Subject: bug#50335: Inheritance vs. ‘this-package’

From: Ludovic Courtès <[email-masked]>
Date: Thu, 02 Sep 2021 15:48:27 +0200

Hi!

We stumbled upon an idiom for package arguments that, combined with
package inheritance, would not work as expected.  It goes like this
(taken from <https://gitlab.inria.fr/guix-hpc/guix-hpc/>):

--8<---------------cut here---------------start------------->8---
(define-public starpu
  (package
    ;; …
    (arguments
      (substitute-keyword-arguments (package-arguments starpu-1.2)
        ((#:configure-flags flags '())
         (match (assoc "mpi" (package-propagated-inputs this-package))
           (("mpi" mpi)
            (if (string=? (package-name mpi) "nmad")
                `(cons "--enable-nmad" ,flags)
                flags))))))
    (propagated-inputs `(("mpi" ,openmpi)))))

(define-public starpu+fxt
  (package
    (inherit starpu)
    ;; …
    (arguments
     (substitute-keyword-arguments (package-arguments starpu)
       ((#:configure-flags flags '())
        `(cons "--with-fxt" ,flags))))))
--8<---------------cut here---------------end--------------->8---

The first package, ‘starpu’, has its arguments depend on:

  (package-propagated-inputs this-package)

This is smart because if you inherit from it without changing the
‘arguments’ field (for instance by doing ‘--with-input=openmpi=nmad’),
it’ll adjust its configure flags based on the MPI package it actually
depends on.

However, this trick doesn’t work with the second package, ‘starpu-fxt’:
if you do:

  guix build starpu-fxt --with-input=openmpi=nmad

the ‘--enable-nmad’ configure flag will not be passed.  This is because
‘starpu-fxt’ calls:

  (package-arguments starpu)

which returns the original arguments of ‘starpu’.

Still here?  :-)

This is admittedly a pretty far-fetched example and there are ways to
work around it¹.  Still, I wonder if we could up with a linguistic
device (ah ha!) to address that.

We don’t want to (package-arguments this-package) here: that would
naturally lead to an infinite loop.  But we want something close to it,
in spirit: (package-arguments parent), where ‘parent’ is the package we
inherit from.

(package-arguments parent) could call the ‘arguments’ thunk of the
parent, but it would pass it the child as its argument.

Or something like that.

Thoughts?

Ludo’.

¹ https://gitlab.inria.fr/guix-hpc/guix-hpc/-/commit/761cfe97d5bb42dbbde538593cef93e803086e3b