Unlike the opaque IO actions that X actions can wrap, regular reads from the XConf and modifications to the XState are fundamentally pure—contrary to the current treatment of such actions in most xmonad code. Pure modifications to the WindowSet can be readily composed, but due to the need for those modifications to be properly handled by windows, other pure changes to the XState cannot be interleaved with those changes to the WindowSet without superfluous refreshes, hence breaking composability.

This module aims to rectify that situation by drawing attention to it and providing PureX: a pure type with the same monadic interface to state as X. The XLike typeclass enables writing actions generic over the two monads; if pure, existing X actions can be generalised with only a change to the type signature. Various other utilities are provided, in particular the defile function which is needed by end-users.

The suggested pattern of usage for this module is to write composable, pure actions as XLike m => m Any or PureX Any values, where the encapsulated Any value encodes whether or not a refresh is needed to properly institute changes. These values can then be combined monoidally (i.e. with <> AKA <+>) or with operators such as <*, *>, <? and &> to build seamless new actions. The end user can run and handle the effects of the pure actions in the X monad by applying the defile function, which you may want to re-export. Alternatively, if an action does not make stackset changes that need to be handled by windows, it can be written with as an XLike m => m () and used directly.

Unfortunately since layouts must handle messages in the X monad, this approach does not quite apply to actions involving them. However a relatively direct translation to impure actions is possible: you can write composable, refresh-tracking actions as X Any values, making sure to eschew refresh-inducing functions like windows and sendMessage in favour of modifyWindowSet and utilities provided by XMonad.Actions.MessageFeedback. The windowBracket_ function recently added to XMonad.Operations is the impure analogue of defile. Note that PureX Any actions can be composed into impure ones after applying toX; don't use defile for this. E.g.

windowBracket_ (composableImpureAction <> toX composablePureAction)

Although both X and PureX have Monoid instances over monoidal values, (XLike m, Monoid a) is not enough to infer Monoid (m a) (due to the open-world assumption). Hence a Monoid (m Any) constraint may need to be used when working with XLike m => m Any where no context is forcing m to unify with X or PureX. This can also be avoided by working with PureX Any values and generalising them with toXLike where necessary.

PureX also enables a more monadic style when writing windowset operations; see the implementation of the utilities in this module for examples. For an example of a whole module written in terms of this one, see XMonad.Hooks.RefocusLast.