I'm trying to make a function that can discriminate typed boxes: I
tried the following in Typed Racket:
(: ensure-number-box (Any -> (Boxof Number)))
(define (ensure-number-box x)
(if (and (box? x) (number? (unbox x)))
x
(error 'ensure-number-box "~s" x)))
Unfortunately, I can't convince the type checker that x is a (Boxof
number). I'm not quite sure how to use the occurrence typing properly
to get this to type correctly.
I want to return the original box. That is, it's not correct for me to do this:
(: ensure-number-box (Any -> (Boxof Number)))
(define (ensure-number-box x)
(if (box? x)
(let ([y (unbox x)])
(if (number? y)
(box y)
(error 'ensure-number-box "~s" x)))
(error 'ensure-number-box "~s" x)))
since this generates a fresh box.
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