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 GHC.TypeLits.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 = …