in reply to do [...], @INC subs, 3-args open to heredoc refs (oh my)

In fact, I partially understand how it works, but not completely. As I told you on IRC, it's a two steps trick.

First, a hook is installed inside @INC. What this sub does is to check whether it's second argument is an arrayref, and derefence it, hence returning the contained list.

Second, do() is passed an arrayref with a filehandle and a coderef inside. As do() also uses @INC, the hook that was installed is invoqued and returns the filehandle which is read and eval'd by do().

The last thing I don't understand is when and how the coderef (the second element of the arrayref given to do() is called. It is obviously called as it plays the role of a filter, but I don't understand how.

And the use of the 3-args open() with a scalarref as file name is just a convenient way to avoid having real files everywhere in the hierarchy (plus, it also avoid path-related problems).

But I think that this test script uses a few features that are not documented anywhere except in the source code.

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Re^2: do [...], @INC subs, 3-args open to heredoc refs (oh my)
by vkon (Curate) on May 01, 2006 at 18:13 UTC
    you understood most essential part.

    Last, and simpliest part is, once hooked by @INC subroutine gets anonymous array, it could do with it whatever it decides to: call its subs, use first argument as file handle, call any found CODE ref, and so on...
    pretty simple, IMO.

[reply]

      I don't agree that it's simple. You haven't explained how the subroutine reference in @INC gets called. Yes, it could in principle do whatever it wants, but it specifically does 

      unshift @INC, sub {
    no warnings 'uninitialized';
    ref $_[1] eq 'ARRAY' ? @{$_[1]} : $_[1];
};

      [download]

      which doesn't call the sub. The sub is somehow passed to the filter when the do reads in the heredoc in the 3-argument open that creates the $fh.

      The docs I pointed to above in perldoc perlfunc say:

      Subroutine references are the simplest case. When the inclusion system walks through @INC and encounters a subroutine, this subroutine gets called with two parameters, the first being a reference to itself, and the second the name of the file to be included (e.g. "Foo/Bar.pm"). The subroutine should return "undef" or a filehandle, from which the file to include will be read. If "undef" is returned, "require" will look at the remaining elements of @INC.

      It says that one value, the filehandle, is returned, but the @INC hook above appears to me to return two values; because the "filename" passed to do is an array ref, and the @INC hook returns @{$_[1]}, those are the items inside the array ref ($fh, sub {...}) passed to do. As the docs say, the @INC hook should return a single $fh, but how is the second return value (sub {...}) called?

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