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

Hi,

Sjoerd Visscher wrote:

...

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) -- Roman I. Cheplyaka :: http://ro-che.info/

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

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

...

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

...

Hi,

Sjoerd Visscher wrote:

...

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

On Apr 26, 2012, at 9:34 PM, Tillmann Rendel wrote:

...

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?

Yes, it is. unfoldr is quite specifically tailored to lists, so it doesn't work well generically. I did include it in the package, but it does a breadth-first search for the first value that has exactly enough positions to store the elements ('a's), and there might not be one.

...

Different unfolders provide different ways of generating values, for example: - Random values - Enumeration of all values (depth-first or breadth-first) - Convert from a list - An implementation of QuickCheck's arbitrary should also be possible (still working on that)

Can this be extended to provide a single API that allows testing à la SmallCheck, LazySmallCheck and/or QuickCheck without duplicating properties or instances?

Well, the idea is to unify all ways of unfolding (i.e. all ways of generating values). So those parts of the checkers could use the same API, but there's a lot more to checking than that. By the way, I uploaded 0.5.0 a few hours ago, which contains a generic arbitrary implementation. greetings, -- Sjoerd Visscher https://github.com/sjoerdvisscher/blog