Re: [Haskell-cafe] ANN: unfoldable-0.4.0

On Apr 28, 2012, at 2:40 AM, wren ng thornton wrote:

On 4/26/12 3:52 PM, Roman Cheplyaka wrote:

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* Tillmann Rendel [2012-04-26 21:34:21+0200]

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Hi,

Sjoerd Visscher wrote:

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Just as there's a Foldable class, there should also be an Unfoldable class. This package provides one:

class Unfoldable t where unfold :: Unfolder f => f a -> f (t a)

Just to be sure: That's not a generalization of Data.List.unfoldr, or is it somehow?

It seems to be -- see https://github.com/sjoerdvisscher/unfoldable/blob/master/src/Data/Unfoldable...

(although that is much more complicated than Data.List.unfoldr)

I must admit I'm a bit weirded out by the (Bounded a, Enum a) restriction on the Either, tuple, and Constant instances. Why not just use Unfoldable a, or have a class specifically devoted to unfolding * types?

I don't like the (Bounded a, Enum a) restrictions very much either. That was basically a quick hack in the first version and I haven't given it much thought after that. The most generic solution would be Biunfoldable I think. class Biunfoldable t where biunfold :: Unfolder f => f a -> f b -> f (t a b) instance Biunfoldable (,) where biunfold fa fb = choose [(,) <$> fa <*> fb] instance Biunfoldable Either where biunfold fa fb = choose [Left <$> fa, Right <$> fb] instance Biunfoldable Constant where biunfold fa _ = choose [Constant <$> fa] But I don't think an unfoldable class for * types is that interesting. Any type that would be an instance could also be in instance of Bounded and Enum: class Unfoldable0 a where unfold0 :: Unfolder f => f a minBoundDef :: Unfoldable0 a => a minBoundDef = fromJust unfold0 maxBoundDef :: Unfoldable0 a => a maxBoundDef = fromJust (getDualA unfold0) toEnumDef :: Unfoldable0 a => Int -> a toEnumDef i = unfold0 !! i fromEnumDef :: (Unfoldable0 a, Eq a) => a -> Int fromEnumDef a = fromJust (elemIndex a unfold0) so having boundedEnum is good enough I think. -- Sjoerd Visscher https://github.com/sjoerdvisscher/blog