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

Why does the execution of the following script print the string MELHARRIS on the last line? I expected perl to evaluate HARRIS, because it evaluated EARL. 
#!/usr/bin/perl -w

use strict;

use CGI qw/:standard/;
use constant EARL => 'MEL';
use constant HARRIS => 'BROOKS';

my @a = map { lc($_) } (EARL, HARRIS);

my $b = { EARL.HARRIS => 55 };

print join(' ', @a)."\n";
print join(' ', keys %{ $b })."\n";

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Replies are listed 'Best First'.
Re: constant not evaluated
by dragonchild (Archbishop) on Sep 20, 2004 at 17:15 UTC 
    use constant EARL => 'MEL';

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    is (functionally) identical to the code 
    sub EARL () { 'MEL' }

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    The problem is that you're using it in a stringifying context - the left-hand side of the => operator. Try the following: 

    my $b = { EARL() . HARRIS() => 55 };

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    And see if it does what you want.

    ------
    We are the carpenters and bricklayers of the Information Age.

    Then there are Damian modules.... *sigh* ... that's not about being less-lazy -- that's about being on some really good drugs -- you know, there is no spoon. - flyingmoose

    I shouldn't have to say this, but any code, unless otherwise stated, is untested

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      To depend less on how constants are implemented, just put the argument that is being stringified (before being constant-evaluated) in parentheses: 
      my $b = {EARL.(HARRIS) => 55 };

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      And of course, using a comma instead of the stringification operator also yields the desired result. 
      my $b = {EARL.HARRIS, 55 };

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      Caution: Contents may have been coded under pressure.
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      Thanks for the reply. This did fix the problem.
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Re: constant not evaluated
by ikegami (Patriarch) on Sep 20, 2004 at 17:18 UTC
    I didn't check, but I bet the order of precedence dictates that
    my $b = { EARL.HARRIS => 55 };
    is
    my $b = { EARL . (HARRIS => 55) };
    which is
    my $b = { EARL . ('HARRIS', 55) };
    Try
    my $b = { (EARL.HARRIS) => 55 };

    mel brooks
MELBROOKS

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    Update: Extra ')' removed

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      ikegami, you have an extra ) after the 55.
      I removed that and it worked.

      TheEnigma

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Re: constant not evaluated
by Fletch (Bishop) on Sep 20, 2004 at 18:08 UTC

    B::Deparse is your friend: 

    freebie:~ 824> perl -MO=Deparse,-p,-q foo                             
+ 14:05:06
sub HARRIS () {
    package constant;
    $scalar;
}
sub EARL () {
    package constant;
    $scalar;
}
(my(@a) = map({lc($_);} 'MEL', 'BROOKS'));
(my $b = {('MEL' . 'HARRIS'), 55});
print((join(' ', @a) . "\n"));
print((join(' ', keys(%{$b;})) . "\n"));
foo syntax OK

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    (The CGI line was removed as it's not the problem here and makes a whole bunch more output.)

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Re: constant not evaluated
by ysth (Canon) on Sep 20, 2004 at 19:46 UTC
    perlop says:
    The "=>" operator is a synonym for the comma, but forces any word to its left to be interpreted as a string (as of 5.001).
    It's not a matter of precedence, since . is higher precendence than , or =>. It appears that the above doc really does mean "any word to its left" rather than "its left operand" as I would have expected.

    I suspect it would be infeasible to change it to "its left operand". It may be possible to give a warning where => does force a word to be interpreted as a string but the word isn't the full left operand. I'd recommend filing a perlbug report on this.

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