Add gptel.el and a README.

README.org

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+#+title: GPTel: A simple ChatGPT client for Emacs
+
+GPTel is a simple, no-frills ChatGPT client for Emacs.
+
+[[file:img/gptel.png]]
+
+- Requires an [[https://platform.openai.com/account/api-keys][OpenAI API key]].
+- No external dependencies, only Emacs. Also, it's async.
+- Interaction is in a Markdown (or text) buffer.
+- Supports conversations (not just one-off queries) and multiple independent sessions.
+- You can go back and edit your previous prompts, or even ChatGPT's previous responses when continuing a conversation. These will be fed back to ChatGPT.
+
+** Installation
+
+*** Package.el
+Clone this repository and run =M-x package-install-file=.
+
+Installing the =markdown-mode= package is optional.
+
+*** Straight
+#+begin_src emacs-lisp
+  (straight-use-package '(gptel :host github :repo "karthink/gptel"))
+#+end_src
+
+Installing the =markdown-mode= package is optional.
+
+*** Manual
+Install =emacs-aio=, (=M-x package-install⏎= =emacs-aio⏎=), then clone this repository and load this file:
+#+begin_src emacs-lisp
+(add-to-list 'load-path "/path/to/gptel/")
+(require 'gptel)
+#+end_src
+
+Installing the =markdown-mode= package is optional.
+
+** Usage
+
+Procure an [[https://platform.openai.com/account/api-keys][OpenAI API key]].
+
+(Optional: Set =gptel-api-key= to the key or to a function that returns the key.)
+
+Run =M-x gptel= to start or switch to the ChatGPT buffer. (It will ask you for the key if you skipped the previous step.)
+
+Run it with a prefix-arg (=C-u M-x gptel=) to start a new session.
+
+In the gptel buffer, send your prompt with =M-x gptel-send=, bound to =C-c RET=.
+
+That's it. You can go back and edit previous prompts and responses if you want.
+
+** Why another ChatGPT client?
+
+Existing Emacs clients don't /reliably/ let me use it the simple way I can in the browser.  (They will get better, but I wanted something for now.)
+
+Also, AI-assisted work is a new way to use Emacs.  It's not yet clear what the best Emacs interface to tools like it is.
+
+- Should it be part of CAPF (=completions-at-point-functions=)?
+- A dispatch menu from anywhere that can act on selected regions?
+- A comint/shell-style REPL?
+- One-off queries in the minibuffer (like =shell-command=)?
+- A refactoring tool in code buffers?
+- An =org-babel= interface?
+
+Maybe all of these, I don't know yet. As a start, I wanted to replicate the web browser usage pattern so I can build from there -- and don't need to switch to the browser every time. As a minimal functional interface, the code is very small right now at ~170 lines.
+
+** Will you add feature X?
+
+Maybe, I'd like to experiment a bit first.
+
+- Support for Org Mode instead of Markdown, including source blocks etc, is planned.
+- I'm experimenting with using it in code buffers.

