采用函数参数的重载泛型方法中的“缺少参数类型”

Question

I am having a problem in my DSL with overloaded generic methods resulting in the compiler wanting me to add explicit parameter types:

def alpha[T](fun: Int => T): String = fun(33).toString

def beta [T](fun: Int => T): String = fun(66).toString
def beta [T](thunk:   => T): String = thunk.toString

alpha { _ + 11 }          // ok
beta { _ + 22 }           // "error: missing parameter type for expanded function"
beta { _: Int => _ + 22 } // ok... ouch.

Any chance I can get rid of the the clutter in the last line?

EDIT:

To demonstrate that the overloading is not an ambiguity problem to scalac per se, here is a version without type parameter which works perfectly fine:

def beta(fun: Int => String): String = fun(66).reverse
def beta(thunk:   => String): String = thunk.reverse

beta(_.toString)  // ok
beta("gaga")      // ok

Answer 1

The problem is that Int => T is also a type. For example, say you defined just the second beta :

def beta[ T ]( thunk: => T ) : String = thunk.toString

And now you pass a function Int => Int to it:

scala> beta((_: Int) + 1)
res0: String = <function1>

So, given that a function fits => T , and that you also have an Int => T , how is Scala supposed to know which one you want? It could be a String , for instance:

scala> beta((_: String) + 11)
res1: String = <function1>

How could Scala assume it was an Int ? The examples you have shown to demonstrate overload isn't to blame don't demonstrate any such thing, because you got rid of the type parameters in them.

Answer 2

As you might have realized, the issue occurs because you have beta function is overloaded. When you define:

beta { _ + 22 }

which beta do you expect it do call? Scala cannot know that _ is an Int just because you are summing it with 22. So for this particular example, you have to define what _ is.