Capability.Constraints

Type family used used to express a conjunction of constraints over a single type.

Examples:

All '[Num, Eq] Int
  -- Equivalent to: (Num Int, Eq Int)

All '[HasReader "foo" Int, HasSink "bar" Float] m
  -- Equivalent to: (HasReader "foo" Int m, HasSink "bar" Float m)

A Capability takes a type constructor Type -> Type (e.g., a monad) and returns a Constraint. Examples of capabilities includ: HasReader "foo" Int, MonadIO, …

The kind of lifted constraints

Values of type Dict p capture a dictionary for a constraint of type p.

e.g.

Dict :: Dict (Eq Int)

captures a dictionary that proves we have an:

instance Eq Int

Pattern matching on the Dict constructor will bring this instance into scope.