Re: The package/inherit trap

[Mark H Weaver <mhw@netris.org>]

Hello again,

I made a mistake in my previous message.

Earlier, I wrote:
> Consider 'kodi/wayland' as an example:
>
> __ (define-public kodi/wayland
> ____ (package
> ______ (inherit kodi)
> ______ (name "kodi-wayland")
> ______ (arguments
> _______ (substitute-keyword-arguments (package-arguments kodi)
> _________ ((#:configure-flags flags)
> __________ `(cons "-DCORE_PLATFORM_NAME=wayland"
> _________________ (delete "-DCORE_PLATFORM_NAME=x11" ,flags)))))
> ______ (inputs
> _______ (modify-inputs (package-inputs kodi)
> _________ (prepend libinput
> __________________ libxkbcommon
> __________________ waylandpp
> __________________ wayland-protocols)))
> ______ (synopsis "Kodi with Wayland rendering backend")))
>
> Suppose, at some future time, 'kodi' were given a replacement named
> 'kodi-with-fixes' to apply a security update.
>
> The question to ask yourself is this: would it make sense for
> 'kodi/wayland' to automatically be given a replacement field defined
> exactly as above except with (inherit kodi) changed to (inherit
> kodi-with-fixes)?

The above paragraph isn't quite right.  It should be:

> The question to ask yourself is this: would it make sense for
> 'kodi/wayland' to automatically be given a replacement field defined
> exactly as above except with all free references to 'kodi' within the
> 'package' (or 'package/inherit') form changed to 'kodi-with-fixes'?

Alternatively, for the sake of those who don't know what it means for a
variable reference to be "free" within a given expression, here's
another way to write it:

> The question to ask yourself is this: would it make sense for
> 'kodi/wayland' to automatically be given a replacement field defined
> exactly as above except with the 'package' (or 'package/inherit') form
> wrapped within (let ((kodi kodi-with-fixes)) ...).

This point is crucial.  Observe that in the 'kodi/wayland' definition,
the OVERRIDES for the 'arguments' and 'inputs' fields are defined as
incremental changes to (package-arguments kodi) and (package-inputs
kodi), respectively.

In order for (package/inherit BASE OVERRIDES ...) to work as intended,
the incremental changes made in OVERRIDES should be based upon the
variable BASE, because that is the only variable that will be rebound to
(package-replacement BASE) when automatically producing the replacement
for the derived package.

      Regards,
        Mark