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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
|
