Zenn
Closed1

wasmgc subtyping お遊戯会場

WebAssembly
tanishikingtanishiking
(module
  (rec
    (type $Base (sub (struct (field $Base_vtable (ref null $Base_vtable)))))
    (type $Base_vtable (sub (struct (field $foo i32))))

    (type $Derived (sub $Base (struct (field $Derived_vtable (ref null $Derived_vtable)))))
    (type $Derived_vtable (sub $Base_vtable (struct (field $foo i32))))
  )
)

これはok immutable fields は covariant

(module
  (rec
    (type $Base (sub (struct (field $Base_vtable (mut (ref null $Base_vtable))))))
    (type $Base_vtable (sub (struct (field $foo i32))))

    (type $Derived (sub $Base (struct (field $Derived_vtable (mut (ref null $Derived_vtable))))))
    (type $Derived_vtable (sub $Base_vtable (struct (field $foo i32))))
  )
)

これはだめ (mut ...) な mutable field は invariant なので Derived <: Base が成り立たない

このスクラップは2024/03/04にクローズされました