[Haskell-cafe] Using promoted lists

Erik Hesselink hesselink at gmail.com
Fri Jun 8 07:42:51 CEST 2012

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].



[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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