Home > Raku > The absence of a riddle

The absence of a riddle

Raku riddles have become popular. By chance, I came across a riddle generated by Rakudo — an emergent riddle so to speak.

class NotNil is Nil {};

class C {
    method NotNil() { NotNil.new }
};

sub s(NotNil()) {};

s(NotNil.new)

# OUTPUT: Impossible coercion from 'Nil' into 'NotNil': method NotNil returned a type object Nil

The culprit here is .new.

dd NotNil.new;
# OUTPUT: Nil

The constructor Nil.new will always return Nil. As lizmat pointed out, we need to provide our own constructor.

class NotNil is Nil {
    method new { self.CREATE }
}

The lesson I learned from this is that I can’t rely on inheritance for object creation. This is something we need to keep in mind when subclassing from a 3rd party module.

Categories: Raku
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  1. May 3, 2021 at 16:16

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