proxy::Proxy i) |
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