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-rw-r--r--module/srfi/srfi-1.scm22
1 files changed, 22 insertions, 0 deletions
diff --git a/module/srfi/srfi-1.scm b/module/srfi/srfi-1.scm
index 2ecdcd7ff..b44eb0341 100644
--- a/module/srfi/srfi-1.scm
+++ b/module/srfi/srfi-1.scm
@@ -841,6 +841,28 @@ the list returned."
;;; Filtering & partitioning
+(define (partition pred lst)
+ "Partition the elements of @var{list} with predicate @var{pred}.
+Return two values: the list of elements satisfying @var{pred} and the
+list of elements @emph{not} satisfying @var{pred}. The order of the
+output lists follows the order of @var{list}. @var{list} is not
+mutated. One of the output lists may share memory with @var{list}."
+ (let ((matches (list #f))
+ (mismatches (list #f)))
+ (let lp ((lst lst)
+ (matches-end matches)
+ (mismatches-end mismatches))
+ (if (null? lst)
+ (values (cdr matches) (cdr mismatches))
+ (let ((x (car lst)))
+ (if (pred x)
+ (begin
+ (set-cdr! matches-end (list x))
+ (lp (cdr lst) (cdr matches-end) mismatches-end))
+ (begin
+ (set-cdr! mismatches-end (list x))
+ (lp (cdr lst) matches-end (cdr mismatches-end)))))))))
+
(define (list-prefix-and-tail lst stop)
(when (eq? lst stop)
(error "Prefix cannot be empty"))