Re^2: Grepping with Perl: How to stop after first match?

"first" ... does exactly what you want

"Exactly" is a strong word. If he's trying to avoid reading through the whole file he won't achive that by using first. To some, first may seem to lazily evaluate its ~~list~~ arguments, but I'd like to emphasize that this is not the case. first will cause the whole file to be slurped into memory.

lodin

Update: changed "list" to "argument" to be more technically (?) correct.

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Re^3: Grepping with Perl: How to stop after first match? by dragonchild (Archbishop) on Dec 06, 2007 at 01:49 UTC
Almost there. `first` itself is (almost) perfectly respectful of a lazy `@_`. In List::Util, the pure-perl version of first() is: `sub first (&@) { my $code = shift; foreach (@_) { return $_ if &{$code}(); } undef; }` [download] The issue is (almost) completely that @_ is not lazy in the slightest. This is one of the biggest changes no-one will notice in Perl6 - the addition of the concept of truly lazy lists. You could code up something lazier with Tie::Array::Lazy. Maybe something like: `open my $fh, '<', $filename or die "Cannot open '$filename' for readin +g: $!\n"; tie my @arr, 'Tie::Array::Lazy', [], sub { scalar <$fh> }; first { <whatever> } @arr;` [download] (Note that this is completely untested - I've never used Tie::Array::Lazy in my life.) My criteria for good software: Does it work? Can someone else come in, make a change, and be reasonably certain no bugs were introduced?	[reply] [d/l] [select]

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Re^3: Grepping with Perl: How to stop after first match?
by dragonchild (Archbishop) on Dec 06, 2007 at 01:49 UTC

first

@_

sub first (&@) {
  my $code = shift;

  foreach (@_) {
    return $_ if &{$code}();
  }

  undef;
}
[download]

Tie::Array::Lazy

open my $fh, '<', $filename or die "Cannot open '$filename' for readin
+g: $!\n";
tie my @arr, 'Tie::Array::Lazy', [], sub { scalar <$fh> };

first { <whatever> } @arr;
[download]

My criteria for good software:

Does it work?
Can someone else come in, make a change, and be reasonably certain no bugs were introduced?

[reply]
[d/l]
[select]