Question

I am having trouble getting Params::Validate to work how I want it to.

#!/usr/local/bin/perl -w  use strict; use Params::Validate qw/:all/; use Data::Dumper;  sub foo {     my %args = validate(@_, {         bar => {             default => 99,             # TODO - Somehow use a callback to return a default if undefined             # ??!!??              # callbacks => { call_me => sub { return 99 if !defined shift }},         },     });      # $args{bar} //= 99; # Don't want to define default in 2 places      print Dumper(\%args); } foo({ bar => undef }); 

So how do I set / test for an undef in the args list and replace with the said ‘default’ value with Params::Validate ??

Answer 1

You need to be setting $_[0]:

call_me => sub { $_[0] = 99 if not defined $_[0] } 

@_ is aliased to the parameters passed in, so you can use this as a reference to the original.

Also,

$args{bar} ||= 99; 

would reset bar even if it were 0 or ” (empty string), which doesn’t sound like what you want. Using

$args{bar} //= 99; 

if you’re using perl 5.10 or later would do what you want, though.

Based on your comment of not duplicating the default, the best I could come up with is:

sub foo {     unshift @_, bar => undef;     my %args = validate(@_,                         {                             bar => {                                 optional => 1,                                 callbacks => {                                     call_me => sub { $_[0] = 99 unless defined $_[0] },                                 },                             },                         },                        );     print Dumper \%args; } 

That seems to work, but I have to admit – it’s ugly.