Ufdjrw

I have this keymap to flush empty lines like this:

(global-set-key (kbd "M-T") (lambda () (interactive) (flush-lines "^[[:space:]]*$")))

But this doesn't work over a region. Unrelated lines out of the region are getting merged. But when I select a region and manually try M-x flush-lines RET ^[[:space:]]*$ RET, it works as expected.
What am I doing wrong ?

A function can be called interactively or non-interactively (this is in fact a little bit more complex, but that's a good enough approximation). The interactive calling mode is used to bind some function arguments from the current environment, or by interacting with the user. The non-interactive mode is the normal calling mode when you write in Emacs Lisp.

Typically, the interactive behaviour is declared using an (interactive ...) form after the docstring, but in the particular case of flush-lines, the behaviour is partly automated by an interactive spec, and partly hardcoded (for the region part). You can ask Emacs whether the current code is executed interactively by calling called-interactively-p, but as the doc say, it is encouraged to have another optional argument that indicates whether the call is interactive.

The function signature is:

(flush-lines REGEXP &optional RSTART REND INTERACTIVE)

RSTART and REND are the starting and ending position of the current region, as integers. They can also be nil. The last parameter indicates whether the call is interactive.

When you call flush-lines from your anonymous function, you are calling it non-interactively, which means you need to pass arguments explicitly to the functions.

For this function, you can simply call it as follows:

(flush-lines "^[[:space:]]*$" nil nil t)

This makes the function behaves as-if called interactively, in which case the region is computed automatically since the start and end argument are nil.

In the general case, what you would do is have your own (interactive "r") declaration, and add two parameters to the anonymous function. For example, in the *scratch* buffer, you can evalue this expression:

(call-interactively
 (lambda (beg end)
 (interactive "r")
 (list beg end)))

Then, the result is a list of cursor positions, that represents your region.
In your case you would call (flush-lines "^[[:space:]]*$" beg end), for example.