Nesting Hydras

Nesting Hydras

As you can see in Basics, a hydra named hydra-zoom generates a function named hydra-zoom/body. This function can immediately be used in another hydra, just like any other function. The only thing that you have to remember is to add :exit t to the appropriate head, since only one hydra can be active at a time.

See hydra-ox.el for an example implementation of a nested hydra.

Order of calls to :post and :body-pre functions

Consider these two hydras:

(defhydra test-a (:post (message "post a"))
  "a"
  ("q" test-b/body "to b" :exit t))

(defhydra test-b (:body-pre (message "pre b"))
  "b"
  ("q" nil "exit" :exit t))

Hitting key q from hydra test-a prints these messages:

post a
pre-b

That is:

• first, the old hydra exits.
• second, test-b/body is called.

Visiting other hydras temporarily

Here's an example of visiting another hydra and going back to the original one on exit. Starting from hydra-a, pressing bcqq will take you to "b", "c", "b", "a". And after one more q, you'll exit.

(defvar hydra-stack nil)

(defun hydra-push (expr)
  (push `(lambda () ,expr) hydra-stack))

(defun hydra-pop ()
  (interactive)
  (let ((x (pop hydra-stack)))
    (when x
      (funcall x))))

(defhydra hydra-a (:color teal)
  "a"
  ("b" (progn
         (hydra-b/body)
         (hydra-push '(hydra-a/body)))
       "visit hydra-b")
  ("c" (progn
         (hydra-c/body)
         (hydra-push '(hydra-a/body)))
       "visit hydra-c")
  ("i" (message "I am a") :exit nil)
  ("q" hydra-pop "exit"))

(defhydra hydra-b (:color teal)
  "b"
  ("a" (progn
         (hydra-a/body)
         (hydra-push '(hydra-b/body)))
       "visit hydra-a")
  ("c" (progn
         (hydra-c/body)
         (hydra-push '(hydra-b/body)))
       "visit hydra-c")
  ("i" (message "I am b") :exit nil)
  ("q" hydra-pop "exit"))

(defhydra hydra-c (:color teal)
  "c"
  ("a" (progn
         (hydra-a/body)
         (hydra-push '(hydra-c/body)))
       "visit hydra-a")
  ("b" (progn
         (hydra-b/body)
         (hydra-push '(hydra-c/body)))
       "visit hydra-b")
  ("i" (message "I am c") :exit nil)
  ("q" hydra-pop "exit"))