Variant is a sum type, i.e. a wrapper for a value which can be of different
types. For instance in the following code
x is a variant whose value can be
Int or a
import Haskus.Utils.Variant x :: V '[Int,Float]
The particularity is that we use a type-level list of types to control the possible value type. Compared to usual sum types (e.g.
Either Int Float) it allows us to manipulate (extend/filter/etc.) the list in a type-safe way and without requiring new data types.
Recursive sum types (e.g.
data List a = Nil | Cons a (List a)) based on
Variant are also supported and are called EADT (for “extensible ADT”).