[Feature Request] support `ephemeron-hash`, remove `Weak-BoxTop`.

[NoahStoryM]

Hello, I noticed that racket has supported ephemeron-hash since version 8.0.0.10, so should we add corresponding types for them?

I sorted out some types and operators that may be involved:

• Types
  • Strong-HashTableTop
  • Strong-HashTable
  • Ephemeron-HashTable
  • Ephemeron-HashTableTop
• Operators

  • hash-strong?

  • hash-ephemeron?

  • make-ephemeron-hash

  • make-ephemeron-hasheq

  • make-ephemeron-hasheqv

  • in-ephemeron-hash

  • in-ephemeron-hash-pairs

  • in-ephemeron-hash-keys

  • in-ephemeron-hash-values

  • hash-copy-clear

  • hash-copy

  • unsafe-ephemeron-hash-iterate-first

  • unsafe-ephemeron-hash-iterate-next

  • unsafe-ephemeron-hash-iterate-pair

  • unsafe-ephemeron-hash-iterate-key

  • unsafe-ephemeron-hash-iterate-value

  • unsafe-ephemeron-hash-iterate-key+value

In addition, I noticed from https://github.com/racket/typed-racket/commit/64b062cb6f60368da80395f6974268f2c3ed50f4 that ephemeron is covariant, and weak-box doesn't seem to have a corresponding set! function. Does this mean that weak-box is also covariant? Can we remove Weak-BoxTop and support the corresponding subtype polymorphism?

[samth]

Weak boxes can be modified with set-box!

[NoahStoryM]

Hello, @samth. I only saw make-weak-box weak-box-value weak-box? in https://docs.racket-lang.org/reference/weakbox.html . I'm not so sure how to use set-box! to modify weak-box, do I miss something?:

In racket:

Welcome to Racket v8.3 [cs].
> (define b (make-weak-box 2))
> (weak-box-value b)
2
> (set-box! b 0)
; set-box!: contract violation
;   expected: (and/c box? (not/c immutable?))
;   given: #<weak-box>
; [,bt for context]

in typed/racket:

Welcome to Racket v8.3 [cs].
> (: b (Weak-Boxof Natural))
> (define b (make-weak-box 8))
> (weak-box-value b)
- : (U Exact-Nonnegative-Integer False)
8
> (set-box! b 0)
; readline-input:4:0: Type Checker: Polymorphic function `set-box!' could not
;   be applied to arguments:
; Argument 1:
;   Expected: (Boxof a)
;   Given:    (Weak-Boxof Nonnegative-Integer)
; Argument 2:
;   Expected: a
;   Given:    Zero
;   in: (set-box! b 0)
; [,bt for context]
> (:print-type set-box!)
(All (a) (-> (Boxof a) a Void))