Scoped Conformances

Evolution Pitches
34

Just copying a relevant insight I had here:

Semantically, all the issues of fileprivate conformances are shared by this example we can produce today:

// Module A
extension UInt: Collection { ... } // the numbers from 0 to `self`

// Module B
extension UInt: Collection { ... } // powers of 2 in self's representation

// Module C, File1.swift
import A
...

// Module C, File2.swift
import B
...

As long as nobody else imports A or B, the conformance from A is effectively private to File1.swift, and the one from B is effectively private to File2.swift, so the above would be equivalent to:

// Module C, File1.swift
extension UInt: fileprivate Collection { ... } // the numbers from 0 to `self`
...

// Module C, File2.swift
extension UInt: fileprivate Collection { ... } // powers of 2 in self's representation
...

Similarly, the semantic issues of internal conformances are shared by this example:

// Module A
extension UInt: Collection { ... } // the numbers from 0 to `self`
// Module A1, all files
import A

// Module B
extension UInt: Collection { ... } // powers of 2 in self's representation
// Module B1, all files
import B

which (as long as nobody else imports A or B), would be equivalent to:

// Module A1
extension UInt: internal Collection { ... } // the numbers from 0 to `self`

// Module B1
extension UInt: internal Collection { ... } // powers of 2 in self's representation

Scoped conformances provide the ability to eliminate:

  • module A
  • module B
  • all the metadata associated with exposing those conformances publicly

The latter goal was the original motivation for this pitch.

[Of course all of this depends on the idea that I understand the intended semantics of distinct imports in different files of the same module, which is currently probably buggy]

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