Maybe someone might find this useful.
In the end I concocted this kind of code. The price of simplicity at the
base program level is paid at the class level, with a specific BUILD
submethod, and with an even greater complexity at role level, where the
accessors are created dynamically with some metaprogramming.
role Creator {
method create(::class, $method, $attribute is rw, &func) {
::class.^add_method($method, my method (::class:) {
Proxy.new(
FETCH => { $attribute },
STORE => -> $, $value { $attribute = &func($value) }
)
});
}
}
class A does Creator {
has $!a;
has $!b;
submethod BUILD {
self.create(::A, 'a', $!a, { $^a * 2 });
self.create(::A, 'b', $!b, { $^a * 3 });
}
}
my A $a .= new;
$a.a = 21;
say $a.a; # 42
$a.b = 21;
say $a.b; # 63
On Tue, Nov 14, 2017 at 7:09 PM, Elizabeth Mattijsen <l...@dijkmat.nl> wrote:
> This might it then:
>
> class A {
> has $!a; # $.a if you want to be able to assign with .new
> method a() {
> Proxy.new(
> FETCH => { $!a },
> STORE => -> $, $value { $!a = $value * 2 }
> )
> }
> }
> my $a = A.new;
> $a.a = 77;
> dd $a.a; # 154
>
> > On 14 Nov 2017, at 18:51, Fernando Santagata <nando.santag...@gmail.com>
> wrote:
> >
> > Hi Liz,
> >
> > What I need is to preprocess the value before assigning it to an
> attribute.
> >
> > I would do that in Perl5/Moose, using "around", like this:
> >
> > package A;
> > use Moose;
> >
> > has 'attribute' => (is => 'rw', isa => 'Str');
> >
> > around [qw(attribute)] => sub {
> > my ($next, $self, $val) = @_;
> > return $self->$next unless $val;
> > return $self->$next(preprocess $val); # Preprocess the value before
> the assignment
> > }
> >
> > In this way I don't have to make an explicit call to the preprocessor
> any time I assign a value to that attribute, effectively removing that from
> the main program.
> >
> > I'm looking for a way to do that in Perl6.
> >
> > On Tue, Nov 14, 2017 at 6:11 PM, Elizabeth Mattijsen <l...@dijkmat.nl>
> wrote:
> > > On 14 Nov 2017, at 18:06, Fernando Santagata <
> nando.santag...@gmail.com> wrote:
> > > I'm converting a program from Perl5/Moose.
> > > I have several classes, each has some attributes that need to be
> processed in the same way before being passed to other objects.
> > >
> > > When I was using Moose, I had some "around" methods that would
> automatically modify the value before delivering it to those attributes, so
> delegating the object to do the needed adjustments.
> > >
> > > Stripped to the bare bones, the thing that in Perl6 looks like this:
> > >
> > > class A {
> > > has $!a;
> > >
> > > method a($val?)
> > > {
> > > if $val.defined {
> > > # Modify $val in some way
> > > $!a = $val;
> > > } else {
> > > $!a;
> > > }
> > > }
> > > }
> > >
> > > my A $a .= new;
> > > # $a.a = 42; # This outputs an error
> > > $a.a(42);
> > > say $a.a;
> > >
> > > Any hint how to make it work as an assignment, instead of a method
> call?
> > > Better yet, is there a way to abstract that behavior in a role?
> >
> > I think you want “is rw” on a public attribute?
> >
> > class A {
> > has $.a is rw;
> > }
> > my $obj = A.new;
> > $obj.a = 42;
> > dd $obj;
> > ===========
> > A $obj = A.new(a => 42)
> >
> >
> >
> > Liz
> >
> >
> >
> > --
> > Fernando Santagata
>
--
Fernando Santagata
