About the closure issue in nested sub

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

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But that article doesn't explain why the issue only exists for closure in nested sub

mod_perl doesn't free global resources,being aware of this, still doesn't explain why the issue only exists in nested sub closure.

Anyone knows the exact reason?

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Re: About the closure issue in nested sub by moritz (Cardinal) on Sep 09, 2011 at 11:48 UTC
The answer is that Perl 5 really doesn't have "nested subs". Subs live in the symbol table, not inside a lexical scope, so the "inner" sub isn't really inside the "outer" sub -- it is in the symbol table, and thus can be called before the outer sub is called. The fact that perl still allows you to write a sub inside a sub comes from a cheat that makes it possible, but that cheat doesn't extend to the closure semantics you seem to expect. (The experience from watching and patching Perl 6 compilers shows that nested named subs and correct closure semantics are a pain to get right (if the inner sub is visible outside the outer sub), no wonder the perl 5 folks haven't felt like fixing this). Perl 6 - second systems done right	[reply]

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Re: About the closure issue in nested sub
by moritz (Cardinal) on Sep 09, 2011 at 11:48 UTC

The answer is that Perl 5 really doesn't have "nested subs".

Subs live in the symbol table, not inside a lexical scope, so the "inner" sub isn't really inside the "outer" sub -- it is in the symbol table, and thus can be called before the outer sub is called.

The fact that perl still allows you to write a sub inside a sub comes from a cheat that makes it possible, but that cheat doesn't extend to the closure semantics you seem to expect.

(The experience from watching and patching Perl 6 compilers shows that nested named subs and correct closure semantics are a pain to get right (if the inner sub is visible outside the outer sub), no wonder the perl 5 folks haven't felt like fixing this).

Perl 6 - second systems done right

[reply]