Add !interpose

README.md

@@ -20,6 +20,7 @@ Or you can just dump `bang.el` in your load path somewhere.
 * [!keep](#keep-fn-list) `(fn list)`
 * [!concat](#concat-rest-lists) `(&rest lists)`
 * [!mapcat](#mapcat-fn-list) `(fn list)`
+* [!interpose](#interpose-sep-list) `(sep list)`
 * [!first](#first-fn-list) `(fn list)`
 * [!partial](#partial-fn-rest-args) `(fn &rest args)`
 * [!rpartial](#rpartial-fn-rest-args) `(fn &rest args)`
@@ -159,6 +160,16 @@ Thus function `fn` should return a collection.
 (!!mapcat (list 0 it) '(1 2 3)) ;; => '(0 1 0 2 0 3)
 ```
 
+### !interpose `(sep list)`
+
+Returns a new list of all elements in `list` separated by `sep`.
+
+```cl
+(!interpose "-" '()) ;; => '()
+(!interpose "-" '("a")) ;; => '("a")
+(!interpose "-" '("a" "b" "c")) ;; => '("a" "-" "b" "-" "c")
+```
+
 ### !first `(fn list)`
 
 Returns the first x in `list` where (`fn` x) is non-nil, else nil.

bang.el

@@ -144,6 +144,17 @@ the supplied LISTS."
 Thus function FN should return a collection."
   (!!mapcat (funcall fn it) list))
 
+(defun !interpose (sep list)
+  "Returns a new list of all elements in LIST separated by SEP."
+  (let (result)
+    (when list
+      (setq result (cons (car list) result))
+      (setq list (cdr list)))
+    (while list
+      (setq result (cons (car list) (cons sep result)))
+      (setq list (cdr list)))
+    (nreverse result)))
+
 (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.

examples.el

@@ -58,6 +58,11 @@
   (!mapcat (lambda (item) (list 0 item)) '(1 2 3)) => '(0 1 0 2 0 3)
   (!!mapcat (list 0 it) '(1 2 3)) => '(0 1 0 2 0 3))
 
+(defexamples !interpose
+  (!interpose "-" '()) => '()
+  (!interpose "-" '("a")) => '("a")
+  (!interpose "-" '("a" "b" "c")) => '("a" "-" "b" "-" "c"))
+
 (defexamples !first
   (!first 'even? '(1 2 3)) => 2
   (!first 'even? '(1 3 5)) => nil