Skip to main content

W0035 - Unnecessary lists:fold/3 to build map

Warning​

main(List) ->
    lists:foldl(fun(K, Acc) -> Acc#{K => []} end, #{}, List).
%%  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 💡 information: Unnecessary explicit fold to construct map from list.
main(List) ->
    lists:foldl(fun({K,V}, Acc) -> Acc#{K => V} end, #{}, List).
%%  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 💡 information: Unnecessary explicit fold to construct map from list.

Explanation​

The warning message indicates an unidiomatic and sub-optimal way to build a map from a list.

Whilst the fold will correctly construct a map from the given list, it is less efficient than it could be, since a BIF exists which can directly construct the required map.

Using the maps:from_keys/2 or maps:from_list/1 BIFs (depending on whether the list is of just keys or of key-value pairs, respectively), is clearer and more performant.

To fix this warning, the the BIFs from the maps module to construct your map directly:

main(List) ->
    maps:from_list(List). % For lists of key-value pairs
main(List) ->
    maps:from_keys(List, []). % For lists of just keys, where every key is given the same value