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Remotely Modifying a Running Program Using Swank

Posted: 2017-10-28. Modified: 2017-10-28. Tags: howto, emacs, LISP.

One of the strengths of Common Lisp is that it includes support for dynamically redefining classes and functions at run-time, while keeping data intact. Theoretically at least, this should enable support for keeping programs continually running in production while changing pieces of them – "Common Lisp Recipes" by Edi Weitz includes the following disclaimer about this functionality

If you'e ever talked to experienced Lispers, you've probably heard "war stories" of huge and complex systems to which substantial modifications were applied while they kept running and without interrupting the services they provided. Although this sometimes has to be taken with a grain of salt, it is in fact true that many COMMON LISP features were designed from the ground up to be dynamic in the sense that they can be changed at run time. This includes CLOS, where an object can change from one class to another, and where classes can be modified, although they already have objects "hanging off" of them.

– "Common Lisp Recipes" by Edi Weitz, section 13-8

My understanding is that some of this functionality at least is difficult/nonstandard to replicate in other languages such as Python and Java (please feel free let me know that I am wrong if that is the case!).

Anyway, I would like to remotely interact in lisp with remote instances of my current project I am working on – dumb-mq – so I figured it would be helpful to start with a small example of remote connection/redefinition.

This example uses sbcl and SLIME-mode in emacs, but it should work for other lisp implementations and other tools that support the swank protocol as well. The easiest way to get SBCL and emacs set up that I am aware currently of is to download the excellent Portacle IDE by Nicolas Hafner. Alternatively just figure out how to install them yourself.

Write the following lisp file: swankdemo.lisp

;; a little common lisp swank demo
;; while this program is running, you can connect to it from another terminal or machine
;; and change the definition of doprint to print something else out!
;; (ql:quickload :swank)
;; (ql:quickload :bordeaux-threads)

(require :swank)
(require :bordeaux-threads)

(defparameter *counter* 0)

(defun dostuff ()
  (format t "hello world ~a!~%" *counter*))

(defun runner ()
  (bt:make-thread (lambda ()
                    (swank:create-server :port 4006)))
  (format t "we are past go!~%")
  (loop while t do
       (sleep 5)
       (incf *counter*)


You can run this program as follows:

sbcl --load swankdemo.lisp

The program will run indefinitely, printing out a message every five seconds and incrementing its counter. Imagine instead that this was program was accepting connections indefinitely and was providing an important service.

By default swank will accept connections only from localhost – if you would like to connect from a different computer you can use ssh tunneling to forward the port on the remote machine to a port on your local computer. For example

ssh -L4006: username@example.com

will securely forward port 4006 on the server at example.com to your local computer's port 4006.

Let's connect to the program. Fire up emacs, type M-x slime-connect, at the prompts select (the default) and port 4006 (type this in). If all went well, you are now connected to the remotely running lisp program! Just to check, see if you can retrieve the current value of the counter:

CL-USER> *counter*

Now let's say you want to change the definition of dostuff and reset the counter while you are at it. Type in the following either in an emacs scratch buffer, select it, and send it to the remote lisp program using M-x slime-eval-region (or an alternate method).

(defun dostuff ()
  (format t "goodbye world ~a!~%" *counter*))
(setf *counter* 0)

Observe swankdemo's output in the console – you will see the output change and the counter be reset. Success!

You can do more complicated redefinitions and changes – refer to the Common Lisp Standard (draft) section 7.2 and 7.3 for some information on modifying objects at run-time.

How to Designate Single Window for Popup Buffers in Emacs

Posted: 2017-08-20. Modified: 2017-08-20. Tags: howto, emacs, emacstips, LISP.

This blogpost is inspired by the approach found here.

One of the things that used to annoy me about programming in emacs with SLIME mode (common lisp), is that SLIME would frequently choose to open up a popup buffer in one of the windows I was trying to use for some other task. For instance, various actions in SLIME will open up a Completion Buffer, Debugger Pane or an Inspection Buffer. I eventually realized that what I really wanted was to designate a given window where all emacs popups would open by default, so my train of thought in the other windows can remain undisturbed. Below is some Emacs Lisp code that enables this functionality:

(defun berry-choose-window-for-popups ()
  "run with your cursor in the window which you want to use to open up 
   all popups from now on!"
  (set-window-parameter (selected-window) 'berrydesignated t)

