Scala strange implicit boxing conversion error

Question

Can someone tell me why the following does not work?

object TestObject {
    def map(f: (Double, Double) => Double, x2: Array[Double]) = {
        val y = x2.zip( x2 )
        val z = y.map(f)
        z
    }
}

Produces this error:

type mismatch; found : (Double, Double) => Double required: ((Double, Double)) => ?

Answer 1

In this snippet, f is a function taking two Double parameters and returning a Double. You are attempting to call f by passing a single argument of type Tuple2[Double,Double]. You can fix this by changing the type of f in the first place:

object TestObject {
    def map(f: ((Double, Double)) => Double, x2: Array[Double]) = {
        val y = x2.zip( x2 )
        val z = y.map(f)
        z
    }
}

You could also declare it as f: Tuple2[Double, Double] => Double to be clearer (this is entirely equivalent).

Conversely, you could change your call like this:

object TestObject {
    def map(f: (Double, Double) => Double, x2: Array[Double]) = {
        val y = x2.zip( x2 )
        val z = y.map(f.tupled)
        z
    }
}

tupled automagically transforms your (Double, Double) => Double function into a Tuple2[Double, Double] => Double function. Keep in mind however that the conversion will be done on each call to TestObject.map

Answer 2

There is a subtle difference between

f: (Double, Double) => Double // two Double arguments -> Double

and

f: ((Double, Double)) => Double // one Tuple2[Double, Double] argument -> Double

y is an Array[(Double, Double)], so it expects a method that takes a tuple and returns something, but the first f defined above doesn't match that.

You can do y.map(f.tupled) to go from the first to the second, or change the signature of f.

Answer 3

I think your issue is that f expects two Double arguments but you're attempting to pass it a single tuple: f((1, 2)) is different from f(1, 2).

Change the type of f to ((Double, Double) => Double) and it should work.