summaryrefslogtreecommitdiffstats
path: root/StackSet.hs
blob: e9006f71ba1236d8d0d780592e12a7af478786bf (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
{-# LANGUAGE PatternGuards #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  StackSet
-- Copyright   :  (c) Don Stewart 2007
-- License     :  BSD3-style (see LICENSE)
--
-- Maintainer  :  dons@galois.com
-- Stability   :  experimental
-- Portability :  portable, Haskell 98
--

module StackSet (
        -- * Introduction
        -- $intro
        StackSet(..), Workspace(..), Screen(..), Stack(..), RationalRect(..),
        -- *  Construction
        -- $construction
        new, view, greedyView,
        -- * Xinerama operations
        -- $xinerama
        lookupWorkspace,
        screens, workspaces, allWindows,
        -- *  Operations on the current stack
        -- $stackOperations
        peek, index, integrate, integrate', differentiate,
        focusUp, focusDown, focusMaster, focusWindow,
        tagMember, renameTag, ensureTags, member, findIndex, mapWorkspace, mapLayout,
        -- * Modifying the stackset
        -- $modifyStackset
        insertUp, delete, delete', filter,
        -- * Setting the master window
        -- $settingMW
        swapUp, swapDown, swapMaster, modify, modify', float, sink, -- needed by users
        -- * Composite operations
        -- $composite
        shift, shiftWin,

        -- for testing
        abort
    ) where

import Prelude hiding (filter)
import Data.Maybe   (listToMaybe,fromJust)
import qualified Data.List as L (deleteBy,find,splitAt,filter,nub)
import Data.List ( (\\) )
import qualified Data.Map  as M (Map,insert,delete,empty)

-- $intro
--
-- The 'StackSet' data type encodes a window manager abstraction. The
-- window manager is a set of virtual workspaces. On each workspace is a
-- stack of windows. A given workspace is always current, and a given
-- window on each workspace has focus. The focused window on the current
-- workspace is the one which will take user input. It can be visualised
-- as follows:
--
-- > Workspace  { 0*}   { 1 }   { 2 }   { 3 }   { 4 }
-- >
-- > Windows    [1      []      [3*     [6*]    []
-- >            ,2*]            ,4
-- >                            ,5]
--
-- Note that workspaces are indexed from 0, windows are numbered
-- uniquely. A '*' indicates the window on each workspace that has
-- focus, and which workspace is current.
--
-- Zipper
--
-- We encode all the focus tracking directly in the data structure, with a 'zipper':
--
--    A Zipper is essentially an `updateable' and yet pure functional
--    cursor into a data structure. Zipper is also a delimited
--    continuation reified as a data structure.
--
--    The Zipper lets us replace an item deep in a complex data
--    structure, e.g., a tree or a term, without an  mutation.  The
--    resulting data structure will share as much of its components with
--    the old structure as possible.
--
--      Oleg Kiselyov, 27 Apr 2005, haskell\@, "Zipper as a delimited continuation"
--
-- We use the zipper to keep track of the focused workspace and the
-- focused window on each workspace, allowing us to have correct focus
-- by construction. We closely follow Huet's original implementation:
--
--      G. Huet, /Functional Pearl: The Zipper/,
--      1997, J. Functional Programming 75(5):549-554.
-- and:
--      R. Hinze and J. Jeuring, /Functional Pearl: The Web/.
--
-- and Conor McBride's zipper differentiation paper.
-- Another good reference is:
--
--      The Zipper, Haskell wikibook
--
-- Xinerama support:
--
-- Xinerama in X11 lets us view multiple virtual workspaces
-- simultaneously. While only one will ever be in focus (i.e. will
-- receive keyboard events), other workspaces may be passively viewable.
-- We thus need to track which virtual workspaces are associated
-- (viewed) on which physical screens. We use a simple Map Workspace
-- Screen for this.
--
-- Master and Focus
--
-- Each stack tracks a focused item, and for tiling purposes also tracks
-- a 'master' position. The connection between 'master' and 'focus'
-- needs to be well defined. Particular in relation to 'insert' and
-- 'delete'.
--

-- |
-- API changes from xmonad 0.1:
-- StackSet constructor arguments changed. StackSet workspace window screen
--
-- * new,                    -- was: empty
--
-- * view,
--
-- * index,
--
-- * peek,                   -- was: peek\/peekStack
--
-- * focusUp, focusDown,  -- was: rotate
--
-- * swapUp, swapDown
--
-- * focus                   -- was: raiseFocus
--
-- * insertUp,             -- was: insert\/push
--
-- * delete,
--
-- * swapMaster,             -- was: promote\/swap
--
-- * member,
--
-- * shift,
--
-- * lookupWorkspace,        -- was: workspace
--
-- * visibleWorkspaces       -- gone.
--
------------------------------------------------------------------------
-- |
-- A cursor into a non-empty list of workspaces.
--
-- We puncture the workspace list, producing a hole in the structure
-- used to track the currently focused workspace. The two other lists
-- that are produced are used to track those workspaces visible as
-- Xinerama screens, and those workspaces not visible anywhere.

