One of my students is trying to be clever, and combine multiple structure
matches in ‘match':
#lang typed/racket
(define-type ArithC (U Number plusC multC))
(struct plusC([l : ArithC] [r : ArithC]) #:transparent)
(struct multC([l : ArithC] [r : ArithC]) #:transparent)
(: num-nums (ArithC -> Number))
(define (num-nums a)
(match a
[(? number? n) 1]
[(or (plusC l r) (multC l r)) (+ (num-nums l) (num-nums r))]))
This doesn’t type check, and it appears that the (standard?) problem is that TR
can’t untangle the expansion of match well enough to determine that ‘l’ and ‘r’
can’t possibly be ‘false’:
Type Checker: type mismatch
expected: ArithC
given: (U ArithC False plusC multC) in: l
Type Checker: type mismatch
expected: ArithC
given: (U ArithC False plusC multC) in: r
Type Checker: Summary: 2 errors encountered in:
l
r
Is this a reasonable summary of this situation?
John
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