develooper Front page | perl.perl6.language | Postings from August 2001

Re: explicitly declare closures???

Thread Previous | Thread Next
From:
Tony Hall
Date:
August 26, 2001 13:33
Subject:
Re: explicitly declare closures???
Message ID:
3.0.6.32.20010826062810.00dc8b60@vcash.com
Dave Mitchell <davem@fdgroup.co.uk> wrote:
> John Porter <jdporter@min.net> wrote:
> > Dave Mitchell wrote:
> > > I think closures are a lot harder (or at least subtler) than
> > > people think ...
> > 
> > ... The scenario you gave seems rather far-fetched to me, in terms
> > of real-world programming.
> 
> Perhaps, although the following code demonstrates it, and isn't (too)
> contrived. Comment out the 'superfluous' "$counters[$i] = 0;" line and
> the code stops working.

We must be very careful not to confuse "closure" with "Perl's
current implementation of closure". You've stumbled onto a bug in
Perl, not discovered a feature of closures. Perl's "closures"
were horribly buggy until release 5.004. (Thanks Chip!)

Closed variables are just local variables. There's nothing special
about refering to a variable from an inner scope. You don't want
to write

  sub foo {
    my $x;

    if (...) { my outer $x; $x = 0 }
    else     { my outer $x; $x = 1 }

    $x;
  }

do you? So why make people using closures do it?

My only closure bug due to scoping was because I named a variable
poorly and accidentally shadowed it in an inner scope. Your
proposal does nothing to eliminate that bug.

A more common problem I have is with reading code that uses a
variable a long way from where it was introduced. Your proposal
adds a nearby declaration, but it's a useless declaration that
doesn't help find the original variable.

IMHO, every declaration we write should make a program more
understandable by *people*. If the only reason we declare
something is to help the computer understand it, we should think
carefully on how to eliminate the declaration.

- Ken




Thread Previous | Thread Next


nntp.perl.org: Perl Programming lists via nntp and http.
Comments to Ask Bjørn Hansen at ask@perl.org | Group listing | About