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

Keep in mind that case also isn't really cheap. There's an infinite number of symbols so there is no cheap jump table. -- Matthias



On Dec 22, 2014, at 10:10 PM, J Arcane wrote:

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