guix system: Simplify closure copy.

* guix/scripts/system.scm (copy-item): Add 'references' argument and
remove 'references*' call.  Turn into a non-monadic procedure.
(copy-closure): Remove initial call to 'references*'.  Only pass ITEM to
'topologically-sorted*' since that's equivalent.  Compute the list of
references corresponding to TO-COPY and pass it to 'copy-item'.

guix/scripts/system.scm

@@ -107,47 +107,44 @@ BODY..., and restore them."
   (store-lift topologically-sorted))
 
 
-(define* (copy-item item target
+(define* (copy-item item references target
                     #:key (log-port (current-error-port)))
-  "Copy ITEM to the store under root directory TARGET and register it."
-  (mlet* %store-monad ((refs (references* item)))
-    (let ((dest  (string-append target item))
-          (state (string-append target "/var/guix")))
-      (format log-port "copying '~a'...~%" item)
+  "Copy ITEM to the store under root directory TARGET and register it with
+REFERENCES as its set of references."
+  (let ((dest  (string-append target item))
+        (state (string-append target "/var/guix")))
+    (format log-port "copying '~a'...~%" item)
 
-      ;; Remove DEST if it exists to make sure that (1) we do not fail badly
-      ;; while trying to overwrite it (see <http://bugs.gnu.org/20722>), and
-      ;; (2) we end up with the right contents.
-      (when (file-exists? dest)
-        (delete-file-recursively dest))
+    ;; Remove DEST if it exists to make sure that (1) we do not fail badly
+    ;; while trying to overwrite it (see <http://bugs.gnu.org/20722>), and
+    ;; (2) we end up with the right contents.
+    (when (file-exists? dest)
+      (delete-file-recursively dest))
 
-      (copy-recursively item dest
-                        #:log (%make-void-port "w"))
+    (copy-recursively item dest
+                      #:log (%make-void-port "w"))
 
-      ;; Register ITEM; as a side-effect, it resets timestamps, etc.
-      ;; Explicitly use "TARGET/var/guix" as the state directory, to avoid
-      ;; reproducing the user's current settings; see
-      ;; <http://bugs.gnu.org/18049>.
-      (unless (register-path item
-                             #:prefix target
-                             #:state-directory state
-                             #:references refs)
-        (leave (G_ "failed to register '~a' under '~a'~%")
-               item target))
-
-      (return #t))))
+    ;; Register ITEM; as a side-effect, it resets timestamps, etc.
+    ;; Explicitly use "TARGET/var/guix" as the state directory, to avoid
+    ;; reproducing the user's current settings; see
+    ;; <http://bugs.gnu.org/18049>.
+    (unless (register-path item
+                           #:prefix target
+                           #:state-directory state
+                           #:references references)
+      (leave (G_ "failed to register '~a' under '~a'~%")
+             item target))))
 
 (define* (copy-closure item target
                        #:key (log-port (current-error-port)))
   "Copy ITEM and all its dependencies to the store under root directory
 TARGET, and register them."
-  (mlet* %store-monad ((refs    (references* item))
-                       (to-copy (topologically-sorted*
-                                 (delete-duplicates (cons item refs)
-                                                    string=?))))
-    (sequence %store-monad
-              (map (cut copy-item <> target #:log-port log-port)
-                   to-copy))))
+  (mlet* %store-monad ((to-copy (topologically-sorted* (list item)))
+                       (refs    (mapm %store-monad references* to-copy)))
+    (for-each (cut copy-item <> <> target #:log-port log-port)
+              to-copy refs)
+
+    (return *unspecified*)))
 
 (define* (install-bootloader installer-drv
                              #:key