flat-map

Map a function over a collection and flatten the result by one-level

It's common to want to map a function on an input list that returns multiple values in a list, but you don't want the output nested in the same way as the input was.

ruby…
["two birds", "three green peas"].
  flat_map {|s| s.split}
# => ["two", "birds", "three", "green", "peas"]
clojure…
(mapcat #(clojure.string/split  % #"\s+") 
        ["two birds" "three green peas"])
;; => ("two" "birds" "three" "green" "peas")

Logically this is the same as a map followed by a single-level flatten.

ruby…
["two birds", "three green peas"].
  map {|s| s.split}.
  flatten (1)
# => ["two", "birds", "three", "green", "peas"]

But it's so commonly needed that many platforms provide a flat-map operation.

You can also consider it to be the same as taking all the results of the map and concatenating the results together - hence the clojure name of "mapcat".

clojure…
(apply concat (map #(clojure.string/split  % #"\s+") 
                   ["two birds" "three green peas"]))
;; => ("two" "birds" "three" "green" "peas")

mapcat

This page describes an operation in the collection pipeline pattern. For more context read: