Neat! Just read the Proxy class doc at https://docs.perl6.org/type/Proxy
Instead of hooking into any accessor magic, this solution creates a Proxy object that lives inside the "a" method. Every time you use the "a" method, it generates a new Proxy object, which will either set or retrieve the $!a attribute from the A object. Two minor quibbles, and since I'm not the author or user, take them with a grain of salt: " $.a if you want to be able to assign with .new " - in that case, $!a won't go through the modifications. class A { has $.a; method a() { Proxy.new( FETCH => { $!a }, STORE => -> $, $value { $!a = $value * 2 } ) } } my $a = A.new(a => 23); dd $a.a; # 23 $a.a = 34; dd $a.a; # 68 A way to fix that, is to have the STORE function be a named routine that both the proxy object & a TWEAK submethod can use, as each sets $!a. The other quibble, is that it creates and discards the proxy object with each call. (Not at all a problem functionally and likely not anything that would affect performance either.) Here's a demo class A { has $!a; method a() { say "baking a new Proxy object for class A"; Proxy.new( FETCH => { $!a }, STORE => -> $, $value { $!a = $value * 2 } ) } } my $a = A.new(); # no output $a.a = 34; # "baking..." dd $a.a; # "baking...", "68" -y 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 >