Prepend a Moose attribute when changed

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

Hi folk:

I'm fairly new to Moose & I cannot seem to figure out this problem: I have a string attribute to which I wish to prepend another string... I try with builder: no luck; I tried w BUILDARGS which works when an instance is initiated, but not when I try to change the attribute...

My testing module:

package Test;
use Moose;

my $prepend = '/some_string/';
has 'fname' => (is => 'rw',
              isa => 'Str');

has 'appended'      => ( is => 'rw',
                      isa => 'Str',
                        #  lazy => 1,
                        #builder => 'set_appended'
                        );
1;

sub BUILDARGS {
    my $class = shift;
    my %args = ref $_[0] ?  %{$_[0]} : @_;
    $args{appended} =  $prepend . $args{appended} if exists $args{appe
+nded};
    return \%args;
}

sub set_appended {
    my $self = shift;
    my $value = $self->appended;
    return $prepend . $value;
}
[download]

My test program


use 5.010;
use strict;
use warnings;

use lib ".";
use Test;
    
my $t  = new Test( fname    => "fred",
                   appended => "this_is_appended_string");

say $t->fname;
say $t->appended;
$t->appended ('another_string');
say $t->appended;
[download]

The output

fred
/some_string/this_is_appended_string
another_string
[download]

Thx in advance..

Comment on Prepend a Moose attribute when changed Select or Download Code

Replies are listed 'Best First'.
Re: Prepend a Moose attribute when changed by 1nickt (Canon) on Feb 08, 2019 at 02:45 UTC
Hi, what you are looking for is a coercion. (I'd also recommend using Type::Tiny for type constraints). This can be done in Moose: `use strict; use warnings; use feature 'say'; package MyClass { use Moose; use Types::Standard 'Str'; has foo => ( is => 'rw', isa => Str->where(sub {/^bar\./})->plus_coercions(Str, sub +{'bar.' . $_}), coerce => 1, ); } my $obj = MyClass->new( foo => 'baz' ); say $obj->foo; # 'bar.baz $obj->foo('qux'); say $obj->foo; # 'bar.qux' __END__` [download] ... but IMHO it's much simpler (as are most things) in Moo: `use strict; use warnings; use feature 'say'; package MyClass { use Moo; use Types::Standard 'Str'; has foo => ( is => 'rw', isa => Str, coerce => sub {'bar.' . shift}, ); } my $obj = MyClass->new( foo => 'baz' ); say $obj->foo; # 'bar.baz' $obj->foo('qux'); say $obj->foo; # 'bar.qux' __END__` [download] Hope this helps! The way forward always starts with a minimal test.	[reply] [d/l] [select]
Re: Prepend a Moose attribute when changed by Haarg (Priest) on Feb 08, 2019 at 13:19 UTC
It may make more sense to prepend the extra string in your accessor. `package Test; use Moose; my $prepend = '/some_string/'; has appended => (is => 'rw'); around appended => sub { my ($orig, $self) = (shift, shift); my $value = $self->$orig(@_); return $prepend . $value; }; 1;` [download]	[reply] [d/l]
Re: Prepend a Moose attribute when changed by tobyink (Canon) on Feb 08, 2019 at 14:45 UTC
This seems the easiest way to me… `use 5.010; use strict; use warnings; { package Test; use Moose; my $prepend = '/some_string/'; has _appended => (is => 'rw', init_arg => 'appended'); sub appended { $prepend . shift->_appended(@_) } } my $t = Test->new(appended => "this_is_appended_string"); say $t->appended; $t->appended('another_string'); say $t->appended;` [download] toby döt ink	[reply] [d/l]