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Subject Author Date
Constants across package boundaries bernie 05-13-2008
Posted by bernie on May 13, 2008, 8:36 am
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 I've been chasing a compile problem and I just saw the light on what
my trouble is: I use a bunch of "use constant this => ...; use
constant that =>...;" to set parameters for the program. BUT: the
program uses a 'package' or two [it has a small embedded objects in
it]. The trouble is that the package can't access the 'constant' --
that section of the program gets [not surprisingly] bareword errors.
I can fix them by using "main::..." but that's really ugly. Is there
any really clean way to set up truly "global" constants? Thanks!

/Bernie\

Posted by Joost Diepenmaat on May 13, 2008, 9:02 am
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 bernie@fantasyfarm.com writes:

> I've been chasing a compile problem and I just saw the light on what
> my trouble is: I use a bunch of "use constant this => ...; use
> constant that =>...;" to set parameters for the program. BUT: the
> program uses a 'package' or two [it has a small embedded objects in
> it]. The trouble is that the package can't access the 'constant' --
> that section of the program gets [not surprisingly] bareword errors.
> I can fix them by using "main::..." but that's really ugly. Is there
> any really clean way to set up truly "global" constants? Thanks!

I think you'd rather just export the constants to any package that
needs them, instead of making them truly global (which would be more
or less equivalent to exporting them to *all* packages)

See perldoc Exporter.



--
Joost Diepenmaat | blog: http://joost.zeekat.nl/ | work: http://zeekat.nl/

Posted by bernie on May 13, 2008, 9:50 am
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> ber...@fantasyfarm.com writes:
> > I've been chasing a compile problem =A0and I just saw the light on what
> > my trouble is: I use a bunch of "use constant this =3D> ...; use
> > constant that =3D>...;" to set parameters for the program. =A0BUT: the
> > program uses a 'package' or two [it has a small embedded objects in
> > it].

I note that this is documented on the 'constant' man page. Sigh. I
guess I could just use ::THIS and ::that but that feels a bit
inelegant...

> I think you'd rather just export the constants to any package that
> needs them, instead of making them truly global (which would be more
> or less equivalent to exporting them to *all* packages)

Can you export from main *to* a package? Using Exporter in this way
looks like it is a little confusing (it might become clearer after
another six readings of the man page..:o)). I guess I could do
something along the lines of putting require Exporter in my main
program, setting up @EXPORT_OK in the main program the "constants" I
want the interior packages to be able to import, and then do
a ::import(stuff I need) after the 'package' for the interior object-
section? You're right: that kind of approach would make the interior
packages be more self-contained and clearer (by making explicit that
they're pulling in those vbls from "main::")

OTOH, I see that there's a "Readonly" package that looks like it will
do what I was originally trying to do ["Readonly \my $CONST =3D> val"
generates a real cross-package constant]. [although now that I see a
way to do what I originally wanted, I'm not so sure it is as clean a
way to go as I'd like...:(]

Thanks...

/Bernie\

Posted by bernie on May 13, 2008, 10:01 am
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On May 13, 9:50=A0am, ber...@fantasyfarm.com wrote:

> > I think you'd rather just export the constants to any package that
> > needs them, instead of making them truly global (which would be more
> > or less equivalent to exporting them to *all* packages)

>... =A0I guess I could do
> something along the lines of putting require Exporter in my main
> program, setting up @EXPORT_OK in the main program the "constants" I
> want the interior packages to be able to import, and then do
> a ::import(stuff I need) after the 'package' for the interior object-
> section?

I'm running down this path to see how it looks and feels to properly
export and import my "global" constants. And I've run into a
problem. The man page for Exporter says:

package YourModule;
use Exporter 'import'; # gives you Exporter's import() method
directly
@EXPORT_OK =3D qw(munge frobnicate); # symbols to export on request

But in my code I have:
--------------------------
#!/usr/bin/perl

use strict ;
use warnings ;

use Exporter 'import' ;
our @ISA =3D qw(Exporter);
use constant AAA =3D> 4 ;
our @EXPORT_OK =3D ('AAA') ;
----------------------------

"import" is not exported by the Exporter module at test.pl line 6

I can't figure out quite what I'm doing wrong... And then I'm
guessing that down in the program in my package I'd do:
package MYPACKAGE;
::import(qw(constants I want))

/Bernie\

Posted by Ben Morrow on May 13, 2008, 10:26 am
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Quoth bernie@fantasyfarm.com:
> On May 13, 9:50 am, ber...@fantasyfarm.com wrote:
>
> > > I think you'd rather just export the constants to any package that
> > > needs them, instead of making them truly global (which would be more
> > > or less equivalent to exporting them to *all* packages)
>
> >...  I guess I could do
> > something along the lines of putting require Exporter in my main
> > program, setting up @EXPORT_OK in the main program the "constants" I
> > want the interior packages to be able to import, and then do
> > a ::import(stuff I need) after the 'package' for the interior object-
> > section?

It's probably a bad idea to export from main::. I'd define a separate
package for your constants, and 'use' it from everywhere else.

> I'm running down this path to see how it looks and feels to properly
> export and import my "global" constants. And I've run into a
> problem. The man page for Exporter says:
>
> package YourModule;
> use Exporter 'import'; # gives you Exporter's import() method
> directly
> @EXPORT_OK = qw(munge frobnicate); # symbols to export on request
>
> But in my code I have:
> --------------------------
> #!/usr/bin/perl
>
> use strict ;
> use warnings ;
>
> use Exporter 'import' ;
> our @ISA = qw(Exporter);
> use constant AAA => 4 ;
> our @EXPORT_OK = ('AAA') ;
> ----------------------------
>
> "import" is not exported by the Exporter module at test.pl line 6

Does the manpage you're using match your version of Exporter? The
ability to import (rather than inherit) Exporter::import was not present
in older versions of Exporter.

Ben

--
Although few may originate a policy, we are all able to judge it.
Pericles of Athens, c.430 B.C.
ben@morrow.me.uk

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