[Feature #196] Add -powerset and -permutations (#203)

dash.el

@@ -1959,6 +1959,22 @@ The test for equality is done with `equal',
 or with `-compare-fn' if that's non-nil."
   (--filter (not (-contains? list2 it)) list))
 
+(defun -powerset (list)
+  "Return the power set of LIST."
+  (if (null list) '(())
+    (let ((last (-powerset (cdr list))))
+      (append (mapcar (lambda (x) (cons (car list) x)) last)
+              last))))
+
+(defun -permutations (list)
+  "Return the permutations of LIST."
+  (if (null list) '(())
+    (apply #'append
+           (mapcar (lambda (x)
+                     (mapcar (lambda (perm) (cons x perm))
+                             (-permutations (remove x list))))
+                   list))))
+
 (defun -contains? (list element)
   "Return non-nil if LIST contains ELEMENT.
 

dev/examples.el

@@ -565,6 +565,15 @@ new list."
     (-intersection '(1 2 3) '(4 5 6)) => '()
     (-intersection '(1 2 3 4) '(3 4 5 6)) => '(3 4))
 
+  (defexamples -powerset
+    (-powerset '()) => '(nil)
+    (-powerset '(x y z)) => '((x y z) (x y) (x z) (x) (y z) (y) (z) nil))
+
+  (defexamples -permutations
+    (-permutations '()) => '(nil)
+    (-permutations '(1 2)) => '((1 2) (2 1))
+    (-permutations '(a b c)) => '((a b c) (a c b) (b a c) (b c a) (c a b) (c b a)))
+
   (defexamples -distinct
     (-distinct '()) => '()
     (-distinct '(1 2 2 4)) => '(1 2 4)))