This is a package for GNU Emacs that can be used to tie related commands into a family of short bindings with a common prefix - a Hydra.

Once you summon the Hydra through the prefixed binding (the body + any one head), all heads can be called in succession with only a short extension.

The Hydra is vanquished once Hercules, any binding that isn't the Hydra's head, arrives. Note that Hercules, besides vanquishing the Hydra, will still serve his orignal purpose, calling his proper command. This makes the Hydra very seamless, it's like a minor mode that disables itself auto-magically.

Here's how I use the examples bundled with Hydra:

(require 'hydra-examples)
(hydra-create "C-M-y" hydra-example-move-window-splitter)
(hydra-create "M-g" hydra-example-goto-error)

You can expand the examples in-place, it still looks elegant:

(hydra-create "<f2>"
  '(("g" text-scale-increase)
    ("l" text-scale-decrease)))

See the introductory blog post for more information.

Using Hydra to define bindings other than global ones

You can use the third optional argument of hydra-create for this (it defaults to global-set-key). Here's an example:

(hydra-create "C-z"
  '(("l" forward-char)
    ("h" backward-char)
    ("j" next-line)
    ("k" previous-line))
  (lambda (key command)
    (define-key lispy-mode-map key command)))

For this simple case, there's even a shortcut: if you give a keymap as the third argument, the lambda will be generated for you:

(hydra-create "C-z"
  '(("l" forward-char)
    ("h" backward-char)
    ("j" next-line)
    ("k" previous-line))
    lispy-mode-map)

Can Hydras can be helpful?

They can, if

(setq hydra-is-helpful t)

In that case, you'll get a hint in the echo area consisting of current Hydra's heads. You can even add comments to the heads like this:

(defvar hydra-example-text-scale
  '(("g" text-scale-increase "zoom in")
    ("l" text-scale-decrease "zoom out"))
  "A two-headed hydra for text scale manipulation.")

With this, you'll see hydra: [g]: zoom in, [l]: zoom out. in your echo area, once the zoom Hydra becomes active.