Understanding arguments to subroutines

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

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Re: Understanding arguments to subroutines by Corion (Patriarch) on May 14, 2012 at 12:31 UTC
See perlop on the Comma-Operator, which documents the "fat comma", `=>`.	[reply] [d/l]
Re^2: Understanding arguments to subroutines by toolic (Bishop) on May 14, 2012 at 14:32 UTC
It seems like the doc is incomplete with regard to the dash character: The => operator is a synonym for the comma except that it causes its left operand 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. `=>` treats the dash as a string, although the doc does not mention it: `use warnings; use strict; use Data::Dumper; my %h = (-v => 5); print Dumper(\%h); print "$h{-v}\n"; __END__ $VAR1 = { '-v' => 5 }; 5` [download]	[reply] [d/l] [select]
Re^3: Understanding arguments to subroutines by Corion (Patriarch) on May 14, 2012 at 14:42 UTC
I think you can explain it without a change to the documentaiton, and B::Deparse supports that: `> perl -MO=Deparse -e "print for -verbose => 2" print $_ foreach (-'verbose', 2); -e syntax OK` [download] ... and the unary minus, when applied to a string, returns "-", prepended to that string, as documented in perlop for Symbolic Unary Operators.	[reply] [d/l]
Re^4: Understanding arguments to subroutines by toolic (Bishop) on May 14, 2012 at 14:52 UTC
Re^2: Understanding arguments to subroutines by uday_sagar (Scribe) on May 14, 2012 at 12:35 UTC
I know the use of fat comma, in the context of a hash, but here, hyphen (-) is also used with "verbose" which confuses me a bit.	[reply]
Re^3: Understanding arguments to subroutines by cdarke (Prior) on May 14, 2012 at 12:45 UTC
The hyphen is a bit of a throw-back to olden-times. This originally meant that the caller did not need to quote the keys – although that has not been required since version 5.6 and that functionality passed to the fat comma. The hyphen is not stripped off by perl, it is passed in @_ so the subroutine has to remove it.	[reply]
Re^3: Understanding arguments to subroutines by toolic (Bishop) on May 14, 2012 at 14:19 UTC
Take a look at the source code perldoc -m Pod::Usage: `sub pod2usage { local($_) = shift; my %opts; # ... ## Now determine default -exitval and -verbose values to use if ((! defined $opts{'-exitval'}) && (! defined $opts{'-verbose'}) +) { $opts{'-exitval'} = 2; $opts{'-verbose'} = 0; }` [download] `-verbose` is used verbatim as a hash key.	[reply] [d/l] [select]
Re^3: Understanding arguments to subroutines by Corion (Patriarch) on May 14, 2012 at 12:41 UTC
You can ask Perl directly to show its interpretation: `> perl -MO=Deparse -e "-verbose" -'verbose'; -e syntax OK` [download] The minus is an unary minus, also documented in perlop.	[reply] [d/l]
Re^3: Understanding arguments to subroutines by DStaal (Chaplain) on May 14, 2012 at 15:11 UTC
It makes it easier to differentiate 'argument tags' from 'argument values', especially in mixed contexts. It doesn't necessarily mean anything to perl, but it means something to the subroutine.	[reply]