[Haskell-cafe] A Proposed Law for Foldable?

[Edward Kmett]

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 <david.feuer at gmail.com> 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
> <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
> > On Thu, Feb 12, 2015 at 04:39:02PM -0500, Edward Kmett wrote:
> >> 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.
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