[Haskell-cafe] Using promoted lists

Yves Parès yves.pares at gmail.com
Thu Jun 7 21:52:44 CEST 2012

The doc page
that lists are now usable as types.

So I'm trying to make a type level function to test if a type list contains
a type. Unless I'm wrong, that calls to the use of a type family.

{-# LANGUAGE DataKinds, TypeOperators, KindSignatures, TypeFamilies #-}

data HBool = HTrue | HFalse  -- Mandatory as Bool type is not currently
promoted to a kind

type family Member x (l :: [*]) :: HBool

type instance Member x (x ': xs) = HTrue
type instance Member x (y ': xs) = Member x xs
type instance Member x (y ': '[]) = HFalse

But the compiler complains about my instance conflicting. Is what I'm
trying to do feasible?

Second question: how can type level tuples (also mentioned in the doc page)
be exploited? Aren't they redundant with type-level lists?
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