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Re: return() in pointy blocks

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From:
Piers Cawley
Date:
June 8, 2005 07:01
Subject:
Re: return() in pointy blocks
Message ID:
m2fyvtkkwl.fsf@obelisk.bofh.org.uk
"TSa (Thomas SandlaƟ)" <Thomas.Sandlass@orthogon.com> writes:

> Piers Cawley wrote:
>> My preference is for:
>>     Boo
>>     Boo
>>     Can't dereferene literal numeric literal 42 as a coderef.
>
> How do you reach the second 'Boo'? Iff -> does not create a Sub
> but a Block instance then Luke's code can be interpreted as a
> much smarter version of

I really wish you'd quote in more detail, it makes it a real PITA to go back
and find the explanatory code.  

    sub foo () {
      return -> { return 42 }
    }

    my $code =   foo();
    #          ^--- continuations points to the RHS of the assignment
    say "Boo!";
    $code();

So, this is what happens.

1. foo() returns a coderef to the RHS of the assignment.
2. The coderef gets assigned to $code.
3. say "Boo!"
4. We invoke the coderef, which returns 14 to continuation which was current
   when it was created.
5. That means 42 gets returned to the RHS of the assignment
6. say "Boo!"
7. Try to invoke the literal 42. 
8. Die.

In other words, it outputs:

   Foo
   Foo
   # dies

>
>     sub foo()
>     {
>         enter: 42;
>         if $?RETURN_LABEL { goto $?RETURN_LABEL }
>         return;
>     }
>
>      say "Boo!";
>      say goto foo::enter; # goto sets up $?RETURN_LABEL
>      say "after goto";
>
> which of course prints
>
>     Boo
>     42
>     after goto
>
> The smartness is in the value that &prefix:{'->'} returns
> while in the snippet above it is explicitly coded.
>
> Or do I completely misunderstand the distinction between
> blocks and closures?

It seems so.

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