utils: Add split procedure.

* guix/utils.scm (split): New procedure.
* tests/utils.scm: Add tests.

guix/utils.scm

@@ -3,6 +3,7 @@
 ;;; Copyright © 2013, 2014, 2015 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2014 Eric Bavier <bavier@member.fsf.org>
 ;;; Copyright © 2014 Ian Denhardt <ian@zenhack.net>
+;;; Copyright © 2015 David Thompson <davet@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -79,6 +80,7 @@
             fold2
             fold-tree
             fold-tree-leaves
+            split
 
             filtered-port
             compressed-port
@@ -684,6 +686,23 @@ are connected to NODE in the tree, or '() or #f if NODE is a leaf node."
        (else result)))
    init children roots))
 
+(define (split lst e)
+  "Return two values, a list containing the elements of the list LST that
+appear before the first occurence of the object E and a list containing the
+elements after E."
+  (define (same? x)
+    (equal? e x))
+
+  (let loop ((rest lst)
+             (acc '()))
+    (match rest
+      (()
+       (values lst '()))
+      (((? same?) . tail)
+       (values (reverse acc) tail))
+      ((head . tail)
+       (loop tail (cons head acc))))))
+
 
 ;;;
 ;;; Source location.

tests/utils.scm

@@ -121,6 +121,20 @@
                '(0 1 2 3)))
     list))
 
+(test-equal "split, element is in list"
+  '((foo) (baz))
+  (call-with-values
+      (lambda ()
+        (split '(foo bar baz) 'bar))
+    list))
+
+(test-equal "split, element is not in list"
+  '((foo bar baz) ())
+  (call-with-values
+      (lambda ()
+        (split '(foo bar baz) 'quux))
+    list))
+
 (test-equal "strip-keyword-arguments"
   '(a #:b b #:c c)
   (strip-keyword-arguments '(#:foo #:bar #:baz)