This is a simple implementation of Promises/A+.
This implementation ported the following Promises/A+ implementation faithfully.
https://github.com/then/promise
For detailed tutorials on its use, see www.promisejs.org (JavaScript).
Promises/A+ for Emacs is used in Async/Await for Emacs, so you can use Async/Await for asynchronous programming.
You can install from MELPA using package.el.
The package name is promise.
See promise-examples.el for details.
(require 'promise)
(defun do-something-async (delay-sec value)
"Return `Promise' to resolve the value asynchronously."
(promise-new (lambda (resolve _reject)
(run-at-time delay-sec
nil
(lambda ()
(funcall resolve value))))))
(defun example4 ()
"All processes are asynchronous Promise chain."
(promise-chain (do-something-async 1 33)
(then (lambda (result)
(message "first result: %s" result)
(do-something-async 1 (* result 2))))
(then (lambda (second-result)
(message "second result: %s" second-result)
(do-something-async 1 (* second-result 2))))
(then (lambda (third-result)
(message "third result: %s" third-result)))))
An example using `url-retrieve 'as a more complicated example.
(require 'promise)
(require 'url-http)
(require 'xml)
(require 'dom)
(defun xml-retrieve (url)
"Return `Promise' to resolve with XML object obtained by HTTP request."
(promise-new
(lambda (resolve reject)
(url-retrieve url
(lambda (status)
;; All errors are reliably captured and rejected with appropriate values.
(if (plist-get status :error)
(funcall reject (plist-get status :error))
(condition-case ex
(with-current-buffer (current-buffer)
(if (not (url-http-parse-headers))
(funcall reject (buffer-string))
(search-forward-regexp "\n\\s-*\n" nil t)
(funcall resolve (xml-parse-region))))
(error (funcall reject ex)))))))))
(defun get-text-first-tag (xml tag)
"Returns the first text that matches TAG in XML."
(decode-coding-string (dom-text (cl-first (dom-by-tag xml tag)))
'utf-8))
(defun get-short-text-first-tag (xml tag)
"Truncate the text obtained with `get-text-first-tag'."
(concat (truncate-string-to-width (get-text-first-tag xml tag) 64)
" ..."))
(defun wait-seconds (seconds fn &rest args)
"Return `Promise' to execute the function after the specified time."
(promise-new (lambda (resolve _reject)
(run-at-time seconds
nil
(lambda ()
(funcall resolve (apply fn args)))))))
(defun example12 ()
"Example using `xml-retrieve'."
(let ((wikipedia-url (concat "https://en.wikipedia.org/w/api.php"
"?format=xml&action=query&prop=extracts"
"&exintro=&explaintext=&titles=")))
(promise-chain (promise-all
(vector
(xml-retrieve (concat wikipedia-url (url-encode-url "GNU")))
;; Request after 2 seconds for load reduction.
(wait-seconds 2
#'xml-retrieve
(concat wikipedia-url (url-encode-url "Emacs")))))
(then (lambda (xmls)
(message "%s" (get-short-text-first-tag (aref xmls 0) 'extract))
(message "%s" (get-short-text-first-tag (aref xmls 1) 'extract))))
(promise-catch (lambda (reason)
(message "promise-catch: %s" reason))))))
$ cask install
$ cask exec ert-runner
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