Savelenko

namespace PhantomTypes

module User =

    type Username<'a> = private Username of string with
        member u.Value = let (Username username) = u in username

    type Short = interface end
    type Full = interface end

Savelenko

module GADTMotivation

(*
Here is a simple motivational example for GADTs and their usefulness for library design and domain modeling. Suppose we
need to work with settings which can be displayed and adjusted in a GUI. The set of possible setting "types" is fixed
and known in advance: integers, strings and booleans (check-boxes).
The GUI should show an example value for each possible setting type, e.g. 1337 for an integer setting and "Hello" for a
string setting. How can we model this small domain of setting types and computing example values?
*)