[Haskell-cafe] Pointed, but not Applicative

roconnor at theorem.ca roconnor at theorem.ca
Wed Aug 31 16:43:27 CEST 2011

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.''

More information about the Haskell-Cafe mailing list