Applicative and Functor
ross at soi.city.ac.uk
Mon Mar 6 04:11:38 EST 2006
On Sun, Mar 05, 2006 at 09:53:02PM -0800, Ashley Yakeley wrote:
> In article <20060305233414.GA8685 at soi.city.ac.uk>,
> Ross Paterson <ross at soi.city.ac.uk> wrote:
> > so I would also need
> > instance Functor ((->) a)
> > but where should it go? (The same instance occurs in
> > Control.Monad.Reader, but that could be removed.)
> The Prelude is to obvious place, since that's where both Functor and
> (->) are introduced.
Yes, but that would break compatibility with Haskell 98.
> > > There may also be a case for a class with ap (<*>) but not return
> > > (pure), which might better be called "Applicative".
> > Do you have both instances and clients for this interface?
> Well, no, but ap gives you (>>) (I'd forgotten this earlier):
OK, one client but no instances.
> > And can an associativity law even be stated?
On second thought, yes it can:
fmap (.) u <*> v <*> w = u <*> (v <*> w)
> Traversable looks like my ExtractableFunctor?
Yes it does, and also Meertens's "pullable functors" (though the
higher-order version is often more efficient, especially for non-regular
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