gexp: Move the package and origin compilers to (guix packages).

From now own, (guix packages) depends on (guix gexps); it was the other
way around now.  This means that (guix packages) code can use gexps.

* guix/gexp.scm (origin-compiler, package-compiler): Remove.
  (default-guile-derivation): New procedure.
  (gexp->derivation): Use it instead of 'default-guile' +
  'package->derivation'.
* guix/packages.scm (default-guile-derivation): New procedure.
  (package-compiler, origin-compiler): New variables.
* doc/guix.texi (G-Expressions): Mention extensibility.

1 files changed, 5 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index fa8d3ab49e..ae2f786111 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -2464,6 +2464,11 @@ processes.
 When a package or derivation is unquoted inside a gexp, the result is as
 if its output file name had been introduced.
 
+Actually this mechanism is not limited to package and derivation
+objects; @dfn{compilers} able to ``lower'' other high-level objects to
+derivations can be defined, such that these objects can also be inserted
+into gexps.
+
 @item
 Gexps carry information about the packages or derivations they refer to,
 and these dependencies are automatically added as inputs to the build