(powerset)~Common Lisp -> Racket

Decided to try my hand at racket, fundamentally the algorithm is the
same and code structure is basically the same anyway.  Just looks cleaner.

powerset.lisp

@@ -1,25 +0,0 @@
-;;; powerset.lisp --- A program to find the power set of some set
-
-;; Copyright (C) 2021  Aryadev Chavali
-
-;; Author: Aryadev Chavali <aryadev@aryadevchavali.com>
-
-;; This program is free software; you can redistribute it and/or modify
-;; it under the terms of the GNU General Public License as published by
-;; the Free Software Foundation, either version 3 of the License, or
-;; (at your option) any later version.
-
-;; This program is distributed in the hope that it will be useful,
-;; but WITHOUT ANY WARRANTY; without even the implied warranty of
-;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-;; GNU General Public License for more details.
-
-;; You should have received a copy of the GNU General Public License
-;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
-
-;;; Commentary:
-;; This program provides a naive counting based approach to finding
-;; specifically sized subsets of some set then using that to generate
-;; the power set.  We start by looking for subsets of size n.
-
-;;; Code:

powerset.rkt

@@ -0,0 +1,24 @@
+#lang racket
+
+(define (subsets-of lst n)
+  (cond
+    [(null? lst) null] ; 0 ways of making anything out of empty set
+    [(= n 0) (list null)] ; 1 way of making a 0 sized subset of something
+    [(= n 1) (map list lst)] ; |lst| ways of making singletons
+    [else
+     (let ([head (car lst)]
+           [tail (cdr lst)])
+       (append
+        ;; HEAD is a part of the subset, so the rest of the subset
+        ;; must be an n-1 sized subset of TAIL
+        (map (lambda (lst) (cons head lst))
+             (subsets-of tail (- n 1)))
+        ;; ... or HEAD is not part of the subset, so the subset is
+        ;; some n sized subset of TAIL
+        (subsets-of tail n)))]))
+
+(define (powerset lst)
+  (append* ; flatten list of n sized subsets to just get subsets
+   (map (lambda (n)
+          (subsets-of lst n)) ; get subset of size n
+        (range 0 (+ 1 (length lst)))))) ; n from 0 to |lst|