Namespace notation might not be optimal, because it's not clear at a glance that this is an aspect and not a regular qualified import of a type (an important distinction if you have multiple versions of the same type imported, e.g. Bytestring). Why not make it look like type application instead? Modifying your example:
allEven :: (Integral a) => [a] -> Bool@A_All_
And for that matter, I'd rather not be forced to write a signature (what if I want to use this in a where clause?) So... does this break anything?
allEven = foldMap@A_All_ even
Which means, for this invocation of foldMap, instances within A_All_ are in effect for all of its constraints. You could chain multiple @'s together so long as they don't produce any relevant overlapping instances.
To avoid surprises, they shouldn't propagate -- when foldMap invokes even, it sees the default Monoid instance for Bool (i.e. none). If it were also a function that needs a Monoid, you'd need another @. There's potential for boilerplate here. Falling back to doing it in the type signature could force propagation? I'm not sure it's ever a safe idea, though.

On Sat, May 6, 2017 at 1:55 PM, MarLinn <monkleyon@gmail.com> wrote:
On 2017-05-06 21:37, Dmitry Olshansky wrote:
How does compiler can infer a type for "allEven = foldMap even" ?

Something like
allEven :: (Monoid (aspect Bool), Integral a) => [a] -> Bool ?

 I hadn't thought about inference actually. But even if the compiler inferred a type, it would still fail with a missing constraint. So I don't strongly care what the implied constraint is, as long as the error message is comprehensible. Maybe it could mention aspects in the future, but that's not even necessary. So basically the inferred type would just be

        allEven :: (Monoid Bool, Integral a) => [a] -> Bool

The compiler would just know "I need a Monoid instance for Bool!" as it does right now. And just as now, it knows where to look – the only difference is that with my proposal that place to look has a different name.

Should all Bool's in function body have the same aspects?

 Yes, if the aspect comes from a type signature. If the type signature is local, it would be local to its region of application. For example:

---
module Test where

    import qualified Data.Bool under (Default, Data.Aspect.Bool.All) as A_All_ (Bool)
    import qualified Data.Bool under (Default, Data.Aspect.Bool.Any) as A_Any_ (Bool)

     test2 :: (Integral a) => [a] -> Bool
    test2 xs = (foldMap even xs :: A_All_.Bool) == not (foldMap odd xs :: A_Any_.Bool)

The type is imported twice with different names and under different aspects. The local type signatures use these names to define which are the right instances.

An alternative would be something like

    test2 :: (Integral a) => [a] -> Bool
    test2 xs = (foldMap even xs :: (Bool under A_All_)) == not (foldMap odd xs :: (Bool under A_Any_))

which would make the aspect-nature clearer. But I didn't want to introduce even more syntax, especially as the naming scheme is already enough.

Was that clarifying?

I have to say, your questions did make me ponder how much of my proposal would already be possible by exploiting type families. I'm not sure yet, but I'll think about it. So thanks!


Cheers,
MarLinn

_______________________________________________
Haskell-Cafe mailing list
To (un)subscribe, modify options or view archives go to:
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
Only members subscribed via the mailman list are allowed to post.