Re: Moose constructor help

You can do that using a default; if I assume you want $results to be a function of $contents, it might look like (untested):

use Moose;
use JSON;

has content =>
    isa => 'Str',
    is  => 'ro',
    required=> 1,
    ;

has results =>
    isa => 'Str',
    is  => 'ro',
    default => sub {
        decode_json($_[0]->content)
    },
    ;
[download]

There are a lot of variations how that would look depending on what you actually intend to do. See Default and builder methods in Moose::Manual::Attributes for more details.

#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

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Re^2: Moose constructor help by greengaroo (Hermit) on Mar 14, 2014 at 18:34 UTC
I'm not sure why you would need this in the constructor, but altering a little the example from kennethk, this is how I would do it: `use Moose; use JSON; has content => ( isa => 'Str', is => 'ro', required=> 1, ); has results => ( isa => 'Str', is => 'ro', lazy => 1, builder => '_build_results', ); sub _build_results { my $self = shift; return decode_json($self->content); }` [download] With lazy and builder, the results attribute will be populated only when needed, only when you call `$obj->results()` Note: the builder method has access to $self, the reference to the instance, because it is called AFTER the instance is created, however, in kennethk's example, I don't think the default sub has access to $self or in this case $_[0]* because the default sub is executed WHILE the instance is being created, therefore, the reference is not yet available. * In a method, $_[0] contains the reference to the instance, same as if you do `my $self = shift;` or `my $self = shift(@_);` or `my $self = $_[0];`. I may be mistaking but I've had problems with this before and since then, I use lazy attributes with a builder method. A for will get you from A to Z; a while will get you everywhere. -- greengaroo	[reply] [d/l] [select]

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Re^2: Moose constructor help
by greengaroo (Hermit) on Mar 14, 2014 at 18:34 UTC

I'm not sure why you would need this in the constructor, but altering a little the example from kennethk, this is how I would do it:

use Moose;
use JSON;

has content => (
    isa => 'Str',
    is  => 'ro',
    required=> 1,
);

has results => (
    isa => 'Str',
    is  => 'ro',
    lazy => 1,
    builder => '_build_results',
);

sub _build_results {
    my $self = shift;
    return decode_json($self->content);
}
[download]

With lazy and builder, the results attribute will be populated only when needed, only when you call $obj->results()

Note: the builder method has access to $self, the reference to the instance, because it is called AFTER the instance is created, however, in kennethk's example, I don't think the default sub has access to $self or in this case $_[0]* because the default sub is executed WHILE the instance is being created, therefore, the reference is not yet available.

* In a method, $_[0] contains the reference to the instance, same as if you do my $self = shift; or my $self = shift(@_); or my $self = $_[0];. I may be mistaking but I've had problems with this before and since then, I use lazy attributes with a builder method.

A for will get you from A to Z; a while will get you everywhere.

-- greengaroo

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