On 3/24/20, mmcdan <mmcdani...@gmail.com> wrote:
> Hello,
> I'm trying to use occurrence typing for (Vectorof Symbol) or (Boxof
> Symbol).
> Creating a custom predicate for (Listof Symbol) seems to work:
> ---
> #lang typed/racket
> (: los? (-> Any Boolean : (Listof Symbol)))
> (define los?
>   (lambda (seq) (and (list? seq) (andmap symbol? seq))))
> ---
> However I haven't been able to create one for vectors or boxes. None of the
> things I've tried will typecheck. Any pointers?

Typed Racket can't be sure that untyped code won't change whats inside
a mutable vector or box

#lang racket
(define v (vector 'A))
(define (set-v!) (vector-set! v 0 0))
(provide v set-v!)

#lang typed/racket
(require/typed "path-to-untyped.rkt"
  (v : (Vectorof Any))
  (set-v! : (-> Void)))

;; suppose this worked
(: vos (-> Any Boolean : (Vectorof Symbol)))
(define (vos x) ....)

  [(vos? v)
    ;; v : (Vectorof Symbol)
    ;; v now contains an integer!

I don't know a work-around.

I was hoping it could work for (Immutable-Vectorof Symbol), but I
don't know what to use in place of `andmap`.

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