Re: INIT blocks and runtime code loading

[broquaint]

Assuming a relatively recent version of perl (5.7.1+ IIRC) you can execute INIT at your discretion. However the trick is in avoiding executing them twice. Here's a very bare bones example of executing INIT blocks from both use and require

package testpkg;

BEGIN { warn "in BEGIN\n" }

{
  no warnings;

  INIT {
    return if $INIT_DONE++;
    warn "in INIT\n";
  }
}

{
  use B;

  $_->object_2svref->() for grep {
    $_->GV->STASH->NAME eq __PACKAGE__
  } B::init_av->ARRAY;
}

1;

And some usage code

$ perl -we 'use testpkg;'
in BEGIN
in INIT
$ perl -we 'require testpkg;'
in BEGIN
in INIT

Unfortunately it uses a rather horrific bodge of a package variable and post-increment behaviour, but nonetheless, it works :) Hopefully there's some very straight-forward solution for determining what state perl is in when a piece of code is executing, but currently I'm out of ideas.

[jdhedden]

One wrinkle on broquaint's kludge is its interaction with an import subroutine.
'use'ing a module normally goes through:

BEGIN
import
INIT

However, adding an import subroutine to the above code produces:

BEGIN
INIT
import

One possible workaround for this would be to move any INIT code to the end of the import subroutine. Yet even this is not a 100% solution as:

use testpkg ();

causes the import subroutine - and hence the moved INIT code - not be be executed.

For the module I'm writing, I have a BEGIN block, an import subroutine, and an INIT block. The INIT block consists of just one function call. Therefore, I warn the user that if they want delayed evaluation of my module, they must do so as follow:

eval {
    require MyModule;
    import MyModule qw/.../;
    MyModule::MyInit();
}