GNU Emacs has a core model for how it operates, and some of its weird seeming limitations are easier to understand if you internalize that model. One of them is what you have to do in GNU Emacs to get the perfectly sensible operation of 'do <X> in a new frame or window'. For instance, one of the things I periodically want to do in MH-E is 'open a folder in a new frame', so that I can go through it while keeping my main MH-E environment on my inbox to process incoming email.

If you dig through existing GNU Emacs ELisp functions, you won't find a 'make-frame-do-operation' function, which is a bit frustrating. GNU Emacs has a whole collection of operations for making a new frame , and I can run mh-visit-folder in the context of this frame, so it seems like there should be a simple function I could invoke to do this and create my own 'C-x 5 v' binding for 'visit MH-E folder in other frame'.

The clue to what's going on is in the description of C-x 5 5 from the Creating Frames page of the manual, with the emphasis mine:

A more general prefix command that affects the buffer displayed by a subsequent command invoked after this prefix command ( other-frame-prefix ). It requests the buffer to be displayed by a subsequent command to be shown in another frame.

GNU Emacs frames (and windows) don't run commands and show their output, they display (GNU Emacs) buffers. In order to create a frame, you must have some buffer to display on that frame, and GNU Emacs must know what it is. GNU Emacs has some relatively complex and magical code to implement the 'C-x 5 5' and 'C-x 4 4' prefix commands, but it's all still fundamentally starting from having some buffer to display, not from running a command. The code basically assumes you're running a command that will at some point try to display a buffer, and it hooks into that 'please display this buffer' operation to make the new frame or window and then display the buffer in it.

(Buffers can be created to show files, but they can also be created for a lot of other purposes, including non-file buffers created by ELisp commands that want to present text to you. All of MH-E 's buffers are non-file ones, as are things like Magit 's information displays.)

The corollary of this is that the most straightforward way to write our own ELisp code to run a command in a new frame is to start out by switching to some buffer in another frame , such as ' *scratch* ', and then run our command. In an extremely minimal form, this looks like:

(defun mh-visit-folder-other-frame (folder &optional argp)
  "...."
  (interactive [...])
  (switch-to-buffer-other-frame "*scratch*")
  (mh-visit-folder folder argp))

If you know that your command displays a specific buffer, ideally you'll check to see if that buffer exists already and switch to it instead of to some scratch buffer that you're only using because you need to tell Emacs to display some buffer (any buffer) in the new frame.

(In normal GNU Emacs environments you can be pretty confident that there's a *scratch* buffer sitting around. GNU Emacs normally creates it on startup and most people don't delete it. And if you're writing your own code, you can definitely not delete it yourself.)

Now that I've written this entry, maybe I'll remember 'C-x 5 5' and also stop feeling vaguely irritated every time I do the equivalent by hand ('C-x 5 b', pick *scratch* , and then run my command in the newly created frame).

PS: It's probably possible to write a general ELisp function to run another function and make any buffers it wants to show come up on another frame, using the machinery that 'C-x 5 5' does. I will leave writing this function as an exercise for my readers (although maybe it already exists somewhere).