Scala get value from lambda - scala

Question

I want to get value from function that passed as parameter and returns Option[Int], after that if I have None throw an exception and in any other case return value
I tried to do like this:
def foo[T](f: T => Option[Int]) = {
def helper(x: T) = f(x)
val res = helper _
res match {
case None => throw new Exception()
case Some(z) => z
}
I call it like this:
val test = foo[String](myFunction(_))
test("Some string")
I have compilation error with mismatched types in match section (Some[A] passed - [T] => Option[Int] required)
As I understood res variable is reference to the function and I cannot match it with optional either call get\gerOrElse methods.
Moreover I probably just dont get how the underscore works and doing something really wrong, I'm using it here to pass a something as parameter to function f, can you explain me where I made a mistake?

Answer 1

helper is a method taking a T and returning an Option[Int].
res is a function T => Option[Int].
Difference between method and function in Scala
You can't match a function T => Option[Int] with None or Some(z).
You should have an Option[Int] (for example the function applied to some T) to make such matching.
Probably you would like to have
def foo[T](f: T => Option[Int]) = {
def helper(x: T) = f(x)
val res = helper _
(t: T) => res(t) match {
case None => throw new Exception()
case Some(z) => z
}
}
or just
def foo[T](f: T => Option[Int]): T => Int = {
t => f(t) match {
case None => throw new Exception()
case Some(z) => z
}
}
or
def foo[T](f: T => Option[Int]): T => Int =
t => f(t).getOrElse(throw new Exception())