reduce-to-map-and-filter.md

Reduce to Map/Filter/Reduce

Relates To

• Recursion to Filter
• Recursion to Map
• Recursion to Reduce

Languages

All

Motivation

Both map and filter can be implemented using reduce. As a result, complex reductions are like to contain some map and/or filter like behaviour. By extracting these out into separate steps, we can create code which clearly documents each transformation step as a pipeline of single responsibilities, rather that creating one complex action.

Description

Study the body reduction function and identify parts which can are either translating all values or skipping values. Extract these parts into separate functions and use the with map or filter to create a pipeline of transformations.

Examples

Clojure

Given

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

Before

(defn order-total
  [items]
  (reduce
   (fn [total item]
    (if (:free-gift? item)
        total
        (+ total (with-tax (:amount item)))))
   0
   items))

After

(defn order-total
  [items]
  (->> items
       (filter (compliment :free-gift?))
       (map :amount)
       (map with-tax)
       (reduce + 0)))