Deep copy a string in Elisp?

Question

I have a propertized string. I want to make a deep copy of it to add more properties, while preserving the properties in the original string. How can I do that (easily)?

Example

Evaluate one-by-one :

(setq test-str-1
      #(";; This `is' a test"
        0 3 (fontified nil face font-lock-comment-delimiter-face)
        3 9 (fontified nil face font-lock-comment-face)
        9 11 (fontified nil face (font-lock-constant-face font-lock-comment-face))
        11 19 (fontified nil face font-lock-comment-face)))
(setq test-str-2 (concat test-str-1))
(add-face-text-property 0 (length test-str-2) 'foobar t test-str-2)

And the result:

test-str-2
;; =>
#(";; This `is' a test" 0 3 (fontified nil face (font-lock-comment-delimiter-face foobar))
  3 9 (fontified nil face (font-lock-comment-face foobar))
  9 11 (fontified nil face (font-lock-constant-face font-lock-comment-face foobar))
  11 19 (fontified nil face (font-lock-comment-face foobar)))
test-str-1
;; =>
#(";; This `is' a test" 0 3 (face font-lock-comment-delimiter-face fontified nil)
  3 9 (face font-lock-comment-face fontified nil)
  9 11 (face (font-lock-constant-face font-lock-comment-face foobar) ; <= foobar is here
        fontified nil)
  11 19 (face font-lock-comment-face fontified nil))

Answer 1

You can use the function font-lock-append-text-property to add the text property. It does not modify the value destructively.

For example:

(setq test-str-1
      #(";; This `is' a test"
        0 3 (fontified nil face font-lock-comment-delimiter-face)
        3 9 (fontified nil face font-lock-comment-face)
        9 11 (fontified nil face (font-lock-constant-face font-lock-comment-face))
        11 19 (fontified nil face font-lock-comment-face)))
(setq test-str-2 (concat test-str-1))
(font-lock-append-text-property 0 (length test-str-2) 'face '(foobar t) test-str-2)


test-str-1
#(";; This `is' a test"
  0 3 (face font-lock-comment-delimiter-face fontified nil)
  3 9 (face font-lock-comment-face fontified nil)
  9 11 (face (font-lock-constant-face font-lock-comment-face) fontified nil)
  11 19 (face font-lock-comment-face fontified nil))

test-str-2
#(";; This `is' a test"
  0 3 (fontified nil face (font-lock-comment-delimiter-face foobar t))
  3 9 (fontified nil face (font-lock-comment-face foobar t))
  9 11 (fontified nil face (font-lock-constant-face font-lock-comment-face foobar t))
  11 19 (fontified nil face (font-lock-comment-face foobar t)))

Here, in test-str-1, has retained its original value.

Answer 2

I found you can do this by iterating over the text properties, copying the underlying property data and overwriting the existing properties with new copies.

(defun deep-copy-text-properties (str)
  (with-temp-buffer
    (insert str)
    (goto-char 1)
    (while (not (eobp))
      (set-text-properties (point)
                           (goto-char (next-char-property-change (point) (point-max)))
                           ;; copy-tree is the important part
                           (copy-tree (text-properties-at (1- (point))))))
    (buffer-string)))

In my tests, this was about 20% faster than your read solution. I also wrote a version that did not use a temp buffer and modified the properties of a string which was less code but was slower.

Looking at the C code it copies the property plists, with copy_sequence which will rebuild the list structure but not copy the elements by value, so the properties like face in your example that have a list value are copied over by reference and modified. Bug or not, I don't know

Answer 3

You can use (concat the-original-string).

For example:

(let ((s "TEXT"))
  (set-text-properties 2 3 '(:foreground "blue") s)
  (let ((q (concat s)))
    (add-text-properties 2 3 '(:background "red") q)
    (cons s q)))
;; Returns:
(#("TEXT" 2 3 (:foreground "blue")) . #("TEXT" 2 3 (:foreground "blue" :background "red")))

Answer 4

Found a (not very efficient) work-around:

(setq test-str-2
      (read (prin1-to-string test-str-1)))