Proposed (guix platforms) lookup procedures API change

[Maxim Cournoyer]

Hello,

There is currently a proposed change [0] to modify the behavior of the
LOOKUP-PLATFORM-BY-SYSTEM, LOOKUP-PLATFORM-BY-TARGET and
LOOKUP-PLATFORM-BY-TARGET-OR-SYSTEM public procedures from (guix
platforms) that would have them return an exception instead of #f when
no platform could be found.

[0]  https://issues.guix.gnu.org/60802

This is motivated so that a clearer error can be produced when these
procedures are used deep in the following reproducer:

--8<---------------cut here---------------start------------->8---
(use-modules (guix packages)
             (gnu packages cross-base))

(define linux-libre-headers-cross-mips64el-linux-gnuabi64
  (cross-kernel-headers "mips64el-linux-gnuabi64"))

(package-arguments linux-libre-headers-cross-mips64el-linux-gnuabi64)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
In procedure struct-vtable: Wrong type argument in position 1 (expecting 
struct): #f

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guix-user) [1]> ,bt
In gnu/packages/cross-base.scm:
   388:33  1 (loop (#:phases (modify-phases %standard-phases (delete (quote 
configure)) (# (…) …) …) …) …)
In ice-9/boot-9.scm:
  1685:16  0 (raise-exception _ #:continuable? _)
--8<---------------cut here---------------end--------------->8---

With the change installed, the backtrace now reads as:

--8<---------------cut here---------------start------------->8---
ice-9/boot-9.scm:1685:16: In procedure raise-exception:
ERROR:
  1. &platform-not-found-error

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guix-user) [1]> ,bt
In gnu/packages/cross-base.scm:
   383:15  3 (arguments #<package 
linux-libre-headers-cross-mips64el-linux-gnuabi64@5.10.35 gnu/packag…>)
In gnu/packages/linux.scm:
   693:27  2 (arguments #<package linux-libre-headers@5.10.35 
gnu/packages/linux.scm:672 7f409be93a50>)
In guix/platform.scm:
    130:6  1 (lookup-platform-by-target-or-system "x86_64-linux")
In ice-9/boot-9.scm:
  1685:16  0 (raise-exception _ #:continuable? _)
--8<---------------cut here---------------end--------------->8---

which more directly communicates the source of the problem.

For more information, see the issue that motivated it:
https://issues.guix.gnu.org/60786

-- 
Thanks,
Maxim