如何修改 XMonad 中的 windowSet？

Question

I have a function that looks like:

sinkFocus :: StackSet i l a s sd -> Maybe (StackSet i l a s sd)
sinkFocus = (fmap . flip sink) <*> peek

However I would like an X () so that I can use it. For example additionalKeys uses an X () .

The documentation says that X is a IO with some state and reader transformers, so I am under the impression that the StackSet is contained within the state of X . So in theory it should be possible to modify the relevant part of the state. However the state accessible is XState not the StackState I want, so I need to be able to turn my function on StackState to one on XState .

This would be easy enough if I had a function of the type

(StackSet i l a s sd -> StackSet i l a s sd) -> X ()

However after digging around in the documentation I haven't been able to piece together a way to do this yet. Is there a way to take a function on StackSet and make an X () that performs that function?

Answer 1

The X monad has an instance of MonadState

instance MonadState XState X

So we can use modify as

modify :: (XState -> XState) -> X ()

So we need to turn out function to one on XState s. And if we look at the definition

XState
    windowset :: !WindowSet
    mapped :: !(Set Window)
    waitingUnmap :: !(Map Window Int)
    dragging :: !(Maybe (Position -> Position -> X (), X ())) 
    numberlockMask :: !KeyMask
    extensibleState :: !(Map String (Either String StateExtension))

we will see WindowSet which is a type alias for a particular StackState . So you can turn a function from StackState s into one on XState s like so:

overWindowSet :: (WindowSet -> WindowSet) -> XState -> XState
overWindowSet f xState = xState { windowset = f (windowset xState) }

This can be combined with modify to make the complete function you would like:

modify . overWindowSet