ui: Factorize user-provided Scheme file loading.

* guix/ui.scm (make-user-module, load*): New procedures.
* guix/scripts/system.scm (%user-module): Define in terms of
  'make-user-module'.
  (read-operating-system): Define in terms of load*'.

guix/scripts/system.scm

@@ -48,28 +48,14 @@
 
 (define %user-module
   ;; Module in which the machine description file is loaded.
-  (let ((module (make-fresh-user-module)))
+  (make-user-module '((gnu system)
-    (for-each (lambda (iface)
-                (module-use! module (resolve-interface iface)))
-              '((gnu system)
                       (gnu services)
-                (gnu system shadow)))
+                      (gnu system shadow))))
-    module))
 
 (define (read-operating-system file)
   "Read the operating-system declaration from FILE and return it."
-  ;; TODO: Factorize.
+  (load* file %user-module))
-  (catch #t
-    (lambda ()
-      ;; Avoid ABI incompatibility with the <operating-system> record.
-      (set! %fresh-auto-compile #t)
 
-      (save-module-excursion
-       (lambda ()
-         (set-current-module %user-module)
-         (primitive-load file))))
-    (lambda args
-      (report-load-error file args))))
 
 
 ;;;

guix/ui.scm

@@ -48,6 +48,8 @@
             P_
             report-error
             leave
+            make-user-module
+            load*
             report-load-error
             warn-about-load-error
             show-version-and-exit
@@ -133,6 +135,28 @@ messages."
     (report-error args ...)
     (exit 1)))
 
+(define (make-user-module modules)
+  "Return a new user module with the additional MODULES loaded."
+  ;; Module in which the machine description file is loaded.
+  (let ((module (make-fresh-user-module)))
+    (for-each (lambda (iface)
+                (module-use! module (resolve-interface iface)))
+              modules)
+    module))
+
+(define (load* file user-module)
+  "Load the user provided Scheme source code FILE."
+  (catch #t
+    (lambda ()
+      (set! %fresh-auto-compile #t)
+
+      (save-module-excursion
+       (lambda ()
+         (set-current-module user-module)
+         (primitive-load file))))
+    (lambda args
+      (report-load-error file args))))
+
 (define (report-load-error file args)
   "Report the failure to load FILE, a user-provided Scheme file, and exit.
 ARGS is the list of arguments received by the 'throw' handler."