How is the result "0" from readdir () not false in the while state?

Question

See also: Where the documentation says that during readdir testing for definition? . (Not a duplicate, just closely related.)

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Many people view the cycle below as idiomatic:

while (defined(my $file = readdir($dir)) { ... } 

instead:

 while (my $file = readdir($dir)) { ... } 

because, presumably, with the latest version, if the file name is “0” (zero), it should break the loop, whereas it returns “undef” when there are no more files.

However, at some point in the past, this test for defined() ceased to be necessary - it seems that there is a special case code that allows the latest version to work independently.

I would like to know how it works?

Curiously, instead of calling readdir() instead of calling foo() instead of foo() replace

 sub foo { my ($dir) = @_; return readdir($dir); } while (my $file = foo($dir)) { ... } 

then the code will really do what I expect and end the loop when a file with the name "0" is found.

(tested with Perl 5.8.9 on MacOS X 10.5.6)

Answer 1

This is magic. In particular, while magic (documented in perlsyn , perlop , and possibly in other places that I don’t remember). Perl allows you to make some abbreviations. If you want to see what Perl is doing behind you, you can use B::Deparse . Here is a file that uses a shorter loop:

 #!/usr/bin/perl use strict; use warnings; opendir my $dir, "/tmp" or die "$!"; while (my $file = readdir($dir)) { print "$file\n"; } 

If you run perl -MO=Deparse filename.pl , you get the Perl code that sees:

 use warnings; use strict 'refs'; die "$!" unless opendir my $dir, '/tmp'; while (defined(my $file = readdir $dir)) { do { print "$file\n" }; } filename.pl syntax OK