Module for storing custom mutable state in xmonad.

To utilize this feature in a contrib module, create a data type and make it an instance of ExtensionClass. You can then use the functions from this module for storing and retrieving your data:

import qualified XMonad.Util.ExtensibleState as XS

data ListStorage = ListStorage [Integer]
instance ExtensionClass ListStorage where
  initialValue = ListStorage []

.. XS.put (ListStorage [23,42])

To retrieve the stored value call:

.. XS.get

If the type can't be inferred from the usage of the retrieved data, you have to add an explicit type signature:

.. XS.get :: X ListStorage

To make your data persistent between restarts, the data type needs to be an instance of Read and Show and the instance declaration has to be changed:

data ListStorage = ListStorage [Integer] deriving (Read,Show)

instance ExtensionClass ListStorage where
  initialValue = ListStorage []
  extensionType = PersistentExtension

One should take care that the string representation of the chosen type is unique among the stored values, otherwise it will be overwritten. Normally these string representations contain fully qualified module names when automatically deriving Typeable, so name collisions should not be a problem in most cases. A module should not try to store common datatypes(e.g. a list of Integers) without a custom data type as a wrapper to avoid collisions with other modules trying to store the same data type without a wrapper.