6.2

Exercise 6.2: Compose yourself

Composing functions means applying them in a chain, so that each function operates on the result of the previous one.

For example, if we had two arbitrary functions outer and inner, then we could compose them directly by writing:

outer(inner())

Your job in this exercise is to write a Compose function that will do just this. Given two functions, outer and inner, and a value of arbitrary type, it should first apply inner to the value, then apply outer to the result returned by inner, then return the result returned by outer.

Got all that? Here's an example. Suppose we had a function double that returns double its input, and another function addOne that returns the result of adding 1 to its input.

And suppose our input value is 1. If we first apply addOne to 1, we should get 2. If we then apply double to 2, we should get 4. So:

fmt.Println(Compose(double, addOne, 1))
// 4

What if we composed the functions in reverse order, so that we applied double first, then addOne to the result? Well:

fmt.Println(Compose(addOne, double, 1))
// 3

If you're having trouble with this, remember that the inner function will need to take a parameter of the same type as the value, and its result type is arbitrary. The outer function will need to take a parameter of whatever type inner returns, but its result type is also arbitrary.

So, all told, there are three type parameters involved here.

If you're still stuck, have a look at one possible way of doing it in Solution 6.2.

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