Coercing type inference using partial unification 

EIP:

We will have a number of new datatypes with coercion functions like Id, unId, Const and unConst. To reduce clutter, we introduce a common notation for such coercions.

In Scala, implicits and type inference take us pretty far. But by dealing with typeclass a lot, we also end up seeing some of the weaknesses of Scala’s type inference. One of the issues we come across frequently is its inability to infer partially applied parameterized type, also known as SI-2712.

This was fixed by Miles Sabin in scala#5102 as ”-Ypartial-unification” flag. See also Explaining Miles’s Magic.

Here’s an example that Daniel uses:

def foo[F[_], A](fa: F[A]): String = fa.toString

foo { x: Int => x * 2 }
// res0: String = "<function1>"

The above did not compile before.

The reason it does not compile is because Function1 takes two type parameters, whereas F[_] only takes one.

With -Ypartial-unification it will now compile, but it’s important to understand that the compiler will now assume that the type constructors can be partially applied from left to right. In short, this will reward right-biased datatypes like Either, but you could end up with wrong answer if the datatype is left-biased.

In 2019, Scala 2.13.0 was released with partial unification enabled by default.