How to avoid the warning "Redundant constraint: Functor f" in the getter?

I have some record SomeRecord and fields like _user, _password. And I want to write a Getter for a "virtual" field, like identity which will look like <user>:<password>:

identity:: Getter SomeRecord String
identity = to $ \srec -> srec^.user <> ":" <> srec^.password

and this getter leads to the warning:

• Redundant constraint: Functor f
• In the type signature for:
       identity :: Getter SomeRecord Stringtypecheck(-Wredundant-constraints)

what's wrong with this getter and how to write it without warnings?

EDIT:

Just found this thread in the Web:

Comment (by ekmett):

In the interest of not getting drowned in complaints by the users of lens on GHC 8, we'll likely just add the constraint to to and rely on {-# INLINE #-} dropping it out in practice. This is a common enough scenario to warrant us eating the noise on our side.

Of course, you'll get the same sort of thing if you go to declare a non- law abiding traversal as a fold, etc. There i'm afraid the only answer will be to squelch the warning. GHC can't see 'laws'.

Solution

This is due to the way that Edward Kmett's lens library works.

Getter is a type synonym:

type Getter s a = forall f. (Contravariant f, Functor f) => (a -> f a) -> s -> f s

Meanwhile the type of to is:

to :: (Profunctor p, Contravariant f) => (s -> a) -> Optic' p f s a

Never mind the Optic', the problem is that Getter has the constraint Functor f but the to doesn't need it. Hence the warning, because otherwise you could use it with a type f that isn't a functor.

The "proper" type for your function is something like:

identity :: (Profunctor p, Contravariant f) => Optic' p f SomeRecord String