SIP-54: Multi-Source Extension Overloads.

Author: sjrd

content/multi-source-extension-overloads.md

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+---
+layout: sip
+permalink: /sips/:title.html
+stage: implementation
+status: waiting-for-implementation
+title: SIP-54 - Multi-Source Extension Overloads
+---
+
+**By: Sébastien Doeraene and Martin Odersky**
+
+## History
+
+| Date          | Version            |
+|---------------|--------------------|
+| Mar 10th 2023 | Initial Draft      |
+
+## Summary
+
+We propose to allow overload resolution of `extension` methods with the same name but imported from several sources.
+For example, given the following definitions:
+
+```scala
+class Foo
+class Bar
+
+object A:
+  extension (foo: Foo) def meth(): Foo = foo
+  def normalMeth(foo: Foo): Foo = foo
+
+object B:
+  extension (bar: Bar) def meth(): Bar = bar
+  def normalMeth(bar: Bar): Bar = bar
+```
+
+and the following use site:
+
+```scala
+import A.*
+import B.*
+
+val foo: Foo = ???
+foo.meth() // works with this SIP; "ambiguous import" without it
+
+// unchanged:
+meth(foo)() // always ambiguous, just like
+regularMeth(foo) // always ambiguous
+```
+
+## Motivation
+
+Extension methods are a great, straightforward way to extend external classes with additional methods.
+One classical example is to add a `/` operation to `Path`:
+
+```scala
+import java.nio.file.*
+
+object PathExtensions:
+  extension (path: Path)
+    def /(child: String): Path = path.resolve(child).nn
+
+def app1(): Unit =
+  import PathExtensions.*
+  val projectDir = Paths.get(".") / "project"
+```
+
+However, as currently specified, they do not compose, and effectively live in a single flat namespace.
+This is understandable from the spec--the *mechanism**, which says that they are just regular methods, but is problematic from an intuitive point of view--the *intent*.
+
+For example, if we also use another extension that provides `/` for `URI`s, we can use it in a separate scope as follows:
+
+```scala
+import java.net.URI
+
+object URIExtensions:
+  extension (uri: URI)
+    def /(child: String): URI = uri.resolve(child)
+
+def app2(): Unit =
+  import URIExtensions.*
+  val rootURI = new URI("https://www.example.com/")
+  val projectURI = rootURI / "project/"
+```
+
+The above does not work anymore if we need to use *both* extensions in the same scope.
+The code below does not compile:
+
+```scala
+def app(): Unit =
+  import PathExtensions.*
+  import URIExtensions.*
+
+  val projectDir = Paths.get(".") / "project"
+  val rootURI = new URI("https://www.example.com/")
+  val projectURI = rootURI / "project/"
+  println(s"$projectDir -> $projectURI")
+end app
+```
+
+*Both* attempts to use `/` result in error messages of the form
+
+```
+Reference to / is ambiguous,
+it is both imported by import PathExtensions._
+and imported subsequently by import URIExtensions._
+```
+
+### Workarounds
+
+The only workarounds that exist are unsatisfactory.
+
+We can avoid using extensions with the same name in the same scope.
+In the above example, that would be annoying enough to defeat the purpose of the extensions in the first place.
+
+The only other possibility is to *define* all extension methods of the same name in the same `object` (or as top-level definitions in the same file).
+This is possible, although cumbersome, if they all come from the same library.
+However, it is impossible to combine extension methods coming from separate libraries in this way.
+
+### Problem for migrating off of implicit classes
+
+Scala 2 implicit classes did not suffer from the above issues, because they were disambiguated by the name of the implicit class (not the name of the method).
+This means that there are libraries that cannot migrate off of implicit classes to use `extension` methods without significantly degrading their usability.
+
+## Proposed solution
+
+We propose to relax the resolution of extension methods, so that they can be resolved from multiple imported sources.
+Instead of rejecting the `/` call outright because of ambiguous imports, the compiler should try the resolution from all the imports, and keep the only one (if any) for which the receiver type matches.
+
+Practically speaking, this means that the above `app()` example would compile and behave as expected.
+
+### Non-goals
+
+It is *not* a goal of this proposal to allow resolution of arbitrary overloads of regular methods coming from multiple imports.
+Only `extension` method calls are concerned by this proposal.
+The complexity budget of relaxing *all* overloads in this way is deemed too high, whereas it is acceptable for `extension` method calls.
+
+For the same reason, we do not propose to change regular calls of methods that happen to be `extension` methods.
+
+### Specification
+
+From the [specification of extension methods](https://docs.scala-lang.org/scala3/reference/contextual/extension-methods.html#translation-of-calls-to-extension-methods), we amend step 1. of "The precise rules for resolving a selection to an extension method are as follows."
+
+Previously:
+
+> Assume a selection `e.m[Ts]` where `m` is not a member of `e`, where the type arguments `[Ts]` are optional, and where `T` is the expected type.
+> The following two rewritings are tried in order:
+>
+> 1. The selection is rewritten to `m[Ts](e)`.
+
+With this SIP:
+
+> 1. The selection is rewritten to `m[Ts](e)` and typechecked, using the following slight modification of the name resolution rules:
+>
+>    - If `m` is imported by several imports which are all on the nesting level, try each import as an extension method instead of failing with an ambiguity.
+>      If only one import leads to an expansion that typechecks without errors, pick that expansion.
+>      If there are several such imports, but only one import which is not a wildcard import, pick the expansion from that import.
+>      Otherwise, report an ambiguous reference error.
+
+### Compatibility
+
+The proposal only alters situations where the previous specification would reject the program with an ambiguous import.
+Therefore, we expect it to be backward source compatible.
+
+The resolved calls could previously be spelled out by hand (with fully-qualified names), so binary and TASTy compatibility are not affected.
+
+## Alternatives
+
+A number of alternatives were mentioned in [the Contributors thread](https://contributors.scala-lang.org/t/change-shadowing-mechanism-of-extension-methods-for-on-par-implicit-class-behavior/5831), but none that passed the bar of "we think this is actually implementable".
+
+## Related work
+
+This section should list prior work related to the proposal, notably:
+
+- [Contributors thread acting as de facto Pre-SIP](https://contributors.scala-lang.org/t/change-shadowing-mechanism-of-extension-methods-for-on-par-implicit-class-behavior/5831)
+- [Pull Request in dotty](https://github.com/lampepfl/dotty/pull/17050) to support it under an experimental import
+
+## FAQ
+
+This section will probably initially be empty. As discussions on the proposal progress, it is likely that some questions will come repeatedly. They should be listed here, with appropriate answers.