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POHow to prevent Emacs from setting an undo boundary?
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copied!<p>I've written an Emacs Lisp function which calls a shell command to process a given string and return the resulting string. Here is a simplified example which just calls <code>tr</code> to convert text to uppercase:</p> <pre><code>(defun test-shell-command (str) "Apply tr to STR to convert lowercase letters to uppercase." (let ((buffer (generate-new-buffer "*temp*"))) (with-current-buffer buffer (insert str) (call-process-region (point-min) (point-max) "tr" t t nil "'a-z'" "'A-Z'") (buffer-string)))) </code></pre> <p>This function creates a temporary buffer, inserts the text, calls <code>tr</code>, replaces the text with the result, and returns the result.</p> <p>The above function works as expected, however, when I write a wrapper around this function to apply the command to the region, two steps are being added to the undo history. Here's another example:</p> <pre><code>(defun test-shell-command-region (begin end) "Apply tr to region from BEGIN to END." (interactive "*r") (insert (test-shell-command (delete-and-extract-region begin end)))) </code></pre> <p>When I call <code>M-x test-shell-command-on-region</code>, the region is replaced with the uppercase text, but when I press <code>C-_</code> (<code>undo</code>), the first step in the undo history is the state with the text deleted. Going two steps back, the original text is restored.</p> <p>My question is, how does one prevent the intermediate step from being added to the undo history? I've read the <a href="http://www.gnu.org/software/emacs/manual/html_node/emacs/Undo.html">Emacs documentation on undo</a>, but it doesn't seem to address this as far as I can tell.</p> <p>Here's a function which accomplishes the same thing by calling the built-in Emacs function <code>upcase</code>, just as before: on the result of <code>delete-and-extract-region</code> with the result being handed off to <code>insert</code>:</p> <pre><code>(defun test-upcase-region (begin end) "Apply upcase to region from BEGIN to END." (interactive "*r") (insert (upcase (delete-and-extract-region begin end)))) </code></pre> <p>When calling <code>M-x test-upcase-region</code>, there is only one step in the undo history, as expected. So, it seems to be the case that calling <code>test-shell-command</code> creates an undo boundary. Can that be avoided somehow?</p>

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