Typed Racket: Subtype of a struct typed where one field has a fixed value

[je...@lisp.sh]

Working in Typed Racket, consider a struct like this:

(struct point

  ([x : Real]

   [y : Real]))

Is there a way to express the type of those points whose y field is, say, 0? In other words, I'd like a talk about point structs where x varies freely, but y is always 0. Obviously, I can make a function that discriminates this subclass, but I'm not sure how to express it as a type. I've hunted around in the docs, but I can't quite find what I'm looking for.

[Sorawee Porncharoenwase]

According to https://docs.racket-lang.org/ts-reference/type-ref.html, `Zero` exists. 

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[je...@lisp.sh]

I have it on good authority that parametric types are the way to go. Look at this:

````

#lang typed/racket

(struct (α β) point ([x : α] [y : β]))

(define-type Point point)

(define-type FixedY (Point Real 0))

````

That's it!