in reply to Re: A cleaner way of scoping variables
in thread A cleaner way of scoping variables

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

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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.

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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__

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    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.

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      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 { };

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      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.

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        Nit: "The Real False" is not the empty string. It's the empty string in string context and 0 in numerical context. You can see the differnce using the following snippet:

        use warnings;
print(("")+1, "\n");
print((!1)+1, "\n");

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        If "The Real False" was simply an empty string, we'd see two warnings instead of

        Argument "" isn't numeric in addition (+) at script.pl line 2.
1
1

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