Dynamically apply and unapply transformers to your window layout. This can be used to rotate your window layout by 90 degrees, or to make the currently focused window occupy the whole screen ("zoom in") then undo the transformation ("zoom out").

The basic idea is to have a base layout and a set of layout transformers, of which at most one is active at any time. Enabling another transformer first disables any currently active transformer; i.e. it works like a group of radio buttons.

To use this module, you need some data types which represent transformers; for some commonly used transformers (including MIRROR, NOBORDERS, and FULL used in the examples below) you can simply import XMonad.Layout.MultiToggle.Instances.

Somewhere else in your file you probably have a definition of layout; the default looks like this:

layout = tiled ||| Mirror tiled ||| Full

After changing this to

layout = mkToggle (single MIRROR) (tiled ||| Full)

you can now dynamically apply the Mirror transformation:

...
  , ((modm,               xK_x     ), sendMessage $ Toggle MIRROR)
...

(That should be part of your key bindings.) When you press mod-x, the active layout is mirrored. Another mod-x and it's back to normal.

It's also possible to stack MultiToggles. For example:

layout = id
    . smartBorders
    . mkToggle (NOBORDERS ?? FULL ?? EOT)
    . mkToggle (single MIRROR)
    $ tiled ||| Grid ||| Circle

By binding a key to (sendMessage $ Toggle FULL) you can temporarily maximize windows, in addition to being able to rotate layouts and remove window borders.

You can also define your own transformers by creating a data type which is an instance of the Transformer class. For example, here is the definition of MIRROR:

data MIRROR = MIRROR deriving (Read, Show, Eq)
instance Transformer MIRROR Window where
    transform _ x k = k (Mirror x) (\(Mirror x') -> x')

Note, you need to put {-# LANGUAGE TypeSynonymInstances, MultiParamTypeClasses #-} at the beginning of your file.