Inheritance vs. ‘this-package’

OpenSubmitted by Ludovic Courtès.
One participant
  • Ludovic Courtès
Ludovic Courtès wrote on 2 Sep 2021 15:48
(address .

We stumbled upon an idiom for package arguments that, combined with
package inheritance, would not work as expected. It goes like this

Toggle snippet (23 lines)
(define-public starpu
;; …
(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)
(propagated-inputs `(("mpi" ,openmpi)))))

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

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.