How to get an insert-text-button 'action property accessing a dynamically-bound buffer-local variable

Question

In realgud I have a text button that needs access to a dynamically-bound buffer-local variable.

The situation occurs like this. Inside one buffer that has the buffer local variable, there is a command that creates another "Describe" buffer with the value of that complex defstruct variable formatted nicely using org mode.

But inside that "Describe" buffer I would like to have a button that calls describe-variable on the variable, but have that use the value from its original buffer location, not the value in the describe buffer.

Currently I have inside insert-text-button an action property like this:

'action '(lambda(button) (describe-variable 'realgud-cmdbuf-info))

However this gives picks up the wrong buffer-local value. I can't change the above say to:

'action '(lambda(button) (with-current-buffer buffer (describe-variable 'realgud-cmdbuf-info)))

because of the quote in front of the lambda, and because with-current-buffer is a macro. Changing that to start `list('lambda(button) would be cumbersome if I could figure it out.

Probably I need to use some macro here, but that also is beyond my comprehension.

Help?

`

Answer 1

You can add not only action but any other attributes to a text-button, that can be referred later with button-get function. So saving the (reference to the) current buffer, together with action, seems a good idea here.

(insert-text-button
     "realgud-cmdbuf-info"
     'buffer buffer
     'action (lambda (b)
           (with-current-buffer (button-get b 'buffer)
             (describe-variable 'realgud-cmdbuf-info)))

Another solution is to enable lexical-binding in your script, that simply disables dynamic-binding.

Answer 2

Start your file with

;; Copyright (C) 2015-2016 Free Software Foundation, Inc -*- lexical-binding: t -*-

Now you're able to use closures in your Lisp code, e.g.

(let ((x 0))
  (lambda () (setq x (1+ x))))

will create a count function. The same applies to your anonymous function, it will close over it's lexical environment, i.e. it knows about the variable buffer.

(insert-text-button
 "realgud-cmdbuf-info"
 'action (lambda (_button) ;; underscore makes compiler happy
           (describe-variable 'realgud-cmdbuf-info buffer))
 'help-echo "mouse-2: help-on-variable")

In rare circumstances, enabling lexical-binding can break your code, but only if you did something stupid to begin with. For example accessing local variables from a function up the call-stack. And of course this won't work, if you're programming for ancient Emacs (I believe it was introduced in 24.1).