Raise on keys for non existing variables in matches

[tomekowal]

Hi! First of all: fantastic project!

Thanks you and sharing how the lib helps my usecase

I was writing a credo check. I needed to match on "pipe operator where the second param is function application". Those kinds of patterns are pretty big:

{:|>, a,
          [b, {{:., c, [{:__aliases__, meta, [module_name]}, function_name]}, d, params}]},

but using the lib makes them more manageable:

  defpat(pipe({:>, meta, [before_pipe, after_pipe]} = ast))

  defpat(
    function_call(
      {{:., dot_meta, [{:__aliases__, module_meta, [module_name]}, function_name]}, params_meta,
       params} = ast
    )
  )

  defp traverse(
         pipe(
           meta: a,
           before_pipe: b,
           after_pipe:
             function_call(
               dot_meta: c,
               module_meta: meta,
               module_name: module_name,
               function_name: function_name,
               params_meta: d,
               params: params
             )
         ),
         ...) do

Thank you very much for making composing pattern matches easier! I wanted to share with you how much easier it makes writing those checks :)

Actual issue

If I make a pattern like this:

  defpat ok({:ok, val})

but then misuse it like this:

  def my_fun(ok(value: value)) do
    value
  end

Notice that the binding is called val but used :value as the key in the keyword list. The error I get is:

warning: variable "value" does not exist and is being expanded to "value()", please use parentheses to remove the ambiguity or change the variable name

If I understand correctly, it means that the key value is ignored when there is no binding called this way. It might be a little bit confusing in case of typos to understand what is happening.

Would you consider it a good idea to raise in case usage of a given pattern tries to access non-existent binding?

If yes, I could try digging into it over the weekend and checking if I understand the lib enough to make that change.