;;; maybe.el --- Library for dealing with nil values -*- lexical-binding: t -*-

;; Author: William Carroll <wpcarro@gmail.com>
;; Version: 0.0.1
;; Package-Requires: ((emacs "24"))
;; Homepage: https://user.git.corp.google.com/wpcarro/briefcase

;;; 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:

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;; Dependencies
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(require 'prelude)
(require 'list)

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;; Constants
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(defvar maybe--run-tests? t
  "When t, run the test suite defined herein.")

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;; Library
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(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))

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;; Tests
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(when maybe--run-tests?
  ;; 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