Re: 'next', 'last' out of subroutine

On Wed, 28 Apr 2004, Jonathan Swartz wrote:

> I have been operating for a while under the assumption that an eval {}
> around a subroutine will always give you a chance to execute code after the
> subroutine is done. However, I've just realized this is not the case: you
> can 'next' or 'last' out of subroutine to a loop in the caller. e.g. this
> program
>
>    sub bar { next; }
>    sub foo { print "starting foo\n"; eval { bar() }; print "finished
> foo\n" }
>    print "starting loop\n";
>    for (1) {
>        foo();
>    }
>    print "finished loop\n";
>
> outputs
>
>    starting loop
>    starting foo
>    finished loop
>
> under perl 5.6.1. I find it odd since the Perl manual for 'next' and 'last'
> says
>
>            [`next'|'last'] cannot be used to exit a block which returns a
> value such
>             as `eval {}', `sub {}' or `do {}', and should not be used to
>             exit a grep() or map() operation.
>
> Am I misinterpreting this statement? I do see that some modules actually
> depend on this behavior (e.g. Test::More for its skip() subroutine).

I think that maybe that statement should be rewritten as

 calling next|last will affect the currently "in-scope" loop.  So if you
 call it in a subroutine called from a loop, it will affect that loop.

> Anyway, the way this relates to Mason is that Request::comp has always
> relied on regaining control after its eval {} so that it can clean up its
> stack. But if you have a loop in one component, and a 'next' or 'last' in
> its child component, you can skip Mason's cleanup code entirely and end up
> with a corrupted stack.

Why the heck would you do that?

> If this is a Mason "bug", it'll be a real pain to fix - we'll have to
> rewrite every bit of code that depends on a reliable exit from eval {}.
> Bleah.

This is not a Mason bug.  People who call "next" or "last" _outside of a
loop_ deserve what they get, or they expect what they get, and either way,
it's not out problem.

Heck, they can call dump too, but we can't fix that either ;)


-dave

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