data StackSet i l a sid sd =
    StackSet { current  :: !(Screen i l a sid sd)    -- ^ currently focused workspace
             , visible  :: [Screen i l a sid sd]     -- ^ non-focused workspaces, visible in xinerama
             , hidden   :: [Workspace i l a]      -- ^ workspaces not visible anywhere
             , floating :: M.Map a RationalRect -- ^ floating windows
             } deriving (Show, Read, Eq)

-- | Visible workspaces, and their Xinerama screens.
data Screen i l a sid sd = Screen { workspace :: !(Workspace i l a)
                                  , screen :: !sid
                                  , screenDetail :: !sd }
    deriving (Show, Read, Eq)

-- |
-- A workspace is just a tag - its index - and a stack
--
data Workspace i l a = Workspace  { tag :: !i, layout :: l, stack :: Maybe (Stack a) }
    deriving (Show, Read, Eq)

-- | A structure for window geometries
data RationalRect = RationalRect Rational Rational Rational Rational
    deriving (Show, Read, Eq)

-- |
-- A stack is a cursor onto a (possibly empty) window list.
-- The data structure tracks focus by construction, and
-- the master window is by convention the top-most item.
-- Focus operations will not reorder the list that results from
-- flattening the cursor. The structure can be envisaged as:
--
-- >    +-- master:  < '7' >
-- > up |            [ '2' ]
-- >    +---------   [ '3' ]
-- > focus:          < '4' >
-- > dn +----------- [ '8' ]
--
-- A 'Stack' can be viewed as a list with a hole punched in it to make
-- the focused position. Under the zipper\/calculus view of such
-- structures, it is the differentiation of a [a], and integrating it
-- back has a natural implementation used in 'index'.
--
data Stack a = Stack { focus  :: !a        -- focused thing in this set
                     , up     :: [a]       -- clowns to the left
                     , down   :: [a] }     -- jokers to the right
    deriving (Show, Read, Eq)


-- | this function indicates to catch that an error is expected
abort :: String -> a
abort x = error $ "xmonad: StackSet: " ++ x

-- ---------------------------------------------------------------------
-- $construction

-- | /O(n)/. Create a new stackset, of empty stacks, with given tags, with
-- 'm' physical screens. 'm' should be less than or equal to the number of
-- workspace tags.  The first workspace in the list will be current.
--
-- Xinerama: Virtual workspaces are assigned to physical screens, starting at 0.
--
new :: (Integral s) => l -> [i] -> [sd] -> StackSet i l a s sd
new l wids m | not (null wids) && length m <= length wids = StackSet cur visi unseen M.empty
  where (seen,unseen) = L.splitAt (length m) $ map (\i -> Workspace i l Nothing) wids
        (cur:visi)    = [ Screen i s sd |  (i, s, sd) <- zip3 seen [0..] m ]
                -- now zip up visibles with their screen id
new _ _ _ = abort "non-positive argument to StackSet.new"

-- |
-- /O(w)/. Set focus to the workspace with index \'i\'.
-- If the index is out of range, return the original StackSet.
--
-- Xinerama: If the workspace is not visible on any Xinerama screen, it
-- becomes the current screen. If it is in the visible list, it becomes
-- current.

view :: (Eq s, Eq i) => i -> StackSet i l a s sd -> StackSet i l a s sd
view i s
    | not (i `tagMember` s)
      || i == tag (workspace (current s)) = s  -- out of bounds or current

    | Just x <- L.find ((i==).tag.workspace) (visible s)
    -- if it is visible, it is just raised
    = s { current = x, visible = current s : L.deleteBy (equating screen) x (visible s) }

    | Just x <- L.find ((i==).tag)           (hidden  s) -- must be hidden then
    -- if it was hidden, it is raised on the xine screen currently used
    = s { current = (current s) { workspace = x }
        , hidden = workspace (current s) : L.deleteBy (equating tag) x (hidden s) }

    | otherwise = s -- can't happen: all workspaces are either invalid, current, visible, or hidden

  where equating f = \x y -> f x == f y

    -- 'Catch'ing this might be hard. Relies on monotonically increasing
    -- workspace tags defined in 'new'
    --
    -- and now tags are not monotonic, what happens here?

-- |
-- Set focus to the given workspace.  If that workspace does not exist
-- in the stackset, the original workspace is returned.  If that workspace is
-- 'hidden', then display that workspace on the current screen, and move the
-- current workspace to 'hidden'.  If that workspace is 'visible' on another
-- screen, the workspