[PATCH 2/6] build-system/emacs: Use "emacs" from native-inputs if specified.

[Federico Beffa <beffa@ieee.org>]

Alex Kost <alezost@gmail.com> writes:

> * guix/build-system/emacs.scm (lower): Do not add "emacs" to
>   build-inputs if it is already specified in the native-inputs.
> @@ -73,8 +75,16 @@
>
>                          ;; Keep the standard inputs of 'gnu-build-system'.
>                          ,@(standard-packages)))
> -         (build-inputs `(("emacs" ,emacs)
> -                         ,@native-inputs))
> +         ;; Add emacs to build-inputs only if native-inputs do not contain
> +         ;; emacs already.  This allows us to use non-default emacs for
> +         ;; building.
> +         (build-inputs (if (find (match-lambda
> +                                   (("emacs" _ ...) #t)
> +                                   (_ #f))
> +                                 native-inputs)
> +                           native-inputs
> +                           `(("emacs" ,emacs)
> +                             ,@native-inputs)))

Note that for the interpreter we normally use a keyword, here #:emacs
(in a similar way as, say, for the python-build-system there is
#:python).  Your code overwrites its effect in a non-transparent way.

Please use the keyword in the packages where you need an emacs package
different from the default one, or remove all the code related to the
keyword.  If you opt for the second one, please be consistent and do the
same for all other build systems.

Fede