On Sat, 2009-09-05 at 23:28 -1000, Shiro Kawai wrote:
> Just a comment on concurrent evaluation of test exprs in cond:
...
> Doesn't that prohibit typical idioms in which the tests assume
> precedent tests are not satisfied? For example, the following
> code assumes when (odd? x) is evaluated x is an integer. If we
> allow concurrent evaluation, (odd? x) may signals an error.
>
> (cond
> [(not (integer? x)) ...]
> [(odd? x) ...])
That is very true. I don't know about anybody else, but I
rely on forms that promise sequential evaluations to prevent
evaluating expressions that will give an error.
This includes
* short-circuit evaluation of 'and'
* short-circuit evaluation of 'or'
* sequential evaluation of test expressions in 'cond'
* 'if' evaluating the test before the consequent or the alternate clause
* 'if' evaluating consequent or alternate but not both
If any of those things changes, a *lot* of code, probably
almost *all* code, will break. Now, it wouldn't be the first
time that a standards body has made a change that invalidated
all programs written before that standard *cough*R6RS*cough*,
but the code in this case would be a heck of a lot harder to
fix.
Bear
Bear
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