Servant.API.UVerb.Union

Type-level code for implementing and using UVerb. Heavily inspired by world-peace.

Documentation

type IsMember (a :: u) (as :: [u]) = (Unique as, CheckElemIsMember a as, UElem a as)

type family Unique xs :: Constraint where ...

Check whether all values in a type-level list are distinct. This will throw a nice error if there are any duplicate elements in the list.

Equations

Unique xs = If (Nubbed xs == 'True) (() :: Constraint) (TypeError (DuplicateElementError xs))

type Union = NS I

inject :: UElem x xs => f x -> NS f xs

eject :: UElem x xs => NS f xs -> Maybe (f x)

foldMapUnion :: forall (c :: Type -> Constraint) (a :: Type) (as :: [Type]). All c as => Proxy c -> (forall x. c x => x -> a) -> Union as -> a

Convenience function to apply a function to an unknown union element using a type class. All elements of the union must have instances in the type class, and the function is applied unconditionally.

See also: matchUnion.

matchUnion :: forall (a :: Type) (as :: [Type]). IsMember a as => Union as -> Maybe a

Convenience function to extract a union element using cast, ie. return the value if the selected type happens to be the actual type of the union in this value, or Nothing otherwise.

See also: foldMapUnion.