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Sort of "do it once" feature request...

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Michele Dondi
September 21, 2005 07:27
Sort of "do it once" feature request...
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Every time I've desired a feature for Perl6 it has turned out that either 
it was already planned to be there or I have been given good resons why it 
would have been better not be there.

Now in Perl(5) {forum,newsgroup}s you can often see people doing stuff 

   my @files=grep !/^\.{1,2}/, readdir $dir;

Letting aside the fact that in the 99% of times they're plainly 
reinventing the wheel of glob() a.k.a. File::Glob, there are indeed 
situations in which one may have stuff like

for (@foo) {
   next if $_ eq 'boo';
   # do something useful here

whereas they know in advance that C<if> can succeed at most once (e.g. foo 
could really be C<keys %hash>).

Or another case is this:

while (<>) {
     if (@buffer < MAX) {
         push @buffer, $_;
     # ...
     shift @buffer;
     push @buffer, $_;

in which one wouldn't like the C<if> condition to be tested any more after 
it first evaluated false. Now efficiency is seldom a real issue in all of 
these situations, and in the rare cases in which it is one may adopt 
alternative strategies, like prefilling @buffer (with apparent referal to 
the previous example). In some cases one could need refactoring into subs 
to avoid duplication of code, but then the original logic may have been 
more clear intuitively to start with...

Whatever, I feel slightly uncomfortable psychologically with the idea of 
something that will be tested even when it's not necessary. So now I 
wonder if Perl6 is already expected to provide syntactical sugar enough to 
do what I want: which something easy enough to type to make a portion of 
code (behave like it) "silently evaporate"(ed) after a condition has been 
verified for the first time (or, say, n times). I think this is similar 
to the functionality of macros, but as I said, this should be -to be 
really useful- only a moderate bunch of keystrokes away from the code that 
doesn't employ it...

I just said it was my conjecture.  That doesn't mean I think it's right.  :-)
- Larry Wall in p6l, "Subject: Re: What do use and require evaluate to?"

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