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Thanks Kang-min. that makes things clear.
btw, I have uploaded my module to metacpan which is used to create my site openmbox.net. In the module I did need to read/write to a large file with high efficiency.
https://metacpan.org/pod/App::OpenMbox
if you have found any bugs in the module please let me know.
regards
Henry
November 21, 2022 at 8:18 AM, "Kang-min Liu" <gugod@gugod.org> wrote:
>
> support@openmbox.net writes:
>
> >
> > please see this ops:
> >
> > $ echo -n 0 > 1.txt
> >
> > 1.txt has only one line without eof.
> >
> > but the script below still got true for matching 0.
> >
> > $ cat test.pl
> > use strict;
> >
> > open HD,"1.txt" or die $!;
> > while(<HD>){
> > print "hi";
> > }
> >
> > which will print hi.
> >
> > can you help further?
> >
>
> I see -- so it's really about how readline operator (<FH>) works, or how
> it works within a while loop. I'm not sure if I could explain it better
> than quoting some documentation.
>
> Indeed the readline operator returns the line and may set $_ as side-effect,
> but when being tested in `while` -- and only in `while` -- perl
> compiler does something extra and put an `defined` operator in there.
>
> If you follow link in the documentation of readline [1] to "perlop: I/O
> Operatorns" [2], you'll find this statement saying:
>
> Thus the following lines are equivalent:
>
> while (defined($_ = <STDIN>)) { print; }
> while ($_ = <STDIN>) { print; }
> while (<STDIN>) { print; }
> for (;<STDIN>;) { print; }
> print while defined($_ = <STDIN>);
> print while ($_ = <STDIN>);
> print while <STDIN>;
>
> [1]: https://perldoc.perl.org/functions/readline
> [2]: https://perldoc.perl.org/perlop#I%2FO-Operators
>
> And the paragraph right before them contains some text describing the
> same thing.
>
> Such effect may be verified by compiling test.pl with -MO=Deparse:
>
> # perl -MO=Deparse test.pl
> use strict;
> die $! unless open HD, '1.txt';
> while (defined($_ = readline HD)) {
> print 'Hi';
> }
> test.pl syntax OK
>
> You could see that <HD> is compiled to be "readline", and an extra
> "defined" operator appears.
>
> (See `perldoc B::Deparse` for more about -MO=Deparse)
>
> You could furthermore play with it and see that no that extra "defined"
> operator would be added to `if (<FH>)`, `unless (<FH>)`, `until (<FH>)`,
> or when the while-condition is a bit more complicated (for
> example: `while(! <FH>)`). This is a special case only for `while (<FH>)`
>
> I guess this is made special to make it convenient for iterating through
> the entire file line by line. Otherwise when line values are false-y by
> chance, the loop ends early and mostly likely that'll be a surprise.
>
> --
> Cheers,
> Kang-min Liu
>
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