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In minor-mode-alist documentation it is described that lighter can be a lisp expression that is evaluated to get text used in the display of minor mode in mode line. It has the form of (:eval ...)

Given a function that makes such a string upon evaluation, for example, called compose-mode-line (no parameters), how minor-mode's lighter can be set to results of evaluation of that function?

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You can do:

(define-minor-mode ...
  :lighter (:eval (compose-mode-line))
  ...)

Note that this will re-compute (compose-mode-line) very frequently (after every key press, typically), so make sure it does not perform longish or blocking computations.

  • i have a branching on a symbol equality there, which is, i believe, cheap enough. – Srv19 Jul 12 '16 at 13:23
  • Oh, yes, that's very cheap. – Stefan Jul 12 '16 at 13:37

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