On Mon, 2011-08-29 at 12:00 +0900, Sebastian Fischer wrote: > On Sun, Aug 28, 2011 at 12:41 AM, Sönke Hahn <[email protected]> wrote: > > I was wondering which > > type could be an instance of Pointed, but not of Applicative. But I can't > > think of one. Any ideas? > > Functional lists: > > type FList a = [a] -> [a] > > they have a Monoid instance for empty and append, a "point" function > for singletons but Applicative or Monad cannot be defined without > converting back and forth to ordinary lists. > > Sebastian
newtype FList a = FList ([a] -> [a])
instance Functor FList where
f `fmap` FList g = ...?
The only possible implementation I can think of:
f `fmap` FList g = _|_
f `fmap` FList g = map id
f `fmap` FList g = map _|_
(+ variation of _|_*)
Neither of them holding fmap id = id.
Regards
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