On Fri, Sep 3, 2010 at 11:47 AM, John Lato <jwlato@gmail.com> wrote:
> On Fri, Sep 3, 2010 at 1:29 PM, Ivan Lazar Miljenovic <ivan.miljenovic@gmail.com> wrote:
>> On 3 September 2010 22:23, John Lato <jwlato@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