Map, Filter, Reduce: Your First Functional Toolkit in Phel

Phel, as many other functional programming languages, comes with three basic tools you should learn right from the beginning:

Map, filter and reduce.

• Map transforms one sequence into another sequence of the same length.
• Filter removes elements from a sequence depending on some predicate function.
• Reduce takes a sequence of elements and aggregates it into some value.

Let's see them in action:

The map function takes two arguments. The first argument is a one-argument function that transforms a single value. The second argument is the sequence that should be transformed.

;; Increment by 1
(map inc [1 2 3]) ; => [2 3 4]
;; Multiply by 2 using fn syntax
(map (fn [x] (* 2 x)) [1 2 3]) ; => [2 4 6]
;; Multiply by 2 using fn shorthand syntax
(map #(* 2 %) [1 2 3]) ; => [2 4 6]

The filter function takes two arguments. The first argument is a one-argument function that returns true if it should keep the value in the list. The second argument is the sequence that should be filtered.

;; keep even numbers
(filter even? [1 2 3]) ; => [2]
;; keep odd numbers
(filter odd? [1 2 3]) ; => [1 3]
;; keep numbers bigger than 2
(filter #(> % 2) [1 2 3]) ; => [3]

The reduce function takes a function and a sequence. The function receives the accumulated value and the next sequence value, then returns the new accumulated value. You can also pass an explicit initial value when you need one.

;; sum all values
(reduce + [1 2 3]) ; => 6
;; multiply all values
(reduce * [2 3 4]) ; => 24
;; concat all numbers to a string
(reduce str "" [1 2 3]) ; => "123"