Mixing parameterized role into enum value causes compile-time error

[landyacht]

The Problem

With more than one value in an enum, mixing a parameterized role into the value yields the obtuse comile-time error "Incompatible MROs in P6opaque rebless for types Int+{MixedInRoleName[Str]} mixin and EnumName". This does not happen when there is only one enum value, nor when the role is unparameterized.

Steps to Reproduce

role MixedInRoleName[$a] { has $.a = $a; }
enum EnumName (
    KeyOne => 1 but MixedInRoleName['Yksi'],
    KeyTwo => 2 but MixedInRoleName['Kaksi']
);

Environment

• Operating system: Linux 6.1.53-gentoo-r1 (reproduced also with raku eval bot in IRC)
• Raku version: Rakudo 2023.12 / MoarVM 2023.12

[lizmat]

ping @vrurg

[vrurg]

It lurks in my memory that values of an enum has to be of the same type. No way for me to recall where I saw this, neither I can find the place at the moment.

Trying to mix a number and a string results in exactly the same error. Apparently, the error itself is LTA, but otherwise it seems to be legal.

The particular example case can be easily fixed with this:

role MixedInRoleName { has $.a; }
enum EnumName (
    KeyOne => (1 but MixedInRoleName('Yksi')),
    KeyTwo => (2 but MixedInRoleName('Kaksi'))
);
say KeyOne.a;

Minor offtop: I still remember myself thinking "what the hell this 'mixin attribute' means and why would I need it?" :)

[landyacht]

Interesting. I suppose technically roles that are parameterized differently do cause the resulting type to be different. Still, might be worth leaving this open to improve the error message?

[vrurg]

Still, might be worth leaving this open to improve the error message?

Absolutely.

[vrurg]

I suppose technically roles that are parameterized differently do cause the resulting type to be different.

Yes, you're correct. This is why the approach you took in your example is not the best one in some scenarios. Imagine one having thousands of objects to be marked with roles-labels. In this case they get thousands of additional types, each consuming memory both to hold its VM representation and to maintain metaobject data related to the type.