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W0010 - Unused Function Argument

Error​

length([]) -> 0;
length([Head|Tail]) -> 1 + length(Tail).
     %% ^^^^ warning: this variable is unused

Explanation​

The message is indicating that the given variable is unused within the function clause body. To fix the warning you should either:

  • remove the unused parameter
  • replace it with an underscore (_)
  • prepend it with an underscore (_)

In the above snippet, we don't need to look at the actual value of the Head to calculate the length of the input list. Therefore, we can replace the variable name with an underscore:

length([]) -> 0;
length([_|Tail]) -> 1 + length(Tail).

Even better, we could keep the variable name and prepend it with an underscore. This way we'll get the best of both worlds: we will silent the warning, while keeping the code readable:

length([]) -> 0;
length([_Head|Tail]) -> 1 + length(Tail).

Let's look at a different scenario:

handle_message(hi, State) ->
                %% ^^^^^ warning: this variable is unused
    hi();
handle_message(bye, State) ->
                 %% ^^^^^ warning: this variable is unused
    bye().

In this case the State variable is not needed by any of the function clauses of the handle_message/2 function. Therefore, we could consider simplifying the function signature getting rid of the un-necessary second argument:

handle_message(hi) ->
    hi();
handle_message(bye) ->
    bye().

Keep in mind that in this case we may need to update callers of the handle_message/1 function!