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L1318 - Expression Updates a Literal

Error​

  -define(DEFAULT, #{a => 1}).

  updated(Value) ->
    ?DEFAULT#{a => Value}.
%%  ^^^^^^^^ warning: expression updates a literal

Explanation​

The warning occurs when a map or a record is updated using the following syntaxes:

> #{a => b}#{c => d}
#{c => d,a => b}
> rd(my_record, {a, b}). %% rd/2 allows you to define an Erlang record from a shell
> #my_record{a = 1}#my_record{a = 2}.
#my_record{a = 2,b = undefined}

While this is valid Erlang syntax, this behaviour is usually not intentional and the result of a missing comma in a list of elements. Consider, for example:

my_list() ->
  [ #{a => 1} %% Missing comma here!
    #{a => 2}
  ].

Which results in [#{a => 2}].

To fix the issue, just add the missing comma. If the update is intentional, a common (but ugly) workaround to silent the linter is to wrap the first map/record in a begin/end block, which will avoid any additional runtime cost. As an example, you could rewrite the following:

  -define(DEFAULT, #{a => 1}).

  updated(Value) ->
    ?DEFAULT#{a => Value}.

Into:

  -define(DEFAULT, #{a => 1}).

  updated(Value) ->
    begin ?DEFAULT end#{a => Value}.