From 9410f7e5a60e4c1aea69e38d78a7ad5710b3707d Mon Sep 17 00:00:00 2001
From: Magnar Sveen <magnars@gmail.com>
Date: Sun, 21 Oct 2012 16:53:37 +0200
Subject: [PATCH] Add !replace-where

---
 README.md   | 13 +++++++++++++
 bang.el     | 20 ++++++++++++++++++++
 examples.el |  5 +++++
 3 files changed, 38 insertions(+)

diff --git a/README.md b/README.md
index f488889..8cbfadd 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,7 @@ Or you can just dump `bang.el` in your load path somewhere.
 * [!concat](#concat-rest-lists) `(&rest lists)`
 * [!mapcat](#mapcat-fn-list) `(fn list)`
 * [!interpose](#interpose-sep-list) `(sep list)`
+* [!replace-where](#replace-where-pred-rep-list) `(pred rep list)`
 * [!first](#first-fn-list) `(fn list)`
 * [!partial](#partial-fn-rest-args) `(fn &rest args)`
 * [!rpartial](#rpartial-fn-rest-args) `(fn &rest args)`
@@ -170,6 +171,18 @@ Returns a new list of all elements in `list` separated by `sep`.
 (!interpose "-" '("a" "b" "c")) ;; => '("a" "-" "b" "-" "c")
 ```
 
+### !replace-where `(pred rep list)`
+
+Returns a new list where the elements in `list` that does not match the `pred` function
+are unchanged, and where the elements in `list` that do match the `pred` function are mapped
+through the `rep` function.
+
+```cl
+(!replace-where 'even? 'square '(1 2 3 4)) ;; => '(1 4 3 16)
+(!replace-where (lambda (n) (= n 3)) (lambda (n) 0) '(1 2 3 4)) ;; => '(1 2 0 4)
+(!!replace-where (> it 2) (* it it) '(1 2 3 4)) ;; => '(1 2 9 16)
+```
+
 ### !first `(fn list)`
 
 Returns the first x in `list` where (`fn` x) is non-nil, else nil.
diff --git a/bang.el b/bang.el
index 78c21a5..065662d 100644
--- a/bang.el
+++ b/bang.el
@@ -155,6 +155,26 @@ Thus function FN should return a collection."
       (setq list (cdr list)))
     (nreverse result)))
 
+(defmacro !!replace-where (pred rep list)
+  "Returns a new list where the elements in LIST that does not match the PRED function
+are unchanged, and where the elements in LIST that do match the PRED function are mapped
+through the REP function."
+  (let ((l (make-symbol "list"))
+        (r (make-symbol "result")))
+    `(let ((,l ,list)
+           (,r '()))
+       (while ,l
+         (let ((it (car ,l)))
+           (setq ,r (cons (if ,pred ,rep it) ,r)))
+         (setq ,l (cdr ,l)))
+       (nreverse ,r))))
+
+(defun !replace-where (pred rep list)
+  "Returns a new list where the elements in LIST that does not match the PRED function
+are unchanged, and where the elements in LIST that do match the PRED function are mapped
+through the REP function."
+  (!!replace-where (funcall pred it) (funcall rep it) list))
+
 (defun !partial (fn &rest args)
   "Takes a function FN and fewer than the normal arguments to FN,
 and returns a fn that takes a variable number of additional ARGS.
diff --git a/examples.el b/examples.el
index 970953d..6bfec6f 100644
--- a/examples.el
+++ b/examples.el
@@ -63,6 +63,11 @@
   (!interpose "-" '("a")) => '("a")
   (!interpose "-" '("a" "b" "c")) => '("a" "-" "b" "-" "c"))
 
+(defexamples !replace-where
+  (!replace-where 'even? 'square '(1 2 3 4)) => '(1 4 3 16)
+  (!replace-where (lambda (n) (= n 3)) (lambda (n) 0) '(1 2 3 4)) => '(1 2 0 4)
+  (!!replace-where (> it 2) (* it it) '(1 2 3 4)) => '(1 2 9 16))
+
 (defexamples !first
   (!first 'even? '(1 2 3)) => 2
   (!first 'even? '(1 3 5)) => nil