Beefy Boxes and Bandwidth Generously Provided by pair Networks
good chemistry is complicated,
and a little bit messy -LW
 
PerlMonks  

Comment on

( #3333=superdoc: print w/ replies, xml ) Need Help??

G'day Lennotoecom,

"... 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}; }

Output:

Ambiguous use of ${shift} resolved to $shift at ./junk line 15. A B C D E

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

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 }]

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

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 reply to Re: shift implicit dereference by kcott
in thread shift implicit dereference by Lennotoecom

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":



  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • Outside of code tags, you may need to use entities for some characters:
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?
    Username:
    Password:

    What's my password?
    Create A New User
    Chatterbox?
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others surveying the Monastery: (18)
    As of 2014-07-31 18:17 GMT
    Sections?
    Information?
    Find Nodes?
    Leftovers?
      Voting Booth?

      My favorite superfluous repetitious redundant duplicative phrase is:









      Results (250 votes), past polls