Dict c a is evidence that there exists an instance of c a.

It is essentially equivalent to Dict (c a) from the constraints package, but because of its kind, it allows us to define things like Dict Show.

Turn a constrained-function into an unconstrained one that uses the packed instance dictionary instead.

Instances of this class provide means to talk about constraints, both at compile-time, using AllB, and at run-time, in the form of Dict, via baddDicts.

A manual definition would look like this:

data T f = A (f Int) (f String) | B (f Bool) (f Int)

instance ConstraintsB T where
  type AllB c T = (c Int, c String, c Bool)

  baddDicts t = case t of
    A x y -> A (Pair Dict x) (Pair Dict y)
    B z w -> B (Pair Dict z) (Pair Dict w)

Now, when we given a T f, if we need to use the Show instance of their fields, we can use:

baddDicts :: AllB Show b => b f -> b (Dict Show `Product' f)

There is a default implementation of ConstraintsB for Generic types, so in practice one will simply do:

derive instance Generic (T f)
instance ConstraintsB T

Like bmap but a constraint is allowed to be required on each element of b

E.g. If all fields of b are Showable then you could store each shown value in it's slot using Const:

showFields :: (AllB Show b, ConstraintsB b) => b Identity -> b (Const String)
showFields = bmapC @Show showField
  where
    showField :: forall a. Show a => Identity a -> Const String a
    showField (Identity a) = Const (show a)

Notice that one can use the (&) class as a way to require several constraiints to hold simultaneously:

bmap @(Show & Eq & Enum) r

Like btraverse but with a constraint on the elements of b.

Similar to AllB but will put the functor argument f between the constraint c and the type a. For example:

  AllB  Show   Person ~ (Show    String,  Show    Int)
  AllBF Show f Person ~ (Show (f String), Show (f Int))
  

ClassF has one universal instance that makes ClassF c f a equivalent to c (f a). However, we have

'ClassF c f :: k -> Constraint

This is useful since it allows to define constraint-constructors like ClassF Monoid Maybe

Like ClassF but for binary relations.