Extensible ADT (EADT)¶
EADTs (extensible algebraic data types) are ADTs whose constructors are defined
independently of the EADT. In the following example, we define two constructors
NilF and then we combine them to form a
import Haskus.Utils.EADT data ConsF a l = ConsF a l deriving (Functor) data NilF l = NilF deriving (Functor) type List a = EADT '[ConsF a, NilF]
We use a type-level list to indicate the valid constructors for the EADT.
Compared to usual ADT (e.g.
data List a = Cons a (List a) | Nil) it allows
us to manipulate (extend/filter/etc.) the list of constructors in a type-safe
way and without requiring new data types. Moreover, the constructors can be
shared between several EADTs. For instance, the following code reuses
constructor to define a binary tree EADT:
data NodeF a l = NodeF a l l deriving (Functor) type Tree a = EADT '[NodeF a, NilF]
EADTs are based on the Variant sum type to which they add support for recursive ADTs as in the examples above.