Hi Yves,
The type level numbers have kind Nat, not Int (and so also can't be
negative). They have to be imported from GHC.TypeLits (I'm not sure if
this will change). So the following code works for me in HEAD:
{-# LANGUAGE TypeFamilies, DataKinds #-}
import GHC.TypeLits
type family Something a
Yves Pare`s wrote:
> 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.
More crucially, list membership also calls for an equality
predicate. Recall, List.elem has the Eq constraint; so the type-level
mem
Thanks for your answers, Anthony and Erik.
I'll try with fundeps. I know about HList, but back at the time when I
looked at it I found quite complex.
Anthony, the link you gave me [1] tends to show that actually Bool type is
promoted.
type family Member x (l :: [*]) :: Bool
type instance Member
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
Yves Parès gmail.com> writes:
>
> 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, th
The doc page
http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/kind-polymorphism-and-promotion.html#promotionshow
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.
