106 lines
3.2 KiB
EmacsLisp
106 lines
3.2 KiB
EmacsLisp
;;; maybe.el --- Library for dealing with nil values -*- lexical-binding: t -*-
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;; Author: William Carroll <wpcarro@gmail.com>
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;; Version: 0.0.1
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;; Package-Requires: ((emacs "24"))
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;; Homepage: https://user.git.corp.google.com/wpcarro/briefcase
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;;; Commentary:
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;; Inspired by Elm's Maybe library.
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;;
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;; For now, a Nothing value will be defined exclusively as a nil value. I'm
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;; uninterested in supported falsiness in this module even at risk of going
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;; against the LISP grain.
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;;
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;; I'm avoiding introducing a struct to handle the creation of Just and Nothing
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;; variants of Maybe. Perhaps this is a mistake in which case this file would
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;; be more aptly named nil.el. I may change that. Because of this limitation,
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;; functions in Elm's Maybe library like andThen, which is the monadic bind for
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;; the Maybe type, doesn't have a home here since we cannot compose multiple
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;; Nothing or Just values without a struct or some other construct.
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;;
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;; Possible names for the variants of a Maybe.
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;; None | Some
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;; Nothing | Something
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;; None | Just
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;; Nil | Set
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;;
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;; NOTE: In Elisp, values like '() (i.e. the empty list) are aliases for nil.
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;; What else in Elisp is an alias in this way?
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;; Examples:
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;; TODO: Provide examples of other nil types in Elisp.
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;;; Code:
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Dependencies
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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(require 'prelude)
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(require 'list)
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Constants
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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(defvar maybe--run-tests? t
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"When t, run the test suite defined herein.")
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Library
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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(defun maybe-nil? (x)
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"Return t if X is nil."
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(eq nil x))
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(defun maybe-some? (x)
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"Return t when X is non-nil."
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(not (maybe-nil? x)))
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(defun maybe-nils? (&rest xs)
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"Return t if all XS are nil."
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(list/all? #'maybe-nil? xs))
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(defun maybe-somes? (&rest xs)
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"Return t if all XS are non-nil."
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(list/all? #'maybe-some? xs))
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(defun maybe-default (default x)
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"Return DEFAULT when X is nil."
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(if (maybe-nil? x) default x))
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(defun maybe-map (f x)
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"Apply F to X if X is not nil."
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(if (maybe-some? x)
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(funcall f x)
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x))
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Tests
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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(when maybe--run-tests?
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;; nil?
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(prelude/assert (maybe-nil? nil))
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(prelude/refute (maybe-nil? t))
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;; some?
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(prelude/assert (maybe-some? 10))
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(prelude/refute (maybe-some? nil))
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;; nils?
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(prelude/assert (maybe-nils? nil nil nil nil))
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(prelude/refute (maybe-nils? nil t nil t))
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;; somes?
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(prelude/assert (maybe-somes? t 10 '(1 2 3) "some"))
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(prelude/refute (maybe-somes? t nil '(1 2 3) "some"))
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;; default
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(prelude/assert
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(and (= 0 (maybe-default 5 0))
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(= 5 (maybe-default 5 nil))))
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;; map
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(prelude/assert
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(and (= 2 (maybe-map #'1+ 1))
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(eq nil (maybe-map #'1+ nil)))))
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(provide 'maybe)
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;;; maybe.el ends here
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