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

Perhaps a duh newbie question, but why will 
use constant k => 2;
print 1-k;
print 1-k();
print 1 -k();

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cause syntax error, whereas 
use constant k => 2;
print 1-&k;
print 1- k();
print 1- k;

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won't? Thanks.

Replies are listed 'Best First'.
Re: syntax error with constant
by BrowserUk (Patriarch) on Aug 03, 2003 at 22:19 UTC

    Probably because -k looks too much like one of the -X filetest operators (eg. -s -e -d etc.).

    If you use upper case for your constant names as is generally advised, the parser has an additional clue which it uses to work out the right way to parse the statement and does so, albeit that it issues a friendly warning in the process. 

    use constant K => 2;
print 1 - K;
-1
print 1 -K;
Ambiguous use of -K resolved as -&K() at ...
-1

    [download]
    Examine what is said, not who speaks.
    "Efficiency is intelligent laziness." -David Dunham
    "When I'm working on a problem, I never think about beauty. I think only how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong." -Richard Buckminster Fuller
    If I understand your problem, I can solve it! Of course, the same can be said for you.
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[d/l]
      ...the parser has an additional clue which it uses to work out the right way to parse the statement...

      I think the parser is reallly straightforward in this respect: known -X filetest operators will give the error, unknown ones will give the warning. Nothing to do with upper case of lower case: -a works but gives the warning even though lowercase, whereas -T will give the compile error.

      perldoc -f -X lists the following uppercase filetest operators: 

       -T  File is an ASCII text file (heuristic guess).
 -B  File is a "binary" file (opposite of -T).

 -M  Script start time minus file modification time, in days.
 -A  Same for access time.
 -C  Same for inode change time (Unix, may differ for other platforms)
      so don't call your constants T, B, M, A or C ;-).

      Liz

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[d/l]
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        But why does it happen with multi-character constant names?

        DB<1> use constant foo => 1
DB<2> print 1-foo
Ambiguous use of -foo resolved as -&foo() at (eval 15)[/usr/lib/perl5/
+5.8.0/perl5db.pl:17] line 2.
        eval '($@, $!, $^E, $,, $/, $\\, $^W) = @saved;package main; $
+^D = $^D | $DB::db_stop;
print 1-foo;
 
;' called at /usr/lib/perl5/5.8.0/perl5db.pl line 17
        DB::eval called at /usr/lib/perl5/5.8.0/perl5db.pl line 1323
        DB::DB called at -e line 1
0

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        Same thing happens with FOO.

        I don't encounter this problem, because I put spaces between terms and operators. Insert invisible 'nyah nyah' here

        --
        TTTATCGGTCGTTATATAGATGTTTGCA

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[d/l]
Re: syntax error with constant
by liz (Monsignor) on Aug 03, 2003 at 22:17 UTC
    The problem is not with use constant: 

    sub k{}
print 1-k;
print 1 -k();
print 1 -k();

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    gives the same error. This happens from at least 5.6.0 until 5.8.1-RC4.

    What I think happens is that the parser gets confused with trying to handle the: 

     -key => value

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    notation.

    Liz

    Update: -k  File has sticky bit set. is of course a much better explanation. Thanks to BrowserUk!

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Re: syntax error with constant
by artist (Parson) on Aug 04, 2003 at 23:04 UTC
    Not the answer to your question, rather than the approach which won't give such question :-),

    Why name your constants like a,b,k or even K, Q etc.. have it some meaningful name like KING or QUEEN :-)

    Said that, nothing wrong against your curiosity.

    artist

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