Package perl-Import-Into: Information

Writing exporters is a pain. Some use the Exporter manpage, some use the Sub::Exporter manpage,
some use the Moose::Exporter manpage, some use the Exporter::Declare manpage ... and some things
are pragmas.

If you want to re-export other things, you have to know which is which.
the Exporter manpage subclasses provide export_to_level, but if they overrode their
import method all bets are off. the Sub::Exporter manpage provides an into parameter
but figuring out something used it isn't trivial. Pragmas need to have
their `import' method called directly since they affect the current unit of
compilation.

It's ... annoying.

However, there is an approach that actually works for all of these types.

  eval "package $target; use $thing;"

will work for anything checking caller, which is everything except pragmas.
But it doesn't work for pragmas - pragmas need:

  $thing->import;

because they're designed to affect the code currently being compiled - so
within an eval, that's the scope of the eval itself, not the module that
just `use'd you - so

  sub import {
    eval "use strict;"
  }

doesn't do what you wanted, but

  sub import {
    strict->import;
  }

will apply the strict manpage to the calling file correctly.

Of course, now you have two new problems - first, that you still need to
know if something's a pragma, and second that you can't use either of
these approaches alone on something like the Moose manpage or the Moo manpage that's both
an exporter and a pragma.

So, the complete solution is:

  my $sub = eval "package $target; sub { shift->import(\@_) }";
  $sub->($thing, @import_args);

which means that import is called from the right place for pragmas to take
effect, and from the right package for caller checking to work - and so
behaves correctly for all types of exporter, for pragmas, and for hybrids.

Remembering all this, however, is excessively irritating. So I wrote a module
so I didn't have to anymore. Loading the Import::Into manpage creates a global method
`import::into' which you can call on any package to import it into another
package. So now you can simply write:

  use Import::Into;

  $thing->import::into($target, @import_args);

This works because of how perl resolves method calls - a call to a simple
method name is resolved against the package of the class or object, so

  $thing->method_name(@args);

is roughly equivalent to:

  my $code_ref = $thing->can('method_name');
  $code_ref->($thing, @args);

while if a `::' is found, the lookup is made relative to the package name
(i.e. everything before the last `::') so

  $thing->Package::Name::method_name(@args);

is roughly equivalent to:

  my $code_ref = Package::Name->can('method_name');
  $code_ref->($thing, @args);

So since the Import::Into manpage defines a method `into' in package `import'
the syntax reliably calls that.

For more craziness of this order, have a look at the article I wrote at
http://shadow.cat/blog/matt-s-trout/madness-with-methods which covers
coderef abuse and the `${\...}' syntax.

Final note: You do still need to ensure that you already loaded `$thing' - if
you're receiving this from a parameter, I recommend using the Module::Runtime manpage:

  use Import::Into;
  use Module::Runtime qw(use_module);

  use_module($thing)->import::into($target, @import_args);

And that's it.

Package perl-Import-Into: Information

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