graph: Add '--expression'.

* guix/scripts/graph.scm (%options, show-help): Add '--expression'.
  (guix-graph): Call 'read/eval-package-expression' for 'expression'
  pairs in OPTS.
* tests/guix-graph.sh: Add tests.
* doc/guix.texi (Invoking guix graph): Document it.

1 files changed, 10 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 6defb0bda7..f943540ac8 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -4417,6 +4417,16 @@ the values listed above.
 
 @item --list-types
 List the supported graph types.
+
+@item --expression=@var{expr}
+@itemx -e @var{expr}
+Consider the package @var{expr} evaluates to.
+
+This is useful to precisely refer to a package, as in this example:
+
+@example
+guix graph -e '(@@@@ (gnu packages commencement) gnu-make-final)'
+@end example
 @end table