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+;;; monoid.el --- Working with Monoids in Elisp -*- lexical-binding: t -*-
+;; Author: William Carroll <wpcarro@gmail.com>
+
+;;; Commentary:
+;; The day has finally arrived where I'm using Monoids in Elisp.
+;;
+;; The monoid typeclass is as follows:
+;; - empty :: a
+;; - concat :: (list a) -> a
+
+;;; Code:
+
+;; TODO: Consider a prelude version that works for all Elisp types.
+(defun monoid/classify (xs)
+  "Return the type of `XS'."
+  (cond
+   ((listp xs) 'list)
+   ((vectorp xs) 'vector)
+   ((stringp xs) 'string)))
+
+
+(defun monoid/empty (xs)
+  "Return the empty monoid for the type `XS'."
+  (pcase (monoid/classify xs)
+    ('list '())
+    ('vector [])
+    ('string "")))
+
+(provide 'monoid)
+;;; monoid.el ends here