capability-0.4.0.0: Extensional capabilities and deriving combinators

Capability.TypeOf

Synopsis

# Documentation

type family TypeOf k (s :: k) :: Type Source #

Type family associating a tag to the corresponding type. It is intended to simplify constraint declarations, by removing the need to redundantly specify the type associated to a tag.

It is poly-kinded, which allows users to define their own kind of tags. Standard haskell types can also be used as tags by specifying the Type kind when defining the type family instance.

Defining TypeOf instances for Symbols (typelevel string literals) is discouraged. Since symbols all belong to the same global namespace, such instances could conflict with others defined in external libraries. More generally, as for typeclasses, TypeOf instances should always be defined in the same module as the tag type to prevent issues due to orphan instances.

Example:

    import Capability.Reader

data Foo
data Bar
type instance TypeOf Type Foo = Int
type instance TypeOf Type Bar = String

-- Same as: foo :: HasReader Foo Int M => …
foo :: HasReader' Foo m => …
foo = …