[xmonad] Managehook which only shifts window if workspace is empty

[Jochen Keil]

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Hi,

On 02/11/2012 11:46 PM, Toby Cubitt wrote:
> I'm trying to write a Managehook that only shifts a window to 
> workspace if that workspace is empty. But I'm quickly realising I 
> don't have a good enough grasp of Xmonad types, and Haskell monads 
> in general, to get it to work.
> 
> So far, the closest I think I've got is the following:
> 
> myManageHook = [ className =? "mutt" --> doShiftEmpty "email" ] 
> where doShiftEmpty t = when (isEmpty t) (doShift t) isEmpty t = do 
> wsl <- gets $ W.workspaces . windowset let mws = find (\ws ->
> W.tag ws == t) wsl return $ maybe True (isNothing . W.stack) mws
> 
> which throws a type error:
> 
> "Couldn't match expected type `Bool' with actual type `m0 Bool' In 
> the return type of a call of `isEmpty'"
you probally want to try "whenX" then.

You can easily the test if a workspace is empty like this:

MonadState XState m => WorkspaceId -> m Bool
\t -> gets $ elem t . map S.tag
           . filter (isNothing . S.stack)
           . S.workspaces . windowset


> I think I vaguely understand the problem, namely that "when" 
> expects a Bool as its first argument, whereas the "return" in 
> isEmpty returns a Bool wrapped inside a monad. But removing the 
> "return" just throws a more confusing type error. Clearly, I'm way 
> out of my depth here, and am doing little better than cargo cult 
> programming.
> 
> Can someone who understands these things properly explain where
> I'm going wrong here?
> 
> More generally, is there a tutorial explaining the Xmonad types
> and data structures, and how they go together to make up Xmoand?
> I've been working
XMonad is documented on xmonad.org as annotated source files. There
are also some documents describing how to write extensions/hooks.
I don't have the links at hand atm, but I think you'll find them
without problems.
Aaaaaaaaand.. there's always the source. :)

> through some basic Haskell tutorials, but there's still quite a 
> step from understanding a little Haskell, to understanding enough 
> of how Xmonad works to write a simple Managehook action.
For learning Haskell from my experiences I can recommend "Real World
Haskell", reading xmonad's source, and hacking on xmonad..

Best wishes,

Jochen

Sorry for the noise. As it happened many times before I replied only
the sender and forgot to cc the list.
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