In newer Getopt::Long How do I set default optional values

Question

In Perl's Getopt::Long version 2.39 I could use

use Getopt::Long qw( :config gnu_getopt );
GetOptions(
   \my %opts, 
   "codon-view|c:20",    # Optional value, default 20
   "consensus|C:50", 
   ...
)

to indicate that if I use -c the default value would be 20 put in %optsunder key codon-view when -c is given but no explicit value for it is there. On the other hand -c or --codon-view is not supplied, then no value in the hash table is stored for in %opts.

In 2.48 this no longer works and I don't see in Getopt::Long's documentation

$ perl -E'
   use Getopt::Long qw( :config gnu_getopt );
   say $Getopt::Long::VERSION;
   GetOptions(\my %opts, "codon-view|c:20");
   say $opts{"codon-view"} // "[undef]"
' -- -c
2.39
20

$ perl -E'
   use Getopt::Long qw( :config gnu_getopt );
   say $Getopt::Long::VERSION;
   GetOptions(\my %opts, "codon-view|c:20");
   say $opts{"codon-view"} // "[undef]"
' -- -c
2.48
[undef]

How can I achieve the old behavior?

Help!

Answer 1

This is a change introduced in 2.48.

$ perl -E'
   use Getopt::Long qw( :config gnu_getopt );
   say $Getopt::Long::VERSION;
   GetOptions(\my %opts, "codon-view|c:20");
   say $opts{"codon-view"} // "[undef]"
' -- -c
2.47
20

$ perl -E'
   use Getopt::Long qw( :config gnu_getopt );
   say $Getopt::Long::VERSION;
   GetOptions(\my %opts, "codon-view|c:20");
   say $opts{"codon-view"} // "[undef]"
' -- -c
2.48
[undef]

I'm not sure, but I think it was done unintentionally, so I filed a bug report.

---

use Getopt::Long qw( :config gnu_getopt );

is short for

use Getopt::Long qw( :config gnu_compat bundling permute no_getopt_compat );

How invested are you in using gnu_compat?

$ perl -E'
   use Getopt::Long qw( :config gnu_getopt );
   say $Getopt::Long::VERSION;
   GetOptions(\my %opts, "codon-view|c:20");
   say $opts{"codon-view"} // "[undef]"
' -- -c
2.48
[undef]

$ perl -E'
   use Getopt::Long qw( :config gnu_compat bundling permute no_getopt_compat );
   say $Getopt::Long::VERSION;
   GetOptions(\my %opts, "codon-view|c:20");
   say $opts{"codon-view"} // "[undef]"
' -- -c
2.48
[undef]

$ perl -E'
   use Getopt::Long qw( :config bundling permute no_getopt_compat );
   say $Getopt::Long::VERSION;
   GetOptions(\my %opts, "codon-view|c:20");
   say $opts{"codon-view"} // "[undef]"
' -- -c
2.48
20

gnu_compat controls whether --opt= is allowed, and what it should do. Without gnu_compat, --opt= gives an error. With gnu_compat, --opt= will give option opt and empty value. This is the way GNU getopt_long() does it.

So if you're ok with --codon-view= assigning zero to $opts{"codon-view"}, simply use

use Getopt::Long qw( :config bundling permute no_getopt_compat );

instead of

use Getopt::Long qw( :config gnu_getopt );

Answer 2

Set the default value before the GetOptions() call. If the option is not provided on the command line, then the default value will not be overwritten.

$ perl -MGetopt::Long -E '$c=20;GetOptions("c=i"=>\$c); say $c' -- -c 14
14

$ perl -MGetopt::Long -E '$c=20;GetOptions("c=i"=>\$c); say $c' --
20

There is a trivial example in the Getopt::Long documentation.