Scala By Example Chapter 5 Solutions - Exercise 5.2.1

Exercise 5.2.1 1. The sum function uses a linear recursion. Can you write a tail- recursive one by filling in the ??’s?

The key is to understand that converting linear recursion to a tail-recursive version involves carrying over a computed value over to the next iteration. Also, note that the call to the iter function at the end need not take in a and b as parameters directly but can be computed values. Thats what facilitates the tail-recursive version. Here's my solution to the exercise.

	object FirstClassFunctions {
	def sum(f: Int => Int)(a: Int, b:Int): Int = {
	def iter(a: Int, result: Int): Int =
	if (a > b) result
	else iter(a + 1, f(a) + result)
	iter(a,0)
	}
	def main(args: Array[String]) {
	println("Sum is: " + sum(x => x)(1,10))
	}
	}

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