Why does Capture::Tiny require loading via BEGIN to successfully function?

Wyrdweaver has asked for the wisdom of the Perl Monks concerning the following question:

Monks,

I ran across a strange bug in the code of one of my modules, and I'm puzzled.

I use the $haveModule = eval { require Module; 1; } idiom fairly frequently, but it broke down for the Capture::Tiny module. Basically, it boils down to Capture::Tiny seemingly requiring module load via a BEGIN block in order to function without error. But I don't understand why, and it chafes me.

Here's code using Capture::Tiny that works:

use strict; use warnings;
BEGIN { require Capture::Tiny; }
my $out = Capture::Tiny::capture { print "STDOUT output to capture"; }
+;
print "Captured STDOUT: [" . ( defined $out ? $out : 'undef' ) ."]\n";
[download]

Output:

Captured STDOUT: [STDOUT output to capture]
[download]

By just removing the BEGIN block around require Capture::Tiny, the code now fails:

use strict; use warnings;
require Capture::Tiny;
my $out = Capture::Tiny::capture { print "STDOUT output to capture"; }
+;
print "Captured STDOUT: [" . ( defined $out ? $out : 'undef' ) ."]\n";
[download]

Output:

Can't call method "Capture::Tiny::capture" without a package or object
+ reference at C:\@local\@projects\@tests\test-Capture-Tiny.pl line 5.
[download]

The source for Capture::Tiny (@ http://cpan.uwinnipeg.ca/htdocs/Capture-Tiny/Capture/Tiny.pm.html) seems reasonably straight-forward, but I must be missing something.

Can someone offer an explanation for this behavior?

Thanks, in advance, for the illumination.

Comment on Why does Capture::Tiny require loading via BEGIN to successfully function? Select or Download Code

Replies are listed 'Best First'.
Re: Why does Capture::Tiny require loading via BEGIN to successfully function? by ikegami (Patriarch) on Nov 10, 2010 at 00:52 UTC
`Capture::Tiny::capture { print "STDOUT output to capture"; };` [download] isn't a normal sub call. Something must define this syntax, and that something must define the syntax before the statement is compiled. (That something is `capture`'s prototype.)	[reply] [d/l] [select]
Re: Why does Capture::Tiny require loading via BEGIN to successfully function? by BrowserUk (Patriarch) on Nov 10, 2010 at 00:56 UTC
I use the $haveModule = eval { require Module; 1; } idiom fairly frequently, I think anonymonk has explained the problem, but I just wanted to ask you what advantages you see in detecting module absence at runtime via require, over compile-time as would happen with use? Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. RIP an inspiration; A true Folk's Guy	[reply]
Re: Why does Capture::Tiny require loading via BEGIN to successfully function? by Anonymous Monk on Nov 10, 2010 at 00:44 UTC
I disagree that this module's code is straightforward. I couldn't actually find the `capture` sub in that module at a glance -- the author redefines `_debug` at run time, for instance, which hints at the possibility for obfuscation-like complexity). However, what appears to be happening is that it's doing something to change parsing rules at compile time. This can be done with a function prototype, like `sub foo (&) {}`, which is how you create functions that act like builtins such as map and grep. What this means is* that when you require it at run time, that compile-time code isn't executed, so the magic of `capture BLOCK` isn't applied. Instead, you need to use `capture( CODEREF )`. Look at this: `# perl -lwE "use Capture::Tiny (); say 'captured: ', Capture::Tiny::ca +pture { say 'foo' }" captured: foo` [download] `# perl -lwE "require Capture::Tiny; say 'captured: ', Capture::Tiny::c +apture { say 'foo' }" foo Can't call method "Capture::Tiny::capture" without a package or object + reference at -e line 1.` [download] `# perl -lwE "require Capture::Tiny; say 'captured: ', Capture::Tiny::c +apture(sub { say 'foo' })" captured: foo` [download] Upshot: without the compile-time-added syntax sugar, you need to call this sub like `capture($coderef);`.	[reply] [d/l] [select]