From e1505232e21b6cb52921fc9bb8ee29f2772ab4a8 Mon Sep 17 00:00:00 2001
From: Arun Isaac
Date: Tue, 1 Jun 2021 13:59:35 +0530
Subject: Add filter-mapi.

* ccwl/utils.scm (filter-mapi): New public function.
---
 ccwl/utils.scm | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/ccwl/utils.scm b/ccwl/utils.scm
index d0db51c..de25fcc 100644
--- a/ccwl/utils.scm
+++ b/ccwl/utils.scm
@@ -33,7 +33,8 @@
             syntax-lambda**
             mapn
             append-mapn
-            foldn))
+            foldn
+            filter-mapi))
 
 (define (pairify lst)
   "Return a list of pairs of successive elements of LST."
@@ -145,6 +146,15 @@ macros that accept keyword arguments."
                ((_ args :::)
                 (unsyntax-keywords #'(args :::))))))))
 
+(define (filter-mapi proc lst)
+  "Indexed filter-map. Like filter-map, but PROC calls are (proc item
+index) where ITEM is an element of list and INDEX is the index of that
+element."
+  (filter-map (lambda (item index)
+                (proc item index))
+              lst
+              (iota (length lst))))
+
 (define (mapn proc lst)
   "Map the procedure PROC over list LST and return a list containing
 the results. PROC can return multiple values, in which case, an equal
-- 
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