Generic nested type inference works with arity-2 b

问题:

Trying to figure out why the code compiles for nested type inference on method with arity-2 but not with currying.

object Test
{
    trait Version
    object VersionOne extends Version

    trait Request[A <: Version]
    trait RequestOne extends Request[VersionOne.type]

    case class HelloWorld() extends RequestOne

    def test1[A <: Version, T <: Request[A]](t : T, a : A): T = t
    def test2[A <: Version, T <: Request[A]](t : T)(a : A): T = t
}

// This works
Test.test1(Test.HelloWorld(), Test.VersionOne)

// This doesn't
Test.test2(Test.HelloWorld())(Test.VersionOne)

test2 fails to compile with the following error:

Error:(22, 73) inferred type arguments [Nothing,A$A96.this.Test.HelloWorld] do not conform to method test2's type parameter bounds [A <: A$A96.this.Test.Version,T <: A$A96.this.Test.Request[A]] def get$$instance$$res1 = /* ###worksheet### generated $$end$$ */ Test.test2(Test.HelloWorld())(Test.VersionOne)

Looking forward to some insights on the same.

回答1:

@DmytroMitin already explained why it does fail.

However, you can solve the problem this way, using the Partially-Applied Type trick, together with Generalized Type Constraints.

def test2[T](t: T): Test2PartiallyApplied[T] = new Test2PartiallyApplied(t)

final class Test2PartiallyApplied[T](private val t: T) extends AnyVal {
  def apply[A <: Version](a: A)(implicit ev: T <:< Request[A]): T = t
}

Which you can use like this.

Test.test2(Test.HelloWorld())(Test.VersionOne)
// res: HelloWorld = HelloWorld()

回答2:

Nothing in compile error usually means that some type parameters were not inferred.

Try to specify them explicitly

Test.test2[Test.VersionOne.type, Test.RequestOne](Test.HelloWorld())(Test.VersionOne)

Difference between test1 and test2 is not only in currying. For example, generally in test2(t: ...)(a: ...) type of a can depend on value of t. So for test2 type inferrence is more complicated than for test1.

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