gptel.el

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+;;; gptel.el --- Interact with ChatGPT from Emacs      -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2023  Karthik Chikmagalur
+
+;; Author: Karthik Chikmagalur
+;; Version: 0.05
+;; Package-Requires: ((emacs "27.1") (aio "1.0"))
+;; Keywords: convenience
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; REQUIREMENTS:
+;; - You need an OpenAI API key. Set the variable `gptel-api-key' to the key or to
+;;   a function of no arguments that returns the key.
+;;
+;; - Install the package `emacs-aio' using `M-x package-install' or however you install packages.
+;;
+;; - Not required but recommended: Install `markdown-mode'.
+;;
+;; USAGE:
+;; - M-x gptel: Start a ChatGPT session
+;; - C-u M-x gptel: Start another or multiple independent ChatGPT sessions
+;;
+;; - In the GPT session: Press `C-c RET' (control + c, followed by return) to send
+;;   your prompt.
+;; - To jump between prompts, use `C-c C-n' and `C-c C-p'.
+
+;;; Code:
+(declare-function markdown-mode "markdown-mode")
+(eval-when-compile
+  (require 'subr-x))
+
+(require 'aio)
+(require 'json)
+(require 'map)
+
+(defcustom gptel-api-key nil
+  "An OpenAI API key (string), or a function of no arguments that
+returns an API key."
+  :type '(choice
+          (string :tag "API key")
+          (function :tag "Function that retuns the API key")))
+
+;;;###autoload
+(defun gptel (name &optional api-key)
+  "Switch to or start ChatGPT session with NAME.
+
+With a prefix arg, query for a (new) session name.
+
+Ask for API-KEY if `gptel-api-key' is unset."
+  (interactive (list (if current-prefix-arg
+                         (read-string "Session name: " (generate-new-buffer-name gptel-default-session))
+                       gptel-default-session)
+                     (or gptel-api-key
+                         (read-string "OpenAI API key: "))))
+  (unless gptel-api-key
+    (user-error "No API key available"))
+  (with-current-buffer (get-buffer-create name)
+    (unless (eq major-mode gptel-default-mode) (funcall gptel-default-mode))
+    (unless gptel-mode (gptel-mode 1))
+    (if (bobp) (insert gptel-prompt-string))
+    (pop-to-buffer (current-buffer))
+    (goto-char (point-max))
+    (skip-chars-backward "\t\r\n")
+    (setq header-line-format
+          (concat (propertize " " 'display '(space :align-to 0))
+                  (format "ChatGPT session (%s)" (buffer-name))))
+    (message "Send your query with %s!"
+             (substitute-command-keys "\\[gptel-send]"))))
+
+(aio-defun gptel-send ()
+  "Submit this prompt to ChatGPT."
+  (interactive)
+  (message "Querying ChatGPT...")
+  (unless (and gptel--prompt-markers
+               (equal (marker-position (car gptel--prompt-markers))
+                      (point-max)))
+    (push (set-marker (make-marker) (point-max))
+          gptel--prompt-markers))
+  (let* ((gptel-buffer (current-buffer))
+         (full-prompt
+          (save-excursion
+            (goto-char (point-min))
+            (cl-loop with role = "user"
+                     for (pm rm . _) on gptel--prompt-markers
+                     collect
+                     (list :role role
+                           :content
+                           (string-trim (buffer-substring-no-properties (or rm (point-min)) pm)
+                                        "[*# \t\n\r]+"))
+                     into prompts
+                     do (setq role (if (equal role "user") "assistant" "user"))
+                     finally return (nreverse prompts))))
+         (response-buffer (aio-await (gptel-get-response full-prompt)))
+         (json-object-type 'plist))
+    (unwind-protect
+        (when-let* ((content-str (gptel-parse-response response-buffer)))
+          (with-current-buffer gptel-buffer
+            (save-excursion
+              (message "Querying ChatGPT... done.")
+              (goto-char (point-max))
+              (display-buffer (current-buffer)
+                              '((display-buffer-reuse-window
+                                 display-buffer-use-some-window)))
+              (unless (bobp) (insert "\n\n"))
+              ;; (if gptel-playback-response
+              ;;     (aio-await (gptel--playback-print content-str))
+              ;;   (insert content-str))
+              (insert content-str)
+              (push (set-marker (make-marker) (point))
+                    gptel--prompt-markers)
+              (insert "\n\n" gptel-prompt-string))))
+      (kill-buffer response-buffer))))
+
+(aio-defun gptel-get-response (prompts)
+  ";;TODO:"
+  (let* ((api-key
+          (cond
+           ((stringp gptel-api-key) gptel-api-key)
+           ((functionp gptel-api-key) (funcall gptel-api-key))))
+         (url-request-method "POST")
+         (url-request-extra-headers
+         `(("Content-Type" . "application/json")
+           ("Authorization" . ,(concat "Bearer " api-key))))
+        (url-request-data
+         (json-encode
+          `(:model "gpt-3.5-turbo"
+            ;; :temperature 1.0
+            ;; :top_p 1.0
+            :messages [,@prompts]))))
+    (pcase-let ((`(,status . ,buffer)
+                  (aio-await
+                   (aio-url-retrieve "https://api.openai.com/v1/chat/completions"))))
+      buffer)))
+
+(defvar-local gptel--prompt-markers nil)
+(defvar gptel-default-session "*ChatGPT*")
+(defvar gptel-default-mode (if (featurep 'markdown-mode)
+                               'markdown-mode
+                             'text-mode))
+(defvar gptel-prompt-string "### ")
+
+;;;###autoload
+(define-minor-mode gptel-mode
+  "Minor mode for interacting with ChatGPT."
+  :glboal nil
+  :lighter " GPT"
+  :keymap
+  (let ((map (make-sparse-keymap)))
+    (define-key map (kbd "C-c RET") #'gptel-send)
+    map))
+
+(defun gptel-parse-response (response-buffer)
+  "Parse response in RESPONSE-BUFFER."
+  (when (buffer-live-p response-buffer)
+    (with-current-buffer response-buffer
+      (if-let* ((status (buffer-substring (line-beginning-position) (line-end-position)))
+                ((string-match "200 OK" status))
+                (response (progn (forward-paragraph)
+                                 (json-read))))
+          (map-nested-elt response '(:choices 0 :message :content))
+        (user-error "Chat failed with status: %S" status)))))
+
+(defvar gptel-playback-response t)
+
+(aio-defun gptel--playback-print (response)
+  (when response
+    (dolist (line (split-string response "\n" nil))
+      (insert line "\n")
+      (aio-await (aio-sleep 0.3)))))
+
+(provide 'gptel)
+;;; gptel.el ends here