note
moritz
<blockquote>But, having dug in to Perl Guts more than I should have, I think that disallowing tied (and probably other magic) variables on the RHS, a built-in smart match would be able to reliably determine if the RHS is a number.</blockquote>
<p>The problem is that Perl doesn't expose the concept "this scalar is a number" to the user (by design). Thus making a decision based on whether a scalar is a number is nearly always wrong.</p>
<p>A piece of code that already makes such a decision is the code that decides whether to warn on a numeric operation:</p>
<code>
$ perl -wE 'say 0 + "1.2"'
1.2
$ perl -wE 'say 0 + "1.2.3"'
Argument "1.2.3" isn't numeric in addition (+) at -e line 1.
1.2
</code>
<p>So, what do I keep complaining about? A real problem are dual vars. Those
aren't just a rare corner case that should be avoided, but for example the
result from boolean operators:</p>
<code>
$ perl -wE 'say 0 + !1' # no warning
0
$ perl -wE 'my $false = !1; say "<<$false>>"' # empty string!
<<>>
</code>
<p>So it's a number, but it's also an empty string. Should smart-matching
against that be numeric or string comparison? My intuitive reaction is "string
comparison", because $false doesn't round-trip when converted to a number and
then to a string. </p>
<p>But you can also construct valid cases where round-tripping to a number is
the wrong criterion; an example is if a user-supplied string is never used as
a number, but happens to look like a number. You certainly don't want those
values to try to coerce your own strings to numbers (and warn).</p>
<p>So, however you decide whether a scalar is number or a string for the
purpose of comparision, I can point out a case where your decision is a big
WTF. Which is precisely the reason that we have separate <c>==</c> and
<c>eq</c> operators.</p>
<p>Whatever will be done about that, the string/number duality will remain a
weakness of any Perl 5 smartmatch proposal.</p>
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