Compiler infering type arguments at call site (when computing common super-type)

Question

Only v2 compiles bellow. I'd like instead to be able to call my apply method as in the v1 example. The compiler seems to be too quick to assign Nothing to my R variable

(inferred type arguments [...A,Nothing] do not conform to method apply's type parameter bounds [T1 <: Nothing,T2 <: Nothing] val v1 = apply(3)(new A)(new B))

class C
class A extends C
class B extends C

object O {
def apply[T1 <: R, T2 <: R, R](d : Int)(succ : T1)(fail : T2): R = if (d<0) succ else fail

  val v1 = apply(3)(new A)(new B)

  val v2 = apply[A, B, C](3)(new A)(new B)
}

The compiler has no easy way to infer R. I'm not sure why you need any type parameters at all though, because nothing is truly constrained. R could always be Any, which doesn't do much good. — Michael Zajac
Because later I can do structural recursion on R (So R is supposed to be a sum type) — Cristian Martin

Cristian Martin Cristian Martin · Accepted Answer · 2017-04-04T09:29:59

So I think I've found a solution for my question. I figured collections do something similar when adding two of different types. So this is something that infers its result automatically :

def apply[S, R >: S](d : Int)(succ : S, fail : R): R = if ( d < 0 ) succ else fail

Oddly only works when succ & fail belong to the same parameter list. I wonder why is that

Compiler infering type arguments at call site (when computing common super-type)

2 Answers