Error index entry for L1318

Summary:
The "update entry" error is un-intuitive, especially in presence of macros. Circumventing the warning via a `begin/end` block is also not intuitive, so let's make the information discoverable by developers.

If we find a better way to circumvent the warning, we can update this page.

Reviewed By: alanz

Differential Revision: D58005331

fbshipit-source-id: 838cbb2160cf04b338684c6ceceb6eb8cd3d63b4

website/docs/erlang-error-index/l/L1318.md

@@ -0,0 +1,60 @@
+---
+sidebar_position: 318
+---
+
+# L1318 - Expression Updates a Literal
+
+## Error
+
+```erlang
+  -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:
+
+```erlang
+> #{a => b}#{c => d}
+#{c => d,a => b}
+```
+
+```erlang
+> 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:
+
+```erlang
+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:
+
+```erlang
+  -define(DEFAULT, #{a => 1}).
+
+  updated(Value) ->
+    ?DEFAULT#{a => Value}.
+```
+
+Into:
+
+```erlang
+  -define(DEFAULT, #{a => 1}).
+
+  updated(Value) ->
+    begin ?DEFAULT end#{a => Value}.
+```