Scala get value from lambda

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?

sorry, can't edit question, after val test = .... I also call the test with some parameters like test("aaaa") — Pot001, Sep 25 '17 at 19:57
You *can* edit the question. There's an edit button underneath it. — Carcigenicate, Sep 25 '17 at 20:00
What does the signature of `myFunction` look like? It seems like it doesn't match what the input to `foo` wants. — Tanjin, Sep 25 '17 at 20:06
You say you want an `Option[Int]` output. What `Int` value do you expect for a given input? — jwvh, Sep 25 '17 at 20:07
@Tanjin def myFunction(digits: String): Option[Int] = if(digits.matches("[0-9]*")) Some(digits.toInt) else None — Pot001, Sep 25 '17 at 20:09
What `Int` **value**? 5? 72? For what string input? "blahblah"? "xxx"? — jwvh, Sep 25 '17 at 20:11
@jwvh For any number string - get corresponding int number, for non-numeric string - None — Pot001, Sep 25 '17 at 20:18

Dmytro Mitin · Accepted Answer · 2017-09-25T20:13:56.113

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())

Scala get value from lambda

1 Answers1