[Haskell-cafe] Using promoted lists

[Erik Hesselink]

If you want to get rid of the overlap in your type families, you have
to add an extra argument indicating if the two types are equal. For
this, you need a type family to indicate equality of types. Sadly, the
naive implementation (TEQ x x = True, TEQ x y = False) overlaps and
isn't allowed. I'm not sure how to work around this, I guess you do
need FunDeps, and then you are pulled into HList land. See also my
attempt at extensible records [1].

Regards,

Erik

[1] https://gist.github.com/2492939

On Thu, Jun 7, 2012 at 9:52 PM, Yves Parès <yves.pares at gmail.com> wrote:
> The doc page
> http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/kind-polymorphism-and-promotion.html#promotion
> show 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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