Re: [Haskell-cafe] Restricted type classes

On Fri, Sep 3, 2010 at 12:01 PM, C. McCann wrote:

On Fri, Sep 3, 2010 at 11:47 AM, John Lato wrote:

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On Fri, Sep 3, 2010 at 1:29 PM, Ivan Lazar Miljenovic < ivan.miljenovic@gmail.com> wrote:

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On 3 September 2010 22:23, John Lato wrote:

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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