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Override Role Attribute

Is it possible to override the role attribute for the default provision?

role A { has $.a; } class B does A { has $.a = "default"; } my $b = B.new;

This results in a compilation error:

 ===SORRY!=== Error while compiling: Attribute '$!a' already exists in the class 'B', but a role also wishes to compose it
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2 answers

Since methods in R can reference $!a , there will be ambiguity about what the attribute refers to.

Use the BUILD subtopics to initialize the inherited / mixedin attributes.

 role R { has $.a }; class C does R { submethod BUILD { $!a = "default" } }; my $c = C.new; dd $c; # OUTPUT«C $c = C.new(a => "default")␤»

Depending on your account, you might be better off setting a default value using the role parameter.

 role R[$d] { has $.a = $d }; class C does R["default"] { }; my $c = C.new; dd $c; # OUTPUT«C $c = C.new(a => "default")␤»
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No, it is not possible to override an attribute, but you can initialize it using the subtext of the BUILD class:

 role A { has $.a; } class B does A { submethod BUILD(:$!a = 'default') {} }

Note that if you just set the value in the body of the BUILD instead of your signature via

 class B does A { submethod BUILD { $!a = 'default' } }

the user cannot overwrite the default value by providing a named initializer through B.new(a => 42) .

Another, more elegant approach, which is especially useful when installing by default, is that you expect to do a lot (that is, the default value can be considered as part of the role interface) makes it a parameter:

 role A[$d] { has $.a = $d; } class B does A['default'] {}
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