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W0033 - Lists Zip With Seq Rather Than Enumerate

Warning​

main(List) ->
    lists:zip(lists:seq(1,length(List)),List).
%%  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 💡 warning: Unnecessary intermediate list allocated.
main(Start, List) ->
    lists:zip(lists:seq(Start,length(List)),List).
%%  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 💡 warning: Unnecessary intermediate list allocated.
main(Start, Stride, List) ->
    lists:zip(lists:seq(Start,Stride,length(List)),List).
%%  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 💡 warning: Unnecessary intermediate list allocated.

Explanation​

The warning message indicates that a list (e.g. [a,b,c,...,z]) is being enumerated by position to (e.g. [{1,a},{2,b},{3,c},...{26,z}]) by creating a parallel list of the index values, which is then zipped with the input list.

Whilst this will correctly pair each list element with it's index, it is not considered idiomatic, and unnecessarily constructs an intermediate list of just the indices (e.g. [1,2,3,...,26]).

Using lists:enumerate functions will avoid constructing an intermediate lists, and hence be more performant. Pick which overload of lists:enumerate depending on whether you need to customise the starting index or stride between index values.

To fix the issue, call lists:enumerate/1, lists:enumerate/2 or lists:enumerate/3 directly on the input list:

main(List) ->
    lists:enumerate(List).
main(Start, List) ->
    lists:enumerate(Start, List).
main(Start, Stride, List) ->
    lists:enumerate(Start, Stride, List).