The macro ->>

Today's exercise is to make use of the macro ->>.

Consider the case where we have a list of numbers [2 5 4 1 3 6].  We wish to drop the first two, take the next five, increase each one and then add them all together.  Recall that the way to apply a function to a list of elements, such as to add up a list of numbers, is to user reduce:-

user=> (reduce + [2 5 4 1 3 6])
21

So the whole task looks like this:-

user=> (reduce + (map inc (take 3 (drop 2 [2 5 4 1 3 6]))))
11

The macro ->> lets us rewrite this so that the function calls are one after another instead of one inside another.  So this same calculation can be written:-

user=> (->> [2 5 4 1 3 6] (drop 2) (take 3) (map inc) (reduce +))
11