Hard-won Scala 3 knowledge: make your type-holes the last type-parameter

som-snytt · October 9, 2025, 11:54am

The smarter comment on leveraging the type alias:

Comment by smarter - Dealias type when type parameters inference occurs

main ← Alex1005a:dealias-in-typeparams

Sorry for the late reply! So having type aliases influence type inference here w…as a conscious choice so that users could customize higher-order unification by defining their own type aliases. The logic proposed in this PR is sound, but I worry it's making the rules around type inference more complex. An alternative here would be to use a curried type alias to get unification to do exactly what you want: ```scala //> using dep org.typelevel::cats-core:2.13.0 import cats.Functor import cats.syntax.all.* type Result[Err, Res] = [F[_]] =>> F[Either[Err, Res]] def test[F[_]: Functor](x: Result[String, Int][F]) = x.map(identity) // ok ```