As Jon said. For that kind of use case, I recommend `match`. It's better than `case` in almost every (all?) respect.
Vincent At Mon, 22 Dec 2014 00:52:48 -0500, Jon Zeppieri wrote: > > `case` is like C's `switch` (but unlike similar constructs in other > C-like languages) in that the case labels must be known at compile > time. This allows the compiler to generate especially efficient code. > If the label values are densely distributed, a decent C compiler will > usually generate a jump table. Racket's `case` can't do that, since > it's not implemented at a low-enough level, but it can generate, in > the best case, a vector lookup followed by an open-coded binary > search. In the worst case, it will use a hash table lookup followed by > an open-coded binary search. (If the number of constant labels is > below a set threshold, then `case` will just test them in order, > exactly as if you'd written a `cond` expression. That's what > `case/sequential-test` does.) > > C's switch does, however, allow the use of constant *expressions* as > case labels, so you can have something like `case FOO % 3`, where `FOO > % 3` can be computed at compile-time. (In your example of `case x % > 3`, unless the value of `x` is known at compile time, it would be > illegal in C, though legal certain languages that use a similar > syntax.) Racket's `case`, on the other hand, only allows plain-old > values. > > This is limiting sometimes. For example, to get the very best > performance out of `case`, you need to use either fixnums or chars, > but if you're creating, say, a state machine, you probably want to use > symbolic names for your states. In C, you'd define an integer constant > and use the symbolic name as your case label. You can't do that with > Racket's `case`, however; if you want to use symbolic names, then > you'll actually be representing your states with symbols rather than > fixnums. > > At any rate, if you actually need to do runtime computation in your > state labels, then neither C's `switch` nor Racket's `case` are > appropriate. > > > > On Sun, Dec 21, 2014 at 11:58 PM, J Arcane <jarc...@gmail.com> 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 > > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
