subtree(users/wpcarro): docking briefcase at '24f5a642'

git-subtree-dir: users/wpcarro
git-subtree-mainline: 464bbcb15c
git-subtree-split: 24f5a642af
Change-Id: I6105b3762b79126b3488359c95978cadb3efa789

users/wpcarro/emacs/.emacs.d/wpc/maybe.el

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+;;; 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:
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Dependencies
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(require 'list)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Constants
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defvar maybe--run-tests? 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))
+
+(provide 'maybe)
+;;; maybe.el ends here