Returning to the parent hydra

Question

I'm looking for a way to return to the parent hydra after using certain commands. In this example

(defhydra hydra-a (:color teal)
  "a"
  ("b" (progn
         (hydra-b/body)
         (hydra-push '(hydra-a/body)))
   "visit hydra-b")
  ("q" hydra-pop "exit"))

(defhydra hydra-b (:color teal)
  "b"
  ("i" forward-line :exit nil)    
  ("q" hydra-pop "exit"))

I need some kind of keyword instead of exit for this line ("i" forward-line :exit nil). If I use :exit t the parent hydra is also closed.

Answer 1

You can do this by maintaining a stack of hydras, as described on the hydra wiki page Nesting Hydras.

The basic mechanism described there is to add your own push/pop commands. See the full example for details, but here are the key pieces:

(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))))

Then wiring this in to your hydra definitions, e.g. define an entry in hydra a that will push on to the stack and switch to hydra b:

"b" (progn
        (hydra-b/body)
        (hydra-push '(hydra-a/body)))
      "visit hydra-b")

and in hydra b defining quit to pop the stack:

 ("q" hydra-pop "exit")

Answer 2

That's what I actually wanted:

(defhydra hydra-b (:color teal)
  "b"
   ("1" (and (call-interactively 'ace-window) (hydra-pop)))
   ("q" hydra-pop "exit"))

I simply have to pop the child hydra after the command in order to return to the parent hydra.