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+;;; maybe.el --- Library for dealing with nil values -*- lexical-binding: t -*-
+;; Author: William Carroll <wpcarro@gmail.com>
+
+;;; Commentary:
+;; Inspired by Elm's Maybe library.
+;;
+;; For now, a Nothing value will be defined exclusively as a nil value.  I'm
+;; uninterested in supported falsiness in this module even at risk of going
+;; against the LISP grain.
+;;
+;; I'm avoiding introducing a struct to handle the creation of Just and Nothing
+;; variants of Maybe.  Perhaps this is a mistake in which case this file would
+;; be more aptly named nil.el.  I may change that.  Because of this limitation,
+;; functions in Elm's Maybe library like andThen, which is the monadic bind for
+;; the Maybe type, doesn't have a home here since we cannot compose multiple
+;; Nothing or Just values without a struct or some other construct.
+;;
+;; Possible names for the variants of a Maybe.
+;; None    | Some
+;; Nothing | Something
+;; None    | Just
+;; Nil     | Set
+;;
+;; NOTE: In Elisp, values like '() (i.e. the empty list) are aliases for nil.
+;; What else in Elisp is an alias in this way?
+;; Examples:
+;; TODO: Provide examples of other nil types in Elisp.
+
+;;; Code:
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Dependencies
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(require 'prelude)
+(require 'list)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Constants
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defvar maybe/test? t
+  "When t, run the test suite defined herein.")
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defun maybe/nil? (x)
+  "Return t if X is nil."
+  (eq nil x))
+
+(defun maybe/some? (x)
+  "Return t when X is non-nil."
+  (not (maybe/nil? x)))
+
+(defun maybe/nils? (&rest xs)
+  "Return t if all XS are nil."
+  (list/all? #'maybe/nil? xs))
+
+(defun maybe/somes? (&rest xs)
+  "Return t if all XS are non-nil."
+  (list/all? #'maybe/some? xs))
+
+(defun maybe/default (default x)
+  "Return DEFAULT when X is nil."
+  (if (maybe/nil? x) default x))
+
+(defun maybe/map (f x)
+  "Apply F to X if X is not nil."
+  (if (maybe/some? x)
+      (funcall f x)
+    x))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Tests
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(when maybe/test?
+  ;; nil?
+  (prelude/assert (maybe/nil? nil))
+  (prelude/refute (maybe/nil? t))
+  ;; some?
+  (prelude/assert (maybe/some? 10))
+  (prelude/refute (maybe/some? nil))
+  ;; nils?
+  (prelude/assert (maybe/nils? nil nil nil nil))
+  (prelude/refute (maybe/nils? nil t nil t))
+  ;; somes?
+  (prelude/assert (maybe/somes? t 10 '(1 2 3) "some"))
+  (prelude/refute (maybe/somes? t nil '(1 2 3) "some"))
+  ;; default
+  (prelude/assert
+   (and (= 0 (maybe/default 5 0))
+        (= 5 (maybe/default 5 nil))))
+  ;; map
+  (prelude/assert
+   (and (= 2 (maybe/map #'1+ 1))
+        (eq nil (maybe/map #'1+ nil)))))
+
+(provide 'maybe)
+;;; maybe.el ends here