On Tuesday 07 September 2010 05:22:55, David Menendez wrote:
> On Mon, Sep 6, 2010 at 10:22 PM, wren ng thornton <w...@freegeek.org>
wrote:
> > On 9/6/10 1:33 PM, David Menendez wrote:
> >> For that matter, can you even describe what pure is intended to do
> >> without reference to<*> or join?
> >
> > As already stated: fmap f . pure = pure . f
>
> That's pretty general. For lists, the functions having that property
> include const [], \x -> [x,x], and repeat.
>
> In fact, I think *every* appropriately-typed function satisfies that
> law. Does anyone know of a counter-example?
class Functor f => Pointed f where
point :: a -> f a
-- satisfying fmap f . point = point . f
notQuitePure :: Pointed f => a -> f a
notQuitePure _ = point undefined
fmap (const True) . notQuitePure = point . const True
But I don't see how to violate that law without introducing undefined on
the RHS.
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