[bug#29296] [PATCH 0/2] gexp: Add 'let-system'

["Ludovic Courtès" <ludo@gnu.org>]

Hello!

This patch adds the ‘let-system’ form to (guix gexp), as discussed with
Mathieu at <https://bugs.gnu.org/29281>.  It allows you to insert
system-dependent code inside a gexp, as in this example:

      #~(system*
         #+(let-system system
             (cond ((string-prefix? "armhf-" system)
                    (file-append qemu "/bin/qemu-system-arm"))
                   ((string-prefix? "x86_64-" system)
                    (file-append qemu "/bin/qemu-system-x86_64"))
                   (else
                    (error "dunno!"))))
         "-net" "user" #$image)

(Using (%current-system) and (%current-target-system) does *not* achieve
this, in case you’re wondering, because at the time the gexp is defined
they carry their default value.)

Feedback welcome!

Ludo’.

Ludovic Courtès (2):
  gexp: Compilers can now return lowerable objects.
  gexp: Add 'let-system'.

 doc/guix.texi  |  26 ++++++++++++++
 guix/gexp.scm  | 105 ++++++++++++++++++++++++++++++++++++++++++++++++---------
 tests/gexp.scm |  50 +++++++++++++++++++++++++++
 3 files changed, 165 insertions(+), 16 deletions(-)

-- 
2.15.0