SLC: Add Function.identity method

hepin1989 · May 17, 2026, 11:50am

Adding this will make it perform great and avoid the eta allocation.

github.com/scala/scala3

Add `scala.Function.identity` returning a shared singleton instance

main ← He-Pin:add-function-identity

opened 11:28AM - 17 May 26 UTC

He-Pin

+50 -1

## Summary Add an `identity` method on the `scala.Function` companion object …that returns a shared, singleton `T => T`. Unlike `scala.Predef.identity` (which is a method and gets eta-expanded into a fresh `Function1` instance every time it is used as a function value, e.g. `xs.map(identity)`), this version returns the same cached function instance regardless of the type parameter, by casting a `private final val constIdentityFunction: Any => Any = x => x`. ```scala private final val constIdentityFunction: Any => Any = x => x /** Returns a function that always returns its input argument as result. */ def identity[T]: T => T = constIdentityFunction.asInstanceOf[T => T] ``` This gives users an allocation-free option when they need an identity function value (`map`, `flatMap`, `andThen`, etc.). https://contributors.scala-lang.org/t/add-function-identity-method/7452 ## Changes - `library/src/scala/Function.scala`: add `constIdentityFunction` private singleton and public `identity[T]` method. - `project/MiMaFilters.scala`: forwards-compat MiMa filter for the newly added method. - `tests/run/function-identity.scala`: directional run test covering: 1. identity returns its input value; 2. **singleton sharing** across distinct type instantiations (`Function.identity[Int] eq Function.identity[String]` after erasure) — the key behavior; 3. usage as a function argument to `map` / `flatMap`; 4. parity with `scala.Predef.identity` for arbitrary inputs; 5. composition via `andThen` / `compose`. ## Test plan - [x] `library/src/scala/Function.scala` compiles (`sbt scala-library-nonbootstrapped/compile`) - [x] `tests/run/function-identity.scala` passes via `sbt 'project scala3-nonbootstrapped; testCompilation function-identity'` (CompilationTests: 16 phases, 0 failed) - [x] Test class also runs cleanly when invoked directly via `java` against the rebuilt library jar (exit 0)

bishabosha · May 18, 2026, 9:01am

i guess to evaluate this utility, we should compare this vs Predef.identity, and bare x => x on megamorphic callsite, and with inliner turned on/off.