Layout where new windows will split the focused window in half, based off of BSPWM

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

import XMonad.Layout.BinarySpacePartition

Then add the layout, using the default BSP (BinarySpacePartition)

myLayout = emptyBSP ||| etc ..

It may be a good idea to use XMonad.Actions.Navigation2D to move between the windows.

This layout responds to SetGeometry and is compatible with e.g. XMonad.Actions.MouseResize or XMonad.Layout.BorderResize. You should probably try both to decide which is better for you, if you want to be able to resize the splits with the mouse.

If you don't want to use the mouse, add the following key bindings to resize the splits with the keyboard:

, ((modm .|. altMask,                 xK_l     ), sendMessage $ ExpandTowards R)
, ((modm .|. altMask,                 xK_h     ), sendMessage $ ExpandTowards L)
, ((modm .|. altMask,                 xK_j     ), sendMessage $ ExpandTowards D)
, ((modm .|. altMask,                 xK_k     ), sendMessage $ ExpandTowards U)
, ((modm .|. altMask .|. ctrlMask ,   xK_l     ), sendMessage $ ShrinkFrom R)
, ((modm .|. altMask .|. ctrlMask ,   xK_h     ), sendMessage $ ShrinkFrom L)
, ((modm .|. altMask .|. ctrlMask ,   xK_j     ), sendMessage $ ShrinkFrom D)
, ((modm .|. altMask .|. ctrlMask ,   xK_k     ), sendMessage $ ShrinkFrom U)
, ((modm,                             xK_r     ), sendMessage Rotate)
, ((modm,                             xK_s     ), sendMessage Swap)
, ((modm,                             xK_n     ), sendMessage FocusParent)
, ((modm .|. ctrlMask,                xK_n     ), sendMessage SelectNode)
, ((modm .|. shiftMask,               xK_n     ), sendMessage MoveNode)
, ((modm .|. shiftMask .|. ctrlMask , xK_j     ), sendMessage $ SplitShift Prev)
, ((modm .|. shiftMask .|. ctrlMask , xK_k     ), sendMessage $ SplitShift Next)

Here's an alternative key mapping, this time using additionalKeysP, arrow keys, and slightly different behavior when resizing windows

, ("M-M1-<Left>",    sendMessage $ ExpandTowards L)
, ("M-M1-<Right>",   sendMessage $ ShrinkFrom L)
, ("M-M1-<Up>",      sendMessage $ ExpandTowards U)
, ("M-M1-<Down>",    sendMessage $ ShrinkFrom U)
, ("M-M1-C-<Left>",  sendMessage $ ShrinkFrom R)
, ("M-M1-C-<Right>", sendMessage $ ExpandTowards R)
, ("M-M1-C-<Up>",    sendMessage $ ShrinkFrom D)
, ("M-M1-C-<Down>",  sendMessage $ ExpandTowards D)
, ("M-s",            sendMessage $ Swap)
, ("M-M1-s",         sendMessage $ Rotate)
, ("M-S-C-j",        sendMessage $ SplitShift Prev)
, ("M-S-C-k",        sendMessage $ SplitShift Next)

Note that ExpandTowards x, ShrinkFrom x, and MoveSplit x are the same as respectively ExpandTowardsBy x 0.05, ShrinkFromBy x 0.05 and MoveSplitBy x 0.05.

If you have many windows open and the layout begins to look too hard to manage, you can Balance the layout, so that the current splittings are discarded and windows are tiled freshly in a way that the split depth is minimized. You can combine this with Equalize, which does not change your tree, but tunes the split ratios in a way that each window gets the same amount of space:

, ((myModMask,               xK_a),     sendMessage Balance)
, ((myModMask .|. shiftMask, xK_a),     sendMessage Equalize)