Conditional Hydra

Code

(defhydra hydra-vi (:hint nil)
  "vi"
  ("j" next-line)
  ("k" previous-line)
  ("n" next-line)
  ("p" previous-line))

(setq hydra-vi/hint
      '(if (evenp (line-number-at-pos))
        (prog1 (eval
                (hydra--format nil '(nil nil :hint nil)
                               "\neven: _j_ _k_\n" hydra-vi/heads))
          (define-key hydra-vi/keymap "n" nil)
          (define-key hydra-vi/keymap "p" nil)
          (define-key hydra-vi/keymap "j" 'hydra-vi/next-line)
          (define-key hydra-vi/keymap "k" 'hydra-vi/previous-line))
        (prog1 (eval
                (hydra--format nil '(nil nil :hint nil)
                               "\nodd: _n_ _p_\n" hydra-vi/heads))
          (define-key hydra-vi/keymap "j" nil)
          (define-key hydra-vi/keymap "k" nil)
          (define-key hydra-vi/keymap "n" 'hydra-vi/next-line)
          (define-key hydra-vi/keymap "p" 'hydra-vi/previous-line))))

Commentary

defhydra defines hydra-vi/hint variable that’s evaled each time for the hint. We re-define it to modify the hydra’s docstring and bindings depending on the line number: on even line numbers j and k will be active, with the corresponding docstring, while on odd line numbers n and p will be active.

Basically we start with a single hydra template, which we modify dynamically depending on the situation.