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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
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