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.

A side effect of this meta-layout is that layout transformers no longer receive any messages; any message not handled by SwitchTrans itself will undo the current layout transformer, pass the message on to the base layout, then reapply the transformer.

To use this module, you first have to define the transformers that you want to be handled by MultiToggle. For example, if the transformer is Mirror:

 data MIRROR = MIRROR deriving (Read, Show, Eq, Typeable)
 instance Transformer MIRROR Window where
     transform _ x k = k (Mirror x)

MIRROR can be any identifier (it has to start with an uppercase letter, of course); I've chosen an all-uppercase version of the transforming function's name here. You need to put {-# OPTIONS_GHC -fglasgow-exts #-} at the beginning of your file to be able to derive Data.Typeable.

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 (MIRROR ?? EOT) (tiled ||| Full)

you can now dynamically apply the Mirror transformation:

 ...
   , ((modMask,               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. Let's define a few more transformers (noBorders is in XMonad.Layout.NoBorders):

 data NOBORDERS = NOBORDERS deriving (Read, Show, Eq, Typeable)
 instance Transformer NOBORDERS Window where
     transform _ x k = k (noBorders x)
 
 data FULL = FULL deriving (Read, Show, Eq, Typeable)
 instance Transformer FULL Window where
     transform _ x k = k Full

 layout = id
     . smartBorders
     . mkToggle (NOBORDERS ?? FULL ?? EOT)
     . mkToggle (MIRROR ?? EOT)
     $ 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.