recursion-to-map.md

Recursion to Map

Relates To

• Recursion to Filter
• Recursion to Reduce

Languages

All

Motivation

In functional programming map, reduce and filter (and their synonyms) become fundamental building blocks for your programs.

map is used when you want create a one-to-one mapping values in a collection to new values via a transformation function.

Using map instead of a loop or recursive function creates more declarative code. It describes the process which you are performing on the data, rather than how you are implementing it.

It also separates to the responsibility of iterating over the collection from the per item transformation rules. This increases reusability and composability.

Description

Any time where you have a recursive function or a loop which iterates over a list and peforms a calcuation on each indiviual item, you have a good case to use map instead.

The in order to refactor you need to extract the transformation function and then pass that to map instead.

Examples

Clojure

Given

(defn tax-multiplier [rate] (+ 1 (/ rate 100)))
(defn with-tax [rate amount] (* amount (tax-multiplier rate)))

Before

(defn amounts-with-tax
  [tax-rate amounts]
  (if (empty? amounts)
      []
      (cons (with-tax (head amounts)) (amounts-with-tax tax-rate (tail amounts)))))

After

(defn amounts-with-tax
  [tax-rate amounts]
  (map (partial with-tax tax-rate) amounts))