ConstraintsB can express constraints over c a

[jcpetruzza]

The ConstraintsB class was defining an associated type ConstraintsOf that one could use to get constraints of the form c (f a). We now use a more general mechanism and make ConstraintsB a type alias for backwards compatiblity.

The way things now work is as follows:

• ConstraintsB has an associated type AllB c b. Given a constraint-constructor c and a barbie b it will produce a constraint (c x1, c x2, ... c xn) for each type xi under an f in b.

• How do we plug an f as ConstraintsOf used to do? The trick is to define the following class, along with its universal instance:

  class c (f a) => ClassF c f a
  instance c (f a) => ClassF c f a

  And now ConstraintsOf c f b becomes essentially AllB (ClassF c f) b.

• Actually, for barbies defined with Wear, ConstraintsOf c f b was also adding constraints of the form (Wear f x1 ~ f x1,... Wear f xn ~ f xn). We add a second associated type to 'ConstraintsB, called 'NotBare b fthat builds this constraint onWear-barbies and()` otherwise.

Credit goes to Csongor Kiss for this very nice formulation.

[jcpetruzza]

@kcsongor want to give it a look?

[kcsongor]

Looks very good 👍