(defun berry-setup-popup-display-handler ()
  "adds an entry to display-buffer-alist which allows you to designate a window 
   for emacs popups. If the buffer is currently being displayed in a given 
   window, it will continue to use that window. Otherwise, it will choose your 
   designated window which should have been already set."
  (add-to-list 'display-buffer-alist
	       `(".*" .
		  ((reusable-frames     . visible)
		   (side                . bottom)
		   (window-height       . 0.50)))

(defun berry-select-window-for-popup (buffer &optional alist)
  "Searches for the a window which the 'berrydesignated parameter set.
    Returns the first such window found. If none is found, returns nil."
  (cl-block berry-select-window-for-popup
    (let* ((winlist (window-list-1 nil nil t))
	   (outindex 0))
      (while (< outindex (length winlist))
	(let ((candidate (elt winlist outindex)))
	  (if (eql t (window-parameter candidate 'berrydesignated))
		(set-window-buffer candidate buffer)
		(cl-return-from berry-select-window-for-popup candidate)))
	  (cl-incf outindex)

(defun berry-clear-popup-setting ()
  "clears the 'berrydesignated flag on all windows, thus removing the designation 
   of any given window to host popups. also removes the popup handler registration"
  (cl-loop for window in (window-list-1 nil nil t) do
	   (set-window-parameter window 'berrydesignated nil))
  (pop display-buffer-alist)

My usual window layout when programming in Emacs looks like the following (note that in emacs a window is more like a frame in most other environments):

|        | second |
|        | code   |
|primary | window |
|code    |--------|
|window  | REPL & |
|        | POPUPS |
|        |        |

So what I do after opening all the windows I want is I put my cursor in the "REPL & POPUPS" window and run berry-choose-window-for-popups. The content of my other windows remains undisturbed by IDE functions unless I tell the computer to change buffers in one of those windows.

Why I prefer emacs+evil to vim

Posted: 2015-10-22. Modified: 2015-12-21. Tags: LISP, Opinion, emacs.
  1. Variable width font support.

    I studied my own reading speed on a variety of documents, and found that I read variable width text about 15% faster than fixed width text. I don't mind writing code in fixed-width fonts, but if I am going to use one text editor for everything, then I very much appreciate having variable-width support.

  2. org-mode > markdown

    Org-mode allows you to write structured content within Emacs, and supports the writer with a variety of useful features and tools. Besides the rich editing, export, and extension possibilities offered by emacs org-mode itself, I find that the org format is superior to markdown for my purposes. Two primary reasons for this are that org provides syntax (such as drawers) for defining all sorts of metadata about your text, and also that org is designed in such a way that it is basically equally usable as variable-width text and fixed-width text. In particular I dislike the extent to which markdown relies on fixed-width text for its display features.

  3. evil >= vim.

    Emacs "Evil" mode pretty much provides a superset of commonly used vim functionality. Evil supports all the commonly used vim editing commands, which allows you to take advantage of vim's ergonomic design as you edit text. Evil actually improves on some vim features – for example, search and replace shows replacements being entered as you type them. Evil also provides access to the full power of Emacs just one M-x away – you get the ergonomics of vim with the power of emacs when you want it.

  4. superior extensibility (> emacs-lisp vimscript)

    Especially for a lisp fan such as myself, Emacs Lisp seems a superior language to Vimscript. Emacs Lisp is kind of like a baby version of common lisp, and supports a rich number of features on its own and with the addition of third-party libraries.

    However the real advantage of Emacs in extensibility is the fact that the majority of emacs is actually written in Emacs Lisp. Emacs' Github mirror indicates that the ratio of elisp to C in the project is ~4:1. I believe most of the C stuff is quite low-level, and is related to multiplatform support, core rendering, and the like. On the other hand Vim's Github repo indicates that Vim's vimscript C ratio is ~0.8:1. Since the vast majority of emacs is written in emacs-lisp, in the emacs environment you can very easily dive in to functionality from within emacs to understand and/or modify how things work.

  5. "self documenting" w/ C-h f, C-h k

    In the Emacs Manual it is stated that Emacs is "extensible, customizable, self-documenting real-time display editor". One feature I really like about emacs is the "self-documenting" component of that description. Emacs makes it very easy to look up the docstring of a given function or command, easy to determine what a keyboard shortcut does, easy to determine what shortcuts are available, easy to determine what functionality is present from various modes, and more. In short, emacs makes it possible to spend a great deal of time within emacs without having to go online to look up how to use a given function or tool.

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