Re: shift implicit dereference

"... what that "+" stuff exactly does."

A unary "+" is used for disambiguation. If you were to code ${shift}, that would be ambiguous: did you mean the variable $shift or a dereferencing operation on the value shifted off @_. Using a "+" indicates you meant the shift function and not a variable name. Consider this code:

#!/usr/bin/env perl -l

use strict;
use warnings;

my $shift = 'E';
my @x = \ (qw{A B C D});
test_shift(@x);

sub test_shift {
    print ${+shift};
    print ${shift()};
    print ${shift @_};
    print ${shift(@_)};
    print ${shift};
}
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Output:

Ambiguous use of ${shift} resolved to $shift at ./junk line 15.
A
B
C
D
E
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You'll find this usage of "+" cropping up in many places. Here's some examples.

Does the "(" following a function name start a list of arguments to the function?

$ perl -Mstrict -Mwarnings -le 'print (1==0) ? "true" : "false"'
print (...) interpreted as function at -e line 1.
Useless use of a constant ("true") in void context at -e line 1.
Useless use of a constant ("false") in void context at -e line 1.

$ perl -Mstrict -Mwarnings -le 'print +(1==0) ? "true" : "false"'
false
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Is "{...}" an anonymous block or a hashref constructor?

$ perl -Mstrict -Mwarnings -MData::Dump -e 'dd [map { my $y = uc; {$y 
+=> 1} } "a".."c"]'
["A", 1, "B", 1, "C", 1]

$ perl -Mstrict -Mwarnings -MData::Dump -e 'dd [map { my $y = uc; +{$y
+ => 1} } "a".."c"]'
[{ A => 1 }, { B => 1 }, { C => 1 }]
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How can I tell a constant bareword and a hash key bareword apart?

$ perl -Mstrict -Mwarnings -le 'use constant X => "a"; my %h = (a => 1
+); print $h{X}'
Use of uninitialized value in print at -e line 1.

$ perl -Mstrict -Mwarnings -le 'use constant X => "a"; my %h = (a => 1
+); print $h{+X}'
1
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Documentation for unary "+" usages appears to be rather scattered. For the three pairs of examples above, see "perlop: Symbolic Unary Operators", "perlref: Making References" and "constant: CAVEATS", respectively. There are other examples documented elsewhere. I couldn't locate any references to the ${+shift} example.

-- Ken

In Section Seekers of Perl Wisdom