Higher-order functions

lists:map

The map function applies a fun to every item and returns the list of results. The input list is untouched.

lists:map(fun(X) -> X * 2 end, [1, 2, 3]).

[2,4,6]

2> lists:map(fun atom_to_list/1, [a, b]).

<<"a","b">>

lists:filter

The filter function keeps the items where the fun answers true. The rest drop away.

lists:filter(fun(X) -> X > 1 end, [1, 2, 3]).

[2,3]

2> lists:filter(fun(S) -> length(S) < 4 end, ["hi", "there"]).

<<"hi">>

lists:foldl

The foldl function walks a list and carries one accumulator through every step. Your fun takes the item and the accumulator, and returns the next accumulator.

lists:foldl(fun(X, Acc) -> Acc + X end, 0, [1, 2, 3]).

6

2> lists:foldl(fun(X, Max) -> max(X, Max) end, 0, [3, 1, 2]).

3

lists:foreach

The foreach function runs a fun for every item and answers the atom ok. Use it for side effects only: printing, logging, sending.

lists:foreach(fun(X) -> io:format("~p~n", [X]) end, [1, 2]).

1

2

ok

Next: Keyword lists

Article author: Arthur Isaev