Re: [Haskell-cafe] A Proposed Law for Foldable?

Gershom's law is an attempt to make rigorous and capture the notion that a Foldable should touch all of the a's in 'f a'. This is a property that Functor and Traversable both share, but which Foldable doesn't. The others are able to attain this property through parametricity. This would rule out the un-extendable `foldMap = mempty` cases, and capture the intuitive definition of what Foldable "does". -Edward On Thu, Feb 12, 2015 at 9:29 PM, David Feuer wrote:

Someone must not have noticed the bit where I mentioned that proposing Functor f => Foldable f in order to avoid the GADT stuff would make everyone reject the idea. But to the extent that Gershom's law is a good idea (I'm very far from convinced, because no one's explained in a way that I can understand what makes it useful), it could by made a conditional law: only Functor instances would have to obey it, in much the same way that (of course) only Functor instances must obey foldMap f = fold . fmap f.

On Thu, Feb 12, 2015 at 7:19 PM, Tom Ellis wrote:

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On Thu, Feb 12, 2015 at 04:39:02PM -0500, Edward Kmett wrote:

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And as been worked through to death in the past, Functor cannot be a superclass of Foldable. ;)

Because, for example, that wouldn't allow a Set to be Foldable. _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

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