guix: utils: Add flatten and flat-map from haunt.

* guix/utils.scm (flatten, flat-map): Add procedures.

Change-Id: I1d7d49fd02115e3de09ed69bcf5f55a10423162e

guix/utils.scm

@@ -139,6 +139,9 @@ (define-module (guix utils)
             with-environment-variables
             arguments-from-environment-variable
 
+            flatten
+            flat-map
+
             config-directory
             cache-directory
 
@@ -1028,6 +1031,29 @@ (define (with-atomic-file-output file proc)
         (false-if-exception (delete-file template))
         (close-port out)))))
 
+;; TODO: bring over other utility procedures from (haunt utils).
+(define* (flatten lst #:optional depth)
+  "Return a list that recursively concatenates the sub-lists of LST,
+up to DEPTH levels deep.  When DEPTH is #f, the entire tree is
+flattened."
+  (if (and (number? depth) (zero? depth))
+      lst
+      (fold-right (match-lambda*
+                   (((sub-list ...) memo)
+                    (append (flatten sub-list (and depth (1- depth)))
+                            memo))
+                   ((elem memo)
+                    (cons elem memo)))
+                  '()
+                  lst)))
+
+(define (flat-map proc . lsts)
+  "Apply PROC to each element of each list in LSTS and return a new
+list in which nested lists are concatenated into the result.
+
+For example, the list (1 2 (3)) would be flattened to (1 2 3)."
+  (flatten (apply map proc lsts) 1))
+
 (define* (xdg-directory variable suffix #:key (ensure? #t))
   "Return the name of the XDG directory that matches VARIABLE and SUFFIX,
 after making sure that it exists if ENSURE? is true.  VARIABLE is an