feat(wpcarro/emacs): Package maybe.el r/4344

(Temporarily) remove dependency on list.el in favor of dash, which I'm not
thrilled about.

Change-Id: Ic4348ee72582dee63ba07a183f3bda65baa7e2d6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5999
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

1 files changed, 57 insertions, 0 deletions

diff --git a/users/wpcarro/emacs/pkgs/maybe/maybe.el b/users/wpcarro/emacs/pkgs/maybe/maybe.el
new file mode 100644
index 000000000000..3c386b531865
--- /dev/null
+++ b/users/wpcarro/emacs/pkgs/maybe/maybe.el
@@ -0,0 +1,57 @@
+;;; 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"))
+
+;;; 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:
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 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-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