[Type Racket] Why doesn't this code type check?

[Weixi Ma]

Hi guys, thanks for your attention. Here is the code.

1 (: n (U False Number))

2 (define n #f)
3 (when n
4   (set! n (sub1 n)))

I suppose there should a refinement at line 3 that narrows down the type of n to be Number.

However typed racket complains that n is expected to be Number and actually given (U Complex False).

[Sam Tobin-Hochstadt]

Typed Racket doesn't refine the type of mutable variables. Doing that soundly would require an effect and escape analysis that seems like more complexity than it's worth. 

Sam

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