utils: Add `fold2'.

* gnu/packages.scm (fold2): Remove.
* guix/utils.scm (fold2): New procedure.  Generalization of the above to
  one and two lists.
* tests/utils.scm ("fold2, 1 list", "fold2, 2 lists"): New tests.

1 files changed, 28 insertions, 1 deletions

diff --git a/guix/utils.scm b/guix/utils.scm
index d7c37e37d1..f13e585e2b 100644
--- a/guix/utils.scm
+++ b/guix/utils.scm
@@ -59,7 +59,8 @@
             %current-system
             version-compare
             version>?
-            package-name->name+version))
+            package-name->name+version
+            fold2))
 
 
 ;;;
@@ -463,6 +464,32 @@ introduce the version part."
       ((head tail ...)
        (loop tail (cons head prefix))))))
 
+(define fold2
+  (case-lambda
+    ((proc seed1 seed2 lst)
+     "Like `fold', but with a single list and two seeds."
+     (let loop ((result1 seed1)
+                (result2 seed2)
+                (lst     lst))
+       (if (null? lst)
+           (values result1 result2)
+           (call-with-values
+               (lambda () (proc (car lst) result1 result2))
+             (lambda (result1 result2)
+               (loop result1 result2 (cdr lst)))))))
+    ((proc seed1 seed2 lst1 lst2)
+     "Like `fold', but with a two lists and two seeds."
+     (let loop ((result1 seed1)
+                (result2 seed2)
+                (lst1    lst1)
+                (lst2    lst2))
+       (if (or (null? lst1) (null? lst2))
+           (values result1 result2)
+           (call-with-values
+               (lambda () (proc (car lst1) (car lst2) result1 result2))
+             (lambda (result1 result2)
+               (fold2 proc result1 result2 (cdr lst1) (cdr lst2)))))))))
+
 
 ;;;
 ;;; Source location.