!keep

Returns a new list of the non-nil results of applying FN to the items in LIST.

README.md

@@ -15,6 +15,7 @@ This is so much a work in progress that you should definitely not be using it ye
 * [!reduce](#reduce-fn-list) `(fn list)`
 * [!filter](#filter-fn-list) `(fn list)`
 * [!remove](#remove-fn-list) `(fn list)`
+* [!keep](#keep-fn-list) `(fn list)`
 * [!concat](#concat-rest-lists) `(&rest lists)`
 * [!mapcat](#mapcat-fn-list) `(fn list)`
 * [!partial](#partial-fn-rest-args) `(fn &rest args)`
@@ -113,6 +114,16 @@ Returns a new list of the items in `list` for which `fn` returns nil.
 (!!remove (= 0 (% it 2)) '(1 2 3 4)) ;; => '(1 3)
 ```
 
+### !keep `(fn list)`
+
+Returns a new list of the non-nil results of applying `fn` to the items in `list`.
+
+```cl
+(!keep 'cdr '((1 2 3) (4 5) (6))) ;; => '((2 3) (5))
+(!keep (lambda (num) (when (> num 3) (* 10 num))) '(1 2 3 4 5 6)) ;; => '(40 50 60)
+(!!keep (when (> it 3) (* 10 it)) '(1 2 3 4 5 6)) ;; => '(40 50 60)
+```
+
 ### !concat `(&rest lists)`
 
 Returns a new list with the concatenation of the elements in

bang.el

@@ -96,6 +96,22 @@ exposed as `acc`."
   "Returns a new list of the items in LIST for which FN returns nil."
   (!!remove (funcall fn it) list))
 
+(defmacro !!keep (form list)
+  "Anaphoric form of `!keep'."
+  `(let ((!--list ,list)
+         (!--result '()))
+     (while !--list
+       (let* ((it (car !--list))
+              (mapped ,form))
+         (when mapped
+           (setq !--result (cons mapped !--result))))
+       (setq !--list (cdr !--list)))
+     (nreverse !--result)))
+
+(defun !keep (fn list)
+  "Returns a new list of the non-nil results of applying FN to the items in LIST."
+  (!!keep (funcall fn it) list))
+
 (defun !concat (&rest lists)
   "Returns a new list with the concatenation of the elements in
 the supplied LISTS."

examples.el

@@ -40,6 +40,11 @@
   (let ((mod 2)) (!remove (lambda (num) (= 0 (% num mod))) '(1 2 3 4))) => '(1 3)
   (let ((mod 2)) (!!remove (= 0 (% it mod)) '(1 2 3 4))) => '(1 3))
 
+(defexamples !keep
+  (!keep 'cdr '((1 2 3) (4 5) (6))) => '((2 3) (5))
+  (!keep (lambda (num) (when (> num 3) (* 10 num))) '(1 2 3 4 5 6)) => '(40 50 60)
+  (!!keep (when (> it 3) (* 10 it)) '(1 2 3 4 5 6)) => '(40 50 60))
+
 (defexamples !concat
   (!concat '(1)) => '(1)
   (!concat '(1) '(2)) => '(1 2)