Nominal type statements

[matthew-healy]

It should be trivial to create a nominal type from another type.

E.g.

type Year wraps Num

In this case this would be fine:

def before |y: Year| y < 2023

let y: Year = 1991 in
before y -- true

This would not be fine:

def before |y: Year| y < 2023

let y = 1991 in
before y -- type mismatch, ideally with a hint about requiring an explicit annotation

And equally:

def add |x: Num, y: Num| y < 2023

let y: Year = 1991 in
add 4 y -- type mismatch, with a hint about requiring an explicit annotation

(Note: the type X wraps Y syntax is bad and should be improved, but type X = Y looks like an alias rather than a newtype).

(Also note: overloading ascryption for this might be a bad idea.)