The mode-line-format can be a list of strings, but it can also include un-evaluated blocks, e.g:

(setq mode-line-format
    '(:eval (my-function-call))

I would like to store a list in a similar format which can be converted into a string.

However I don't want to use format-mode-line as I don't want %p etc.. to have special meanings.

Is there a utility function for doing this? Or would I need to implement the logic to extract/evaluate a string from the list myself?

  • format-mode-line can be used to store any portion of the mode-line as a string -- saving it to a local/global variable -- it is not just for special mode-line codes .... I use it to store an alphabetically sorted minor-mode-alist so that I can use the stored value when I need to save time; e.g., rapid fire movements ... scrolling up/down, moving cursor left/right, etc. The disadvantage, is that the mouse interaction with a saved string will be limited .... To store a value, you can use (setq my-variable (format-mode-line COMPONENT)) – lawlist Feb 3 at 7:16
  • Is there a way to make it not evaluate %p - and similar characters though? (besides replacing % with %%). I'm wondering if the ability to merge a string and evaluate :eval property is available in a more generic, less mode-line spesific function. – ideasman42 Feb 3 at 8:49

If you really just want a list (and not a tree like format-mode-line) that can be evaluated takes element-wise to strings and then concatenated mapconcat and eval are your friends:

(let ((my-list '("Hello, I am " (number-to-string (user-uid)) " and it is " (format-time-string " %T, %F"))))
  (mapconcat #'eval my-list ""))

The output is:

Hello, I am 1000 and it is  11:47:45, 2021-02-03

A more general version can also be implemented quite simple in a recursive way:

(defun tree-to-string (tree &optional lexical)
  "Convert TREE recursively to a string.
TREE can be one of the following:
- lists with car :eval : the cdr is evaluated and the result is passed to `tree-to-string'
- other lists: element-wise processed with `tree-to-string'
- any other element: transformed to string with `prin1-to-string'
The optional argument LEXICAL is passed to `eval'."
  (if (listp tree)
      (if (eq (car tree) :eval)
      (tree-to-string (eval (cons 'progn (cdr tree)) lexical))
    (mapconcat (lambda (item) (tree-to-string item lexical)) tree ""))
    (prin1-to-string tree t)))

(tree-to-string '("Hello, I am " (:eval (user-uid)) (" and it is " (:eval (format-time-string "%T, %F"))) "."))

The output is:

Hello, I am 1000 and it is 11:47:45, 2021-02-03.
  • 1
    I suggest calling eval with a non-nil second argument for lexical binding. – Basil Feb 3 at 10:29
  • @Basil Can you provide a practical example where this becomes relevant? I already tried some cases where I thought this would play a role. But the only example where I could get some advantage was with passing a specific lexical environment to eval. – Tobias Feb 5 at 10:45
  • It's relevant wherever Elisp is evaluated. Most such places (*scratch*, --eval, *ielm*, ...) are transitioning or have already transitioned to defaulting to lexical-binding, so it should be used and encouraged in general. You never know when a user might write a form that depends on lexical-binding (e.g. by using a closure) and subsequently be surprised that it doesn't work as expected. – Basil Feb 5 at 11:34

Adding an answer based on @Tobias's answer with changes.

  • Support :propertize (as mode-line-format does).
  • Support symbols (which are resolved into strings).
  • Call eval with lexical binding set to t.
  • Raise an error for unsupported values (instead of converting them into strings), this means mistakes are more easily caught.
(defun tree-to-string (tree)
  "Convert TREE recursively to a string.
TREE can be one of the following:
- lists with `car' `:eval'
  - the `cdr' is evaluated and the result is passed to `tree-to-string'
- lists with `car' `:propertize'
  - the `caar' is passed to `tree-to-string'.
  - the `cddr' is passed to as properties to `propertize'.
- other lists: element-wise processed with `tree-to-string'
- a symbol, it's value is transformed to string with `prin1-to-string'.
- any other element must be a string.
    ((stringp tree)
    ((null tree)
    ((symbolp tree)
      ;; Support non-string symbols, allows numbers etc to be included.
      (prin1-to-string (symbol-value tree) t))
    ((listp tree)
      (let ((tree-type (car tree)))
          ((eq tree-type :eval)
            (tree-to-string (eval (cons 'progn (cdr tree)) t)))
          ((eq tree-type :propertize)
            (pcase-let ((`(,item . ,rest) (cdr tree)))
              (apply 'propertize (cons (tree-to-string item) rest))))
            (mapconcat #'tree-to-string tree "")))))
        "Found '%S' (%S), not a string, nil or `:eval' cons or list of these"
        (type-of tree)

Example usage:

;; Define a value that can be evaluated into a string at any time.
(defvar my-custom-format
    "Hello, User files are in "
    " and it is "
    '(:eval (format-time-string "%T, %F "))
    '(:propertize "This text is green" face (:background "green"))

;; Evaluate and show the string.
(message "%s" (tree-to-string my-custom-format))

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