[Haskell-cafe] Restricted type classes

[John Lato]

On Fri, Sep 3, 2010 at 12:01 PM, C. McCann <cam at uptoisomorphism.net> wrote:

> On Fri, Sep 3, 2010 at 11:47 AM, John Lato <jwlato at gmail.com> wrote:
> > On Fri, Sep 3, 2010 at 1:29 PM, Ivan Lazar Miljenovic <
> ivan.miljenovic at gmail.com> wrote:
> >> On 3 September 2010 22:23, John Lato <jwlato at gmail.com> wrote:
> >> > Do you have a kind * implementation of Foldable?  I'd be interested in
> >> > seeing it, because I was unable to create a usable implementation
> (based
> >> > upon the RMonad scheme) on my last attempt.
> >>
> >> I was going to make it a subset of Foldable: fold, foldr, foldl, etc.
> >
> > So you don't have a working implementation yet?  I ended up thinking this
> is
> > impossible, although I don't remember the reasoning that led me to that
> > conclusion (and I could very well be wrong).
> > I would suggest that you check this before going too far along the
> > restricted-monad path.
>
> This sounds odd to me. An RMonad-style version of Foldable is
> straightforward:
>
>    class RFoldable t where
>        rfold :: Control.RMonad.Suitable t a => (a -> b -> b) -> b -> t a ->
> b
>
>    instance RFoldable Data.Set.Set where
>        rfold = Data.Set.fold
>
> A similar class for types of kind * is also straightforward:
>
>    class Reduce t where
>        type Elem t
>        reduce :: (Elem t -> r -> r) -> r -> t -> r
>
>    instance Reduce Data.ByteString.ByteString where
>        type Elem Data.ByteString.ByteString = Word8
>        reduce = Data.ByteString.foldr
>
> Both seem to work as I'd expect. Am I missing something? Foldable is
> pretty trivial--perhaps it was Traversable that you found problematic?
>

This certainly does seem to work just fine in ghc-6.12, but not 6.10.4.  I
wonder if that was the source of my problems last time.

John
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