Pass Search term and Operator to Scala Polymorphic Function

Question

I have this polymorphic function:

def findFirst[A](as: Array[A], p: A => Boolean): Int = {
    @annotation.tailrec
    def loop(n: Int): Int = {
        if(n >= as.length) -1
        else if(p(as(n))) n
        else loop(n + 1)
    }
    loop(0)
}

From functional programming in Scala and I want to pass the operator < Is it equal to a specific value >. How Would I do this? I currently have this:

println(findFirst(Array("abc", "def", "ghi"), == "def"))

Here the operator is < == "def">. Scala doesn't like that but I can't seem to figure out a way to pass in that value, operator pair. Something tells me currying could be used here?

Joe K · Accepted Answer · 2017-09-21 21:02:59Z

0

The full syntax to specify this function would be:

(x: String) => x == "def"

So you can do:

println(findFirst(Array("abc", "def", "ghi"), (x: String) => x == "def"))

However, if you change the findFirst definition to be two separate parameter lists:

def findFirst[A](as: Array[A])(p: A => Boolean): Int = {
  ...
}

Then scala can look at the first parameter (the array), see that A must be String, and infer that the type of the second parameter must be String => Boolean, so you can omit the type annotation:

println(findFirst(Array("abc", "def", "ghi"))(x => x == "def"))

Or better yet:

println(findFirst(Array("abc", "def", "ghi"))(_ == "def"))

answered Sep 21, 2017 at 21:02

Joe K

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ConorR · Accepted Answer · 2017-09-21 21:01:55Z

0

You just need an underscore:

println(findFirst(Array("abc", "def", "ghi"), _== "def"))

answered Sep 21, 2017 at 21:01

ConorR

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