Skip to main content

W0000 - Bound Variable in Pattern

Error​

caution

This error code is not emitted any longer. Semantic tokens are used instead.

Explanation​

Inspired by Elixir's pinning operator, this diagnostic marks variables in patterns, if they are already bound. This can be problematic if the binding is not intentional and it can lead to subtle bugs.

Consider the following code snippet:

handle_request(Message) ->
  case next_action() of
    {send, Message} ->
      ...
    {error, Error}
      ...
  end

The pattern on line 3 will only be matched if and only if the Message returned by the call to next_action/1 is the same as the one on line 1. 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:

handle_request(Message) ->
  case next_action() of
    {send, ActionMessage} when ActionMessage =:= Message ->
      ...
    {error, Error}
      ...
  end