(select case ...) does not evaluate test clauses

[jarcane]

This is a limitation, apparently, of the Racket version. Racket's (case ...) quotes its matching clauses, so they are not actually evaluated, just compared to the value expression with equal?

So this works:

(select case (mod 5 2)
  ((1) 'foo)
  ((0) 'bar))

; returns 'foo

But this doesn't:

(select case 1
  ((mod 5 2) 'foo)
  ((mod 2 2) 'bar))

; expect 'foo, but falls through instead.void

I think the case can be made for writing our own (select case ...) to allow for this usage. It's a common enough behavior in other language's case statements, so if it's possible to do with Racket's macros I think it's worth implementing.

[jarcane]

Racket apparently does provide an alternate case in the old mzscheme libraries that works more like our previous second example. With evcase you can do this for example:

(require mzlib/etc)

(for ([n (in-range 1 101)])
  (evcase 0
          ((+ (modulo n 5)
              (modulo n 3)) (displayln "FizzBuzz"))
          ((modulo n 5) (displayln "Buzz"))
          ((modulo n 3) (displayln "Fizz"))
          (else (displayln n))))

I've mocked up a simple case macro that supports the best of both features, but the discussion in the mailing list seems to indicate there may be performance costs to allowing both a list of matching values and matching expressions in the same block. I don't consider performance as high on the list of priorities as ease of use and syntax power though, so this may not necessarily be worth worrying about yet.