[racket] Why does (case ...) quote its matching value expressions?

From: J Arcane (jarcane at gmail.com)
Date: Mon Dec 22 23:10:29 EST 2014

Thanks for the insight. I hadn't considered the performance implications
involved I suppose (the perpetual problem of the young Lisper ...)

And evcase does indeed work more like I expected, for example:

(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))))

Works exactly as I'd expect. I'll have a look at the code and see if any
further insights might be found there for Heresy's (select case ...)


On Mon, Dec 22, 2014 at 5:20 PM, Matthias Felleisen <matthias at ccs.neu.edu>

> Look for evcase. -- Matthias
> On Dec 21, 2014, at 11:58 PM, J Arcane wrote:
> > Up with a horrible ear-ache this morning I decided to include a FizzBuzz
> example in Heresy, the Racket #lang I've been working on, and ran into an
> unexpected behavior in the (case ...) statement.
> >
> > In many languages with case, you can make the testing value a constant,
> and then make the matching clauses actual calculations which then match
> against that constant. So when doing FizzBuzz in C-like languages you can
> do something like "switch 0" and then "case x % 3" for the matching clauses.
> >
> > It turns out this doesn't work in Racket, because Racket quotes the
> values in the matching clauses so they do not evaluate. Specifically, it
> narrows down to doing this in (case/sequential-test ...): #`(equal? v 'k)
> >
> > I can implement an alternate version that works as I expect (and will
> probably include it in Heresy) just by removing that quote in my version,
> but I was curious as to the reasoning behind this behavior and if perhaps
> there's some explanation for it that I may've missed.
> >
> > Any insights appreciated,
> > John Berry
> >
> > ____________________
> >  Racket Users list:
> >  http://lists.racket-lang.org/users
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