On Sat, 27 Aug 2011, Sönke Hahn wrote:
Hi!
I was reading through the Typeclassopedia ([1]) and I was wondering which
type could be an instance of Pointed, but not of Applicative. But I can't
think of one. Any ideas?
(Identity :+: Store s) is a comonad that is also instance of Pointed, but
I do not believe it is an instance Applicative.
newtype SemiStore s a = (Identity :+: Store s) a
instance Pointed (SemiStore s) where
pure a = Inl (Identity a)
Coalgebras of the (Identity :+: Store s) comonad form the type of partial
lenses so this isn't just an academic functor.
--
Russell O'Connor <http://r6.ca/>
``All talk about `theft,''' the general counsel of the American Graphophone
Company wrote, ``is the merest claptrap, for there exists no property in
ideas musical, literary or artistic, except as defined by statute.''
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