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W0060 - Bound Variable in LHS

Error​

handle_request(Message) ->
  Message = next_action().
%% ^^^^^^^ 💡 warning: W0060: Match on a bound variable

Explanation​

This diagnostic flags cases where a variable that is already bound appears on the left-hand side (LHS) of a match expression. This can be problematic if the binding is not intentional and can lead to subtle bugs.

Consider the following code snippet:

foo() ->
    AA = foo(),
    AA = bar().

The pattern on line 3 will only match if and only if the result of the call to bar/0 is the same as the call to foo/0. This behaviour could be intentional or not. If not, it can easily lead to bugs.

An alternative, more explicit, way to express that behaviour - when intentional - could be:

foo() ->
    AA = foo(),
    BB = bar(),
    AA = BB.

Or using an assertion:

foo() ->
    AA = foo(),
    ?assertEqual(AA, bar()).

Semantic highlighting​

Note that we also have semantic highlighting of the more general case, where a bound variable appears in any pattern.