Re: Why are case implementations so slow?

[R. Kent Dybvig <dyb@iuvax.cs.indiana.edu>]

One reason is that the "natural" expansion of

    (case e0 ((k1 k2 ...) e1 e2 ...) ...)

is

    (let ([tag e0]) (cond ((memv? tag '(k1 k2 ...)) e1 e2 ...) ...))

where "tag" does not occur free in e1 e2 ....

This is surely slower than using the "equivalent" eq? tests when there
is only one key per clause and that key is a symbol.  The Chez Scheme
expander looks for these special cases and expands them "appropriately",
so you should find case as fast as cond.  You could write your own case
macro that does the same thing.

However, it is not practical to turn case into a computed goto unless
there are several keys within a relatively small range of fixnums or
characters.  It could be beneficial, certainly, but Chez Scheme doesn't
bother, and I suspect that most other implementations don't either.

R. Kent Dybvig                 | arpa: dyb@iuvax.cs.indiana.edu
Computer Science Department    | csnet: dyb@indiana
Indiana University             | usenet: ...!ihnp4!iuvax!dyb
Bloomington, IN 47405          | phone: 812/335-6486