LYR-SEM0124

an extend block on an interface

error · spec 05-interfaces

An 'extend' block names an interface as its target.

Explanation

Methods for every conformer of an interface are added through the generic form, extend<T :: [Walker]> T { … }. extend Walker { … } would be a second spelling of it, and a conformance of an interface to an interface has no meaning either, so the form is refused.

Write the generic form, or put the method into the interface as a default.

Wrong

interface Walker { fn walk(): string; }

extend Walker {
    fn describe(): string { return "w"; }
}

fn main(): void {
}

Right

interface Walker { fn walk(): string; }

extend<T :: [Walker]> T {
    fn describe(): string { return "w"; }
}

fn main(): void {
}