[Haskell-cafe] class instances for kinds with finite ty constrs, does this make sense?
José Pedro Magalhães
jpm at cs.uu.nl
Thu Jun 7 21:03:25 CEST 2012
This has been discussed before:
Feel free to open a feature request for this. I think it's something we
addressing, but at the moment it's not immediately clear how to do it.
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?
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> Haskell-Cafe at haskell.org
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