[e-lang] Is the SuchThatPattern always a bad idea?
darius at accesscom.com
Mon Aug 8 01:44:23 EDT 2005
Mark Miller <markm at cs.jhu.edu> wrote:
> If we dispense with getSynEnv(), then there's no reason to keep the guard
> qualifier coupled to a variable definition pattern. Instead, without
> increasing the size of the language, and while being upwards compatible from
> the present language, we could generalize the guard qualifier to appear to the
> right of any pattern.
> If '<p1>' is the pattern '<p2> :<e2>', then the meaning is, first evaluate
> <e2> to a guard, coerce the specimen by this guard, and then match the
> coercion result against <p2>.
Doesn't this break left-to-right evaluation if <p2> happens to have
embedded expressions? I guess you could handle this by statically
disallowing patterns where this would matter -- I think you've already
made that sort of rule for other Kernel-E expansions.
Or, ugh, you might evaluate-but-don't-yet-match-on <p2> before
evaluating <e2>. I think there's no precedent in E for that kind of
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