feat(wpcarro/emacs): Package maybe.el

(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

users/wpcarro/emacs/pkgs/maybe/default.nix

@@ -0,0 +1,24 @@
+{ pkgs, depot, ... }:
+
+let
+  maybe = pkgs.callPackage
+    ({ emacsPackages }:
+      emacsPackages.trivialBuild {
+        pname = "maybe";
+        version = "1.0.0";
+        src = ./maybe.el;
+        packageRequires = [ ];
+      })
+    { };
+
+  emacs = (pkgs.emacsPackagesFor pkgs.emacs28).emacsWithPackages (epkgs: [
+    maybe
+  ]);
+in
+maybe.overrideAttrs (_old: {
+  doCheck = true;
+  checkPhase = ''
+    ${emacs}/bin/emacs -batch \
+      -l ert -l ${./tests.el} -f ert-run-tests-batch-and-exit
+  '';
+})

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

users/wpcarro/emacs/pkgs/maybe/tests.el

@@ -0,0 +1,28 @@
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Dependencies
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(require 'maybe)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Tests
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(ert-deftest maybe-nil? ()
+  (and
+   (maybe-nil? nil)
+   (not (maybe-nil? t))))
+
+(ert-deftest maybe-some? ()
+  (and
+   (maybe-some? '(1 2 3))
+   (not (maybe-some? nil))))
+
+(ert-deftest maybe-default ()
+  (and
+   (string= "some" (maybe-default "some" nil))
+   (= 10 (maybe-default 1 10))))
+
+(ert-deftest maybe-map ()
+  (eq nil (maybe-map (lambda (x) (* x 2)) nil))
+  (= 4 (maybe-map (lambda (x) (* x 2)) 2)))