[bug#67172] [PATCH 0/2] Turning <gexp-input> into lowerable objects

[Ludovic Courtès]

Hello there!

These patches address a long-standing issue with gexps: the gexp
writer has full control over the type of references used in the gexp
(they get to choose between ‘ungexp’ and ‘ungexp-native’, they also
choose which output of the file-like to refer to), but whoever passes
a value that ends up in the gexp has no power over the type of
reference.

The goal here is to provide a more control over that, as shown in
this manual excerpt added here:

--8<---------------cut here---------------start------------->8---
-- Procedure: gexp-input OBJ [OUTPUT] [#:native? #f]
     Return a “gexp input” record for the given OUTPUT of file-like
     object OBJ, with ‘#:native?’ determining whether this is a native
     reference (as with ‘ungexp-native’) or not.

     This procedure is helpful when you want to pass a reference to a
     specific output of an object to some procedure that may not know
     about that output.  For example, assume you have this procedure,
     which takes one file-like object:

          (define (make-symlink target)
            (computed-file "the-symlink"
                           #~(symlink #$target #$output)))

     Here ‘make-symlink’ can only ever refer to the default output of
     TARGET—the ‘"out"’ output (*note Packages with Multiple Outputs::).
     To have it refer to, say, the ‘"lib"’ output of the ‘hwloc’
     package, you can call it like so:

          (make-symlink (gexp-input hwloc "lib"))

     You can also compose it like any other file-like object:

          (make-symlink
            (file-append (gexp-input hwloc "lib") "/lib/libhwloc.so"))
--8<---------------cut here---------------end--------------->8---

Thoughts?

Ludo’.

Ludovic Courtès (2):
  gexp: Add compiler for <gexp-input>.
  gexp: #:references-graphs accepts and honors <gexp-input> records.

 doc/guix.texi  | 45 ++++++++++++++++++++++++++++++++++++----
 guix/gexp.scm  | 31 +++++++++++++++++++++++-----
 tests/gexp.scm | 56 +++++++++++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 122 insertions(+), 10 deletions(-)


base-commit: 08d94fe20eca47b69678b3eced8749dd02c700a4
-- 
2.41.0