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, 20 insertions, 0 deletions

diff --git a/guix/packages.scm b/guix/packages.scm
index df56286c5b..ec0e79d08b 100644
--- a/guix/packages.scm
+++ b/guix/packages.scm
@@ -22,6 +22,7 @@
   #:use-module (guix records)
   #:use-module (guix store)
   #:use-module (guix monads)
+  #:use-module (guix gexp)
   #:use-module (guix base32)
   #:use-module (guix derivations)
   #:use-module (guix build-system)
@@ -111,6 +112,7 @@
             bag-transitive-target-inputs
 
             default-guile
+            default-guile-derivation
             set-guile-for-build
             package-file
             package->derivation
@@ -341,6 +343,12 @@ derivations."
   (let ((distro (resolve-interface '(gnu packages commencement))))
     (module-ref distro 'guile-final)))
 
+(define* (default-guile-derivation #:optional (system (%current-system)))
+  "Return the derivation for SYSTEM of the default Guile package used to run
+the build code of derivation."
+  (package->derivation (default-guile) system
+                       #:graft? #f))
+
 ;; TODO: Rewrite using %STORE-MONAD and gexps.
 (define* (patch-and-repack store source patches
                            #:key
@@ -939,6 +947,13 @@ cross-compilation target triplet."
 (define package->cross-derivation
   (store-lift package-cross-derivation))
 
+(define-gexp-compiler (package-compiler (package package?) system target)
+  ;; Compile PACKAGE to a derivation for SYSTEM, optionally cross-compiled for
+  ;; TARGET.  This is used when referring to a package from within a gexp.
+  (if target
+      (package->cross-derivation package target system)
+      (package->derivation package system)))
+
 (define patch-and-repack*
   (store-lift patch-and-repack))
 
@@ -976,5 +991,10 @@ outside of the store) or SOURCE itself (if SOURCE is already a store item.)"
      (interned-file file (basename file)
                     #:recursive? #t))))
 
+(define-gexp-compiler (origin-compiler (origin origin?) system target)
+  ;; Compile ORIGIN to a derivation for SYSTEM.  This is used when referring
+  ;; to an origin from within a gexp.
+  (origin->derivation origin system))
+
 (define package-source-derivation
   (store-lower origin->derivation))