monads, gexp: Remove unintended dependency on (gnu packages …).

* guix/gexp.scm (gexp->derivation, gexp->script): Use 'default-guile'
  instead of an explicit reference to 'guile-final'.
  (default-guile): New procedure.
* guix/monads.scm (run-with-store)[default-guile]: New procedure.
  Use it.

guix/gexp.scm

@@ -129,8 +129,7 @@ The other arguments are as for 'derivation'."
                                      (return #f)))
                        (guile    (if guile-for-build
                                      (return guile-for-build)
-                                     (package->derivation
-                                      (@ (gnu packages base) guile-final)
+                                     (package->derivation (default-guile)
                                                           system))))
     (raw-derivation name
                     (string-append (derivation->output-path guile)
@@ -336,9 +335,14 @@ package/derivation references."
 ;;; Convenience procedures.
 ;;;
 
+(define (default-guile)
+  ;; Lazily resolve 'guile-final'.  This module must not refer to (gnu …)
+  ;; modules directly, to avoid circular dependencies, hence this hack.
+  (module-ref (resolve-interface '(gnu packages base))
+              'guile-final))
+
 (define* (gexp->script name exp
-                       #:key (modules '())
-                       (guile (@ (gnu packages base) guile-final)))
+                       #:key (modules '()) (guile (default-guile)))
   "Return an executable script NAME that runs EXP using GUILE with MODULES in
 its search path."
   (mlet %store-monad ((modules  (imported-modules modules))

guix/monads.scm

@@ -414,10 +414,15 @@ input list as a monadic value."
                          (system (%current-system)))
   "Run MVAL, a monadic value in the store monad, in STORE, an open store
 connection."
+  (define (default-guile)
+    ;; Lazily resolve 'guile-final'.  This module must not refer to (gnu …)
+    ;; modules directly, to avoid circular dependencies, hence this hack.
+    (module-ref (resolve-interface '(gnu packages base))
+                'guile-final))
+
   (parameterize ((%guile-for-build (or guile-for-build
                                        (package-derivation store
-                                                           (@ (gnu packages base)
-                                                              guile-final)
+                                                           (default-guile)
                                                            system)))
                  (%current-system system))
     (mval store)))