Clarify relationship between Functor and Applicative

[Edward Kmett]

On Fri, Feb 25, 2011 at 5:42 PM, Ross Paterson <ross at soi.city.ac.uk> wrote:

> On Fri, Feb 25, 2011 at 05:34:18PM -0500, roconnor at theorem.ca wrote:
> > In the applicative documentation, it says for an Applicative functor f:
> >
> > The Functor instance should satisfy
> >
> >       fmap f x = pure f <*> x
> >
> > I think the documentation should be clarified that this does not
> > need to be checked because it is a consequence of the other
> > applicative laws.
> >
> > See <http://hpaste.org/44315/applicative_implies_functor>.
>
> Do you have a proof that Functor instances are uniquely determined?


Given fmap id = id, the uniqueness of fmap for a given type follows
trivially from the free theorem.

-Edward Kmett
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