Haskell Language Role Representational Role
Example
An example of a parametric parameter of a type function:
data List a = Nil | Cons a (List a)
type family DoNotInspect x
type instance DoNotInspect x = List x
Here x is parametric because to determine the outcome of applying DoNotInspect to a type argument, the type function do not need to inspect x.
In this case, the role of x is representational. We can declare the role explicitly with the RoleAnnotations extension:
type role DoNotInspect representational
