Passing arguments to higher order functions - scala

Question

how do I pass different parameters for my function inside a higher order function. my code below.
val sum = (a: Int, b:Int) => a + b
val x = 7
val y = 9
def sq (x: Int, y: Int, f: (Int, Int)=> Int): Int = {
x + y * f(a,b)
}
println(sq(x,y,sum(2,3)))
This doesn't work and throws an error that 'a' is not defined.

Answer 1

The parameter list for sq looks ok.
The call to sq isn’t right, the function would get evaluated before it gets passed in. Assuming it got by the compiler, which of course it doesn’t.
Passing in a function does not mean its arguments come along for the ride. Passing in a function means the caller may not know what arguments to use or even if the function should be called. The sq function has to supply arguments when it calls it. a and b mean nothing outside the definition of sum. sq could supply x and y as arguments to sum.

Answer 2

Just as an addition to #Nathan answer. This example would make sense:
val sum = (a: Int, b:Int) => a + b
def sq (x: Int, y: Int, f: (Int, Int)=> Int): Int = {
x + y * f(x, y)
}
val x = 7
val y = 9
println(sq(x,y,sum))
This would be evaluated in your sgfunction like:
7 + 9 * sum(7, 8)
I hope this makes sense.

Answer 3

In addition to the #Nathan Hughes answer, if you want to pass the different argument to the sum function on each call you need to change your sq method like
def sq (x: Int, y: Int, f: (Int, Int)=> Int): (Int, Int)=> Int = {
x + y * f(_: Int, _:Int)
}
Insted of expecting the int from the sq method return partially applied function of type (Int, Int) => Int from `sq' then pass other argument to that function like,
sq(10, 20, sum)(1, 2) // 1st approach
//2nd approach would be
//You can hold the function in another variable and call that function with other arguments
val partialSum = sq(10,20, sum)
partialSum(1, 2)
and you will get your result.
Or if you still want that sq method should return Int, you can define your sq method like
def sq (x: Int, y: Int, f: (Int, Int)=> Int)(a:Int, b:Int):Int = {
x + y * f(a, b)
}
scala> sq(10,20, sum)(1,2)
res2: Int = 70