Confusing error: "undefined function" when actually private

[nathanl]

Environment

• Elixir version (elixir -v): 1.3.0-dev
• Operating system: OS X 10.11

Current behavior

apply(__MODULE__, :name_of_private_function, [some,args] gives error "undefined function"

Expected behavior

I was confused by this. I figured out that apply is inlined to __MODULE__.name_of_function..., but it took a minute to understand that the function definition I was looking at wasn't being found because it was private.

I think that something like "undefined (or private) function Foo.bar/2" would be a clearer error.

[josevalim]

We could use something like String.module_info(:functions) to find out if it is private or not and show a proper error but I am really not sure if we should.

[lexmag]

I'm 👎 on doing so.

[sunny-mittal]

For what it's worth, I would agree that actually catering the error to private vs undefined functions would be a little unsafe, but amending the existing message to convey that the function is either undefined or private might be useful.

[nathanl]

I'd be happy with just amending the message as a friendly reminder.

[Qqwy]

I agree with @sunny-mittal on this. I find behaviour like Ruby, where it is explixitly stated when a private function is called. Although that is understandable there, with the way everything ought to be monkey patchable, I really think that that approach does not fit well in Elixir.

Changing the error message to hint at the possibility of a private method seems a lot cleaner.