Scala, Composing Function with two values

Question

def adder(a:Int,b:Int):Int = {a+b}
def doubler(a:Int):Int = {a*2}
def doubleAdd = doubler _ compose adder

I get the error: type mismatch found: (Int,Int)=>Int required: ? => Int

Then if I just try doubleAdd = doubler(adder _) I get the same error except required Int instead of ? => Int

Is there a way of composing a function with two parameters? Sorry if this is pretty basic, I'm pretty new to the language, and I couldn't find an example with two parameters anywhere.

Answer 1

You're attempting to compose a Function2 (adder) with a Function1, hence the issue. One workaround is to change your definition of Adder to a curried version:

def adder(a: Int)(b: Int):Int = a + b

Then doubleAdd to partially apply adder like this:

def doubleAdd(x: Int) = doubler _ compose adder(x)

What's happening under the hood is transforming adder from a Function2 (Int, Int) => Int, to a Function1 (Int) => (Int) => Int, or a function that returns a function. You're then able to compose the function returned from adder with the first parameter already applied.