Re: A cleaner way of scoping variables

There is a way you can avoid declaring $bar once and assining it an other time:

use strict;
my $foo = 1;
{
        my $bar = do { 
                if ($foo) 
                        { "true"; }
                };
print $bar; 
}
[download]

You still need a pair of braces around the whole construct. If you want $bar to be local, you need a block, how else would perl know where the variable is local.

The block is not ugly imo. I often use blocks in perl just to declare local variables. There is a way to avoid such blocks, but that involves changing programming style: you have to write short (50 lines most) subroutines that do one thing and do it well, and you can declare local variables at the top of each subroutine, with one my() statement perl sub (and preferable at most 7 variables). This is a good programming style in C imo (see Documentation/CodingStyle in the linux source), but doesn't always worth in Perl.

This is a bit OT, but note that I write the curlies of a block inline if the block contains only one line of code.

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Re^2: A cleaner way of scoping variables by Aristotle (Chancellor) on Aug 09, 2004 at 13:44 UTC
This code has a problem: if the condition is false, the condition is the last expression evaluated in the `do` block, so whatever the condition evaluates to becomes the result of the `do` block. `$ perl -le'print do { "x" if 1 }' x $ perl -le'print do { "x" if 0 }' 0` [download] In your case, the value of `$foo` is assigned to `$bar` if `$foo` is false. This could be particularly nasty because you'll never notice if you don't run tests with `$foo` being 0 or the empty string rather than undefined. You must make sure that your `do` blocks always evaluate to an intended result. In this case, you have to add an `else { undef }` clause. Makeshifts last the longest.	[reply] [d/l]
Re^3: A cleaner way of scoping variables by ambrus (Abbot) on Aug 09, 2004 at 13:59 UTC
You must make sure that your do blocks always evaluate to an intended result. In this case, you have to add an else { undef } clause. Indeed. In your case, the value of $foo is assigned to $bar if $foo is false. Perl is a bit wierd about the return value of a loop of which the body never runs. This code: #!/usr/bin/perl -w my $true = "foo"; my $false = "0"; my $dc = "bar"; warn do { $dc if $false }; warn do { $dc unless $true }; warn do { $dc while $false }; warn do { $dc until $true }; warn do { if ($false) { $dc } }; warn do { unless ($true) { $dc } }; warn do { while ($false) { $dc } }; warn do { until ($true) { $dc } }; warn do { $dc if "0" }; warn do { $dc unless "foo" }; warn do { $dc while "0" }; warn do { $dc until "foo" }; warn do { if ("0") { $dc } }; warn do { unless ("foo") { $dc } }; warn do { while ("0") { $dc } }; warn do { until ("foo") { $dc } }; __END__ [download] prints: Useless use of private variable in void context at a line 9. Useless use of private variable in void context at a line 10. Useless use of private variable in void context at a line 13. Useless use of private variable in void context at a line 14. Useless use of private variable in void context at a line 21. Useless use of private variable in void context at a line 22. 0 at a line 7. foo at a line 8. 0 at a line 9. foo at a line 10. 0 at a line 11. foo at a line 12. 0 at a line 13. foo at a line 14. 0 at a line 15. foo at a line 16. 0 at a line 17. Warning: something's wrong at a line 18. 0 at a line 19. Warning: something's wrong at a line 20. Warning: something's wrong at a line 21. Warning: something's wrong at a line 22. [download]	[reply] [d/l] [select]
Re^4: A cleaner way of scoping variables by Aristotle (Chancellor) on Aug 09, 2004 at 14:24 UTC
As often, Deparse comes to the rescue. With formatting adjusted for comparability with yours, the output is `#!/usr/bin/perl -w my $true = "foo"; my $false = "0"; my $dc = "bar"; warn do { $dc if $false }; warn do { $dc unless $true }; warn do { $dc while $false }; warn do { $dc until $true }; warn do { if ($false) { $dc; } }; warn do { unless ($true) { $dc; } }; warn do { while ($false) { $dc; } }; warn do { until ($true) { $dc; } }; warn do { '0' }; warn do { 'foo' }; warn do { do { '0' } }; warn do { do { !1 } }; warn do { '0' }; warn do { !1 }; warn do { }; warn do { };` [download] Note the `!1` — ie "not true", ie "The Real False", which is an empty string in Perl5. That explains your `something's wrong` messages: the loops are getting folded away at compile time, but apparently the compiler doesn't evaluate hard enough. Makeshifts last the longest.	[reply] [d/l]
Re^5: A cleaner way of scoping variables by ikegami (Patriarch) on Oct 21, 2005 at 15:09 UTC
Re^4: A cleaner way of scoping variables by ambrus (Abbot) on Nov 03, 2007 at 17:01 UTC
See also Strange behaviour when returning the value of "do {}" - possible perl bug? for an example when someone is actually bitten by this bug. See also Twin-lines japh for an obfu where I use a perl feature somewhat similar to this bug. Update 2008 jan 16: yet another monk bitten by this bug: unless versus if ( ! ) inside a subroutine. Update 2008 nov 3: see also Re^2: When doesn't the flip-flop operator work in all scalar contexts?. Update 2009-06-23: and another: return if 0.	[reply]