Blog post about given prioritization changes

Author: bracevac

_posts/2024-08-01-given-priority-change-3.5.md

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+---
+layout: blog-detail
+post-type: blog
+by: Oliver Bračevac, EPFL
+title: "Changes to Givens Prioritization in Scala 3.5"
+---
+
+## Motivation
+
+Starting with Scala 3.5, the prioritization of givens has been
+improved to better handle inheritance triangles, resulting in enhanced
+typeclass support.
+
+Consider a scenario with the following inheritance triangle of type classes:
+```scala
+trait Functor[F[_]]:
+  extension [A, B](x: F[A]) def map(f: A => B): F[B]
+trait Monad[F[_]] extends Functor[F] { ... }
+trait Traverse[F[_]] extends Functor[F] { ... }
+```
+and corresponding instances:
+```scala
+given a:Functor[List]  = ...
+given b:Monad[List]    = ...
+given c:Traverse[List] = ...
+```
+Now, let’s use these in the following context:
+```scala
+def fmap[F[_] : Functor, A, B](c: F[A])(f: A => B): F[B] = c.map(f)
+fmap(List(1,2,3))(_.toString)
+// ^ rejected by Scala < 3.5, now accepted by Scala 3.5
+```
+
+Before Scala 3.5, the compiler would reject the `fmap` call due to
+ambiguity. Since it prioritizes the `given` instance with the _most
+specific_ subtype of the context bound `Functor`, both `c` and `b` are
+valid candidates for `Functor[List]`, but neither is more specific
+than the other. However, all we really need is the functionality of
+`a:Functor[List]`!
+
+In Scala 3.5, the compiler now selects the instance with the _most
+general_ subtype that satisfies the context bound of `fmap`. In this
+case, it chooses `a:Functor[List]`.
+
+Inheritance triangles like this are common in practice, and the
+prioritization change in Scala 3.5 makes working with them more
+intuitive and straightforward.
+
+## Tips for Migrating to 3.5
+
+Based on our evaluation using the [open community
+build](https://github.com/VirtusLab/community-build3), the impact of
+this change on existing Scala 3 projects has been minimal. However,
+there may still be cases where the behavior of existing programs
+changes due to the new prioritization of givens.
+
+In some cases, the new prioritization might silently select the wrong
+`given`. For example, consider a library that provides a default
+`given` for a component:
+```scala
+// library code
+class LibComponent:
+    def msg = "library-defined"
+
+// default provided by library
+given libComponent: LibComponent = LibComponent()
+
+def printComponent(using c:LibComponent) = println(c.msg)
+```
+
+Clients of the library might have relied on the “most specific”
+prioritization to override the default given with a user-defined one:
+```scala
+// client code
+class UserComponent extends LibComponent:
+    override def msg = "user-defined"
+
+given userComponent: UserComponent = UserComponent()
+
+@main def run = printComponent 
+// Scala < 3.5: prints "user-defined"
+// Scala 3.5: prints "library-defined"
+```
+
+To detect such "silent" changes, we recommend compiling under Scala
+3.5 with the `-source:3.6-migration` flag:
+```bash
+scalac client.scala -source:3.6-migration
+```
+This will issue warnings when the choice of `given` has changed:
+```scala
+-- Warning: client.scala:11:30 ------------------------------------------
+11 |@main def run = printComponent
+   |                              ^
+   |           Given search preference for LibComponent between alternatives
+   |             (userComponent : UserComponent)
+   |           and
+   |             (libComponent : LibComponent)
+   |           has changed.
+   |           Previous choice          : the first alternative
+   |           New choice from Scala 3.6: the second alternative
+```
+
+### Explicit Parameters
+
+If the pre-3.5 behavior is preferred, you can explicitly pass the
+desired given:
+```scala
+@main def run = printComponent(using libComponent)
+```
+
+To determine the correct explicit parameter (which could involve a
+complex expression), it can be helpful to compile with Scala 3.4 (or
+earlier) using the `-Xprint:typer` flag:
+```scala
+scalac client.scala -Xprint:typer
+```
+This will output all parameters explicitly:
+```scala
+...
+@main def run: Unit = printComponent(libComponent)
+...
+```
+
+### Outlook
+
+We are considering adding `-rewrite` rules that automatically insert
+explicit parameters when a change in choice is detected.