Hello,
Does the following code work for you?
-Jeff
---------------------------
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances
-fallow-overlapping-instances #-}
data Nil = Nil
data x ::: xs = x ::: xs
infixr 5 :::
data HTrue = HTrue deriving Show
data HFalse = HFalse deriving Show
class Member x xs b | x xs -> b where member :: x -> xs -> b
instance Member x Nil HFalse where member _ _ = HFalse
instance Member x xs b => Member x (x ::: xs) HTrue where member _ _ =
HTrue
instance Member x xs b => Member x (y ::: xs) b where member x (_ ::: xs)
= member x xs
{-
member 'a' (() ::: "a" ::: '1' ::: Nil) ==> HTrue
member 'a' (() ::: "a" ::: Nil) ==> HFalse
-}
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