Improve error message when variables are undefined

[rs86]

If I try to use an undefined variable, the compiler will give out an error saying there is no function/0 defined. It is trying to parse the variable as a 0-arity function.

== Compilation error on file lib/x.ex ==
** (CompileError) lib/x.ex:36: function somevariable/0 undefined
(stdlib) lists.erl:1337: :lists.foreach/2
(stdlib) erl_eval.erl:669: :erl_eval.do_apply/6

This makes the programmer think at first that there is a problem with a function.

Is there any way to give a friendlier error message? Suggestion: if the compiler can't find a null-arity function, add to the message:

"This might be an undefined variable called xxxx".

[ericmj]

I agree, the error message can be confusing and should be improved to also include that an undefined variable can be the cause.

[pdilyard]

Where might the code for this be found? I've found the definition of CompileError in lib/elixir/lib, but I can't seem to find where this is being raised with the given message. I'm assuming it's in Erlang code somewhere?

I also notice that this error is slightly different in iex:

** (CompileError) iex:1: undefined function abc/0

as opposed to:

** (CompileError) undefined.exs:3: function abc/0 undefined

If someone could give me a little pointer, I'd love to get started contributing and figured this would be a good issue to begin with.

[josevalim]

The error is coming from within Erlang, so we need to investigate what is the format and wrap it in the elixir_errors.erl file.

[josevalim]

Good point @pdilyard. our error messages are inconsistent here. Are you ok if I take this issue and fix this and other error messages?

[pdilyard]

Go ahead, thanks!

@josevalim tag me in your pull request or something, I want to make sure I understand the changes you make.

[josevalim]

Actually, we can't fix this. Since the error is coming from Erlang, we no longer have a distinction between a variable and a function call, which means changing this would mean we would change the message for both x and x(). This will be a won't fix for now. Sorry.