A name for a product of `Const` and a functor?

Question

Is SF already defined anywhere, or does it at least have a name?

data SF a f x = SF a (f x)

instance Functor f => Functor (SF a f) where
  fmap g (SF a fx) = SF a (fmap g fx)

It looks like you are declaring a functor which contains another functor. — Code-Apprentice, Oct 02 '14 at 19:22
AFAIK there isn't one already out there, but that doesn't mean that it doesn't exist in the corner of some random package I've never used. — bheklilr, Oct 02 '14 at 19:28
@Code-Apprentice, I'm defining a functor transformer. `SF a f` is the same as functor `f`, but it carries an additional value of type `a` attached to it. — Emily, Oct 02 '14 at 19:37
@ArtyomKazak Since functors (and applicative functors, [unlike monads](http://stackoverflow.com/q/13034229/1333025)) compose, there is no need for something like (applicative) functor transformer. See `Data.Functor.*` modules in [transformers](http://hackage.haskell.org/package/transformers-0.4.1.0). — Petr, Oct 02 '14 at 20:24

score 4 · Accepted Answer · answered Oct 02 '14 at 19:59

4

Your functor looks like

type SF a f = (,) a :. f

using functor-combo notation.

(I somehow prefer to look at it using composition, rather than using product and Const.)

answered Oct 02 '14 at 19:59

chi

2

A more mainstream name for your `:.` is `Compose` from the `transformer`'s package. – Ørjan Johansen Oct 03 '14 at 12:32

score 1 · Answer 2 · answered Oct 03 '14 at 15:10

1

You could just define functor products

data (f :* g) a = P (f a) (g a) deriving Functor

and then write it directly

type SF a f = Const a :* f

answered Oct 03 '14 at 15:10

J. Abrahamson

2 Answers2