[Haskell-cafe] class instances for kinds with finite ty constrs, does this make sense?

[José Pedro Magalhães]

Hi,

This has been discussed before:
http://haskell.1045720.n5.nabble.com/Data-Kinds-and-superfluous-in-my-opinion-constraints-contexts-td5689436.html

Feel free to open a feature request for this. I think it's something we
should consider
addressing, but at the moment it's not immediately clear how to do it.


Cheers,
Pedro

On Thu, Jun 7, 2012 at 7:32 PM, ex falso <0slemi0 at gmail.com> wrote:

> Hi, i've been using DataKinds for a while and there seems to be a
> recurring annoyance that could be possibly eliminated with a language
> extension. It goes something like this:
>
> data Tuple (l :: [*]) where
>  Unit :: Tuple '[]
>  Comma :: a -> Tuple as -> Tuple (a ': as)
>
> data Proxy k = Proxy
>
> class TupleLength (l :: [*]) where
>  tupleLength :: Proxy l -> Integer
> instance TupleLength '[] where
>  tupleLength _ = 0
> instance (TupleLength as) => TupleLength '(a : as) where
>  tupleLength _ = 1 + tupleLength (Proxy :: Proxy as)
>
> so far so good. but now if we want to use TupleLength:
>
> printLength :: (TupleLength l) => Tuple l -> IO ()
> printLength t = print $ tupleLength t
>
> we always have to put the class restriction (TupleLength l) there,
> even though all possible type constructors of [*] have a TupleLength
> instance defined!
>
> If we had a way to signal that all ty constrs are catered for then the
> compiler could check this, and deduce that l::[*] is always an
> instance. Well that's the hypothesis.
> Of course this would only make sense for kinds with finite no. of ty
> constructors, namely DataKinds-lifted types.
>
> Does this make sense?
> ex
>
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