How do I depend on macros from other packages?

Question

I'm writing a package that provides some Python functionality. I want to use python-rx, which is part of Emacs' python.el.

(eval-when-compile (require 'python))

(defun python-foo-starts-defun-p ()
  "Return non-nil if the current statement opens a defun."
  (save-excursion
    (python-nav-beginning-of-statement)
    (looking-at (python-rx defun))))

(provide 'python-foo)

However, this gives me byte compilation errors:

the function `python-nav-beginning-of-statement' might not be defined at runtime.

the function `python-rx' is not known to be defined.

This error is correct: when I run my code in a fresh Emacs, I do get an error about python-rx. However, I can open python.el and run M-x eval-buffer and then my code works.

I believe this is because python-rx is wrapped in an eval-when-compile in python.el. How can I use it in my code?

Answer 1

Firstly, that eval-when-compile method works fine for macros. It is a perfectly good way to use macros (not functions) from another package.

The problems you report are two different things.

the function python-nav-beginning-of-statement' might not be defined at runtime.

This is happening because python-nav-beginning-of-statement is a function, not a macro. Unlike macros (which are expanded at compile-time), functions have to be available at runtime. So that eval-when-compile doesn't work here. You need to use a plain (require 'python) to make sure that this function is available.

the function `python-rx' is not known to be defined.

This is a different warning. This is saying that python-rx wasn't found at all. The current stable Emacs (version 24.5) does not have this macro available.

Technically, the macro is there in the file, but it's defined in such a way that makes it unavailable for other files to use. This has been fixed on the development snapshots (futurely Emacs 25).

Answer 2

Regarding those warnings in general (Malabarba's answer covers your specific question nicely)...

When you know that a "Warning: the function X is not known to be defined." warning is unnecessary, you can use declare-function:

C-hig (elisp) Declaring Functions RET

C-hf declare-function RET

More generally, you want to read:

C-hig (elisp) Compiler Errors RET