This is a layout modifier that will make a layout change the size of the window that has focus.

Example screenshot using magnifiercz' 1.3 with one of the two stack windows focused.

You can use this module with the following in your xmonad.hs:

import XMonad.Layout.Magnifier

Then edit your layoutHook by e.g. adding the magnifier layout modifier to some layout:

myLayout = magnifier (Tall 1 (3/100) (1/2))  ||| Full ||| etc..
main = xmonad def { layoutHook = myLayout }

By default magnifier increases the focused window's size by 1.5.

You can also use magnifiercz 1.2 or magnifierxy 1 1000 to use a custom level of magnification. You can even make the focused window smaller for a pop in effect. There's also the possibility of starting out not magnifying anything at all (magnifierOff); see below for ways to toggle this on while in use.

The most general combinator available is magnify—all of the other functions in this module are essentially just creative applications of it.

For more detailed instructions on editing the layoutHook see the tutorial and XMonad.Doc.Extending.

Magnifier supports some commands, see MagnifyMsg. To use them add something like this to your key bindings:

   , ((modm .|. controlMask              , xK_plus ), sendMessage MagnifyMore)
   , ((modm .|. controlMask              , xK_minus), sendMessage MagnifyLess)
   , ((modm .|. controlMask              , xK_o    ), sendMessage ToggleOff  )
   , ((modm .|. controlMask .|. shiftMask, xK_o    ), sendMessage ToggleOn   )
   , ((modm .|. controlMask              , xK_m    ), sendMessage Toggle     )

Note that a few other extension modules, such as XMonad.Layout.MultiToggle and XMonad.Layout.ToggleLayouts, also define a message named Toggle. To avoid conflicts when using these modules together, you can import Magnifier qualified, like this:

import qualified XMonad.Layout.Magnifier as Mag

and then prefix Mag to the front of everything from this module, like Mag.Toggle, Mag.magnifier, and so on.

For detailed instruction on editing the key binding see the tutorial.