Beefy Boxes and Bandwidth Generously Provided by pair Networks
Clear questions and runnable code
get the best and fastest answer

if only blocks were avoidable...

by yosefm (Friar)
on Sep 21, 2003 at 19:05 UTC ( #293020=perlquestion: print w/ replies, xml ) Need Help??
yosefm has asked for the wisdom of the Perl Monks concerning the following question:

Or are they? I'm trying to write a module that uses only if it was requested in the import list, like - use module qw(LegacyEncoding and other options);

The problem is that if I use if, bytes will only be scoped to the if block. And anyway if I manage to avoid that (like with $legacy and use bytes) it will still be scped to the import sub.

Any way around it?

Comment on if only blocks were avoidable...
Select or Download Code
Re: if only blocks were avoidable...
by jeffa (Chancellor) on Sep 21, 2003 at 20:22 UTC
    Not quite sure if i am following you, but have you tried require instead? The "problem" with use is that no matter where it is, the module will be loaded. Consider this:
    if (0) { use CGI qw(:standard); } print header,"\n";
    Which does load and no compile error happens because there really is a header() method available. Try using require instead.


    (the triplet paradiddle with high-hat)
Re: if only blocks were avoidable...
by liz (Monsignor) on Sep 21, 2003 at 20:58 UTC
    The use bytes is basically setting $^H at compile time.

    According to that documentation, you should be able to do:

    BEGIN { require bytes; bytes->import if $condition }
    inside the block where you want the use bytes to be conditionally set. Of course, if you don't put it in a block, the file in which the source sits, will be your scope.

    This seems the right thing according to the docs and seems to work with stricture:

    $foo = $a; BEGIN { require strict; strict->import if 1 } $foo = $a; __END__ Variable "$foo" is not imported at x line 3. Global symbol "$foo" requires explicit package name at x line 3. x had compilation errors.
    If the import is not done, this runs without any errors:
    $foo = $a; BEGIN { require strict; strict->import if 0 } $foo = $a; __END__

    Hope this made sense and helps.


      So if I understand what you (and perlvar) say correctly, the BEGIN block is what's going to make my import globally scoped? If that's the case then I guess that's what I was looking for.
        No. Wrapping an import call in a BEGIN block will make it happen at compile time.
      OK, it turns out that this isn't what I need, because if I use BEGIN, I can't use parameters passed to sub in order to decide on the condition. I can only use environment variables, perl variables and other stuff that already exists before anything happens.

      What I need is this:

      sub import { $_[1] eq 'StoneAge' and use bytes globally; #no such thing. }

      BEGIN can't achieve that.

        Ok, I guess I missed that particular point. Well, I think it is possible, but it's rather yucky and it suffers from the same problems that AutoLoader and the likes have. Basically, you compile your code in a two step process. First to compile the "import" routine. Then, when the import routine is called, the rest of the source is compiled Consider module Foo:
        package Foo; require bytes; # make sure the module is loaded without (un)import $bytes = 0; # flag: whether bytes should be enforced $evalled = 0; # flag: whether rest of source has been compiled sub import { unless ($evalled++) { # we haven't evalled the rest +before $bytes = ($_[1] eq "usebytes"); # should bytes be enforced? local $/; # enable slurp mode eval <DATA>; # compile rest of source (afte +r __DATA__) die $@ if $@; # die now if an error occurred } }; 1; # in order for require of to be + successful __DATA__ # actual source of module starts here BEGIN {bytes->import if $bytes} # activate bytes if flag set sub new { bless {},shift } # example routine
        Then, in an example program, you would do:
        use Foo 'usebytes'; # or something else to not use bytes my $foo = Foo->new;
        Hope this will help you in your quest.


Log In?

What's my password?
Create A New User
Node Status?
node history
Node Type: perlquestion [id://293020]
Approved by herveus
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others surveying the Monastery: (7)
As of 2014-12-27 15:51 GMT
Find Nodes?
    Voting Booth?

    Is guessing a good strategy for surviving in the IT business?

    Results (177 votes), past polls