Extensions

• Rust has impl blocks for as the sole mechanism for attaching members to nominal targets

• TS++ has its as an additional mechanism

• like impl in Rust but a little broader

• inherent, anonymous, named extensions

• E / T, T may be local or imported

• extension of T

• extension E of T

• export extension of T

• export extension E of T

• an inhernt extension beside its target is visible wherever the target is visible

• an anonymous extension on a foreign target is visible only in its declaring file

• a named foreign extension must be imported explicitly

• extension<T> of T: blanket extension

• structural types, unions, and intersections cannot receive extensions

• rustc coherence

• overlapping implementations of one interface for one type, including blanket overlap, are errors

• no orphan rule?

• extension members are lexical, but implements contributes a program-wide relation whenever its module is in the program

• global extensions considered for impls (not import order)

• member overloading only within a single declaration block (extension or itme declaration)

• also for @unsafe impls

1newtype UserId = string;2 3extension of UserId {4    value(this): string {5        string(this)6    }7}

• extension members are lexical and import-scoped
• interface implementations participate in the whole program (even if not imported/exported)