[xmonad] Strange workspace switching bug in my config

[Norbert Zeh]

Leo Alekseyev [2012.05.13 1923 -0400]:
> Hi Norbert,
> Yes, my original math was kind of dodgy...  Your solution seems
> better, but it doesn't compile for me...  any ideas?
> 
> xmonad.hs:452:14:
>     No instance for (MonadState
>                        XState ((->) (W.StackSet [Char] l3 a3 s3 sd3)))
>       arising from a use of `gets'
>     Possible fix:
>       add an instance declaration for
>       (MonadState XState ((->) (W.StackSet [Char] l3 a3 s3 sd3)))
>     In the first argument of `(>>=)', namely
>       `gets (tag . W.workspace . W.current . windowset)'
>     In the expression:
>       gets (tag . W.workspace . W.current . windowset)
>       >>= W.greedyView . toMain
>     In an equation for `jumpToMain':
>         jumpToMain
>           = gets (tag . W.workspace . W.current . windowset)
>             >>= W.greedyView . toMain
>           where
>               toMain ws = [head ws]
> 
> 
> xmonad.hs:456:17:
>     No instance for (MonadState
>                        XState ((->) (W.StackSet [Char] l1 a1 s1 sd1)))
>       arising from a use of `gets'
>     Possible fix:
>       add an instance declaration for
>       (MonadState XState ((->) (W.StackSet [Char] l1 a1 s1 sd1)))
>     In the first argument of `(>>=)', namely
>       `gets (tag . W.workspace . W.current . windowset)'
>     In the expression:
>       gets (tag . W.workspace . W.current . windowset)
>       >>= W.greedyView . toScratch
>     In an equation for `jumpToScratch':
>         jumpToScratch
>           = gets (tag . W.workspace . W.current . windowset)
>             >>= W.greedyView . toScratch
>           where
>               toScratch ws
>                 | length ws == 1 = ws ++ "SCR"
>                 | otherwise = ws

Sorry.  I didn't check whether it compiles and got the type of greedyView wrong.
Here's a correct (and slightly cleaner version):

jumpToMain = withWindowSet (windows . W.greedyView . toMain . W.tag . W.workspace . W.current)
  where
    toMain ws = [head ws]

jumpToScratch = withWindowSet (windows . W.greedyView . toScratch . W.tag . W.workspace . W.current)
  where
    toScratch ws | length ws == 1 = ws ++ "SCR"
                 | otherwise      = ws