Hello Andrew, you write:
> Apologies if this is a dumb question, but I have repeatedly tried to > understand this section of the documentation, and I need some help: > > http://search.cpan.org/~ether/Moose-2.1404/lib/Moose/Manual/Attributes. > pod#Accessor_methods > > I want to create a class that has some attributes. However, there’s > some logic that needs to be executed when setting values on these > attributes. In some cases, I’ll need to throw an error like “you can’t > set that value on this object attribute”, I also need to be able to > modify the value on the way into the object sort of like a DECODE() > statement in SQL, etc. You'll want to scroll half a dozen screenfuls downward on the same page and check out the section titled "Triggers". These can be used to do exactly the things you want. Setting writer / reader, on the other hand, is just providing names to the accessor routines which Moose creates automatically for you. > This seems like I’d just need to declare a writer sub for the > attribute, do my logic in there, then set the attribute value or throw > an error, right? > > Why doesn't this work? > > package Bogus { > use Moose; > has ‘value’, is => “rw”, writer => “_value”, isa => “Num”; make that trigger => "_value" > sub _value { > my ($self, $value) = @_; ...you'll get ($self,$value,$previous_value) = @_; > ## insert some logic here Your logic can check and modify $current_value, and if changed, pass it to the writer (again), or even reinstate the previous value. > $self->value($value); > return(1); > } > } > > > use Bogus; > my $a = new Bogus(value => 1); > > Can what I’m trying to do even be done in Moose? > again, sorry for the dumb question. I’m quite sure I’ve just missed > something in my understanding. I'm also a Moose beginner (more or less) and know this feeling very well. There's a lot of stuff to read, and I was lucky to have a gentle introduction at our Perl monger's meeting. I have never tried the more sophisticated methods to achieve your goal: declare how to check your value as described in `perldoc Moose::Manual::Types`: type constraints can valuate the input against the properties of a custom class, and coercion can modify the value on the fly. -- Cheers, haj
