This works though:
#lang typed/racket
(require mzlib/etc)
(define-type NDigit (U 0 1 2 3 4 5 6 7 8 9))
(define-type SDigit (U 'one 'two 'three 'four 'five 'six 'seven 'eight 'nine))
(: f (-> NDigit SDigit))
(define (f i)
(evcase i
[(ann 0 0) (displayln i) 'two]
[(ann 1 1) (displayln i) 'two]
[(ann 2 2) (displayln i) 'two]
[(ann 3 3) (displayln i) 'two]
[else 'one]))
This is because the original version didn't annotate the numeric literals,
which typecheck as Positive-Byte (except for 0 and 1 which are special).
Using = here won't work because the typechecking rule that considers filters
for equality doesn't work for =. The comment in the code says
;; `=' is not included. Its type is more useful than this typing rule.
Could we get the benefits of both approaches somehow?
Cheers,
Asumu
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