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

Hello Monks
I am not able to understand the following line of code.
use constant x => 1/3 in the context that the following line of code gives a scalar assignment error
use constant x = 1/3
From what i have read in perldoc -f use, the former is a pragma. I tried to check in the perlpragma for this usage reference, but i could only such case where the => was mentioned , without a clear reference as to its usage.
The background is im writing a file to a PDF, and while reading a document on how to use PDF:API2 and i came across this code instance which seemed relevant to fully understand

The temporal difficulty with perl is u need to know C well to know the awesome.else u just keep *using* it and writing inefficient code

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Re: use constant usage clarification
by Loops (Curate) on Oct 31, 2014 at 06:19 UTC

    Hi perlron,

    from the perlop page:
    The => operator is a synonym for the comma except that it causes a word on its left to be interpreted as a string if it begins with a letter or underscore and is composed only of letters, digits and underscores. This includes operands that might otherwise be interpreted as operators, constants, single number v-strings or function calls. If in doubt about this behavior, the left operand can be quoted explicitly.

    So there is no assignment going on (at that point), just a list of values being passed to the pragma. It's the same as this:

    use constant 'x',  1/3;

    The pragma then does the magic of turning each identifier,value pair into a usable constant.

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      Ok .thanks . nothing to worry for me about => then..

      The temporal difficulty with perl is u need to know C well to know the awesome.else u just keep *using* it and writing inefficient code
[reply]
Re: use constant usage clarification (Deparse)
by Anonymous Monk on Oct 31, 2014 at 07:38 UTC

    I am not able to understand the following line of code.

    Its right there in Basic debugging checklist , B::Deparse 

    $ perl -MO=Deparse,-p -e " use constant X => 1/3; "
use constant ('X', 0.33333333333333331);
-e syntax OK

    [download]

    When you're not sure about perl syntax, get App::PPI::Dumper

    ppi_dumper / wxPPIxregexplain.pl 

    $ cat 2
use constant X => 1/3;


$ ppi_dumper 2
PPI::Document
  PPI::Statement::Include
    PPI::Token::Word    'use'
    PPI::Token::Whitespace      ' '
    PPI::Token::Word    'constant'
    PPI::Token::Whitespace      ' '
    PPI::Token::Word    'X'
    PPI::Token::Whitespace      ' '
    PPI::Token::Operator        '=>'
    PPI::Token::Whitespace      ' '
    PPI::Token::Number          '1'
    PPI::Token::Operator        '/'
    PPI::Token::Number          '3'
    PPI::Token::Structure       ';'
  PPI::Token::Whitespace        '\n'
  PPI::Token::Whitespace        '\n'

    [download]
    So what is  =>? Its an PPI::Token::Operator ... see perlop
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Re: use constant usage clarification
by BrowserUk (Patriarch) on Oct 31, 2014 at 06:21 UTC
    [this]' gives a scalar assignment error use constant x = 1/3

    Really? Cos when I try it, the error is: Unquoted string "x" may clash with future reserved word.

    Care to post a short script that produces that error when run?

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
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      Hello BrowserUK i'm receiving too an assignement error using the wrong syntax with the not stringified left value:
      #(something like:)
perl -MConfig -e "BEGIN{print 'Perl ', map {$Config{$_},' '}qw/version
+ os archname/ }; use constant X = 1/3;print qq(\n),+X,qq(\n)"

  ----------------------------------------·······
 | Can't modify constant item in scalar assignment at -e line 1, near 
+"3;"
 |  BEGIN not safe after errors--compilation aborted at -e line 1.
 |  Perl 5.14.2  MSWin32-x86-multi-thread
  --------------------------------------------------------------------
+---·······

  ----------------------------------------·······
 | Can't modify constant item in scalar assignment at -e line 1, near 
+"3;"
 |  BEGIN not safe after errors--compilation aborted at -e line 1.
 |  Perl 5.20.0  MSWin32-x86-multi-thread-64int
  --------------------------------------------------------------------
+---·······

  ----------------------------------------·······
 | Can't modify constant item in scalar assignment at -e line 1, near 
+"3;"
 |  BEGIN not safe after errors--compilation aborted at -e line 1.
 |  Perl 5.16.2  MSWin32-x64-multi-thread
  --------------------------------------------------------------------
+---·······

      [download]

      L*
      There are no rules, there are no thumbs..
      Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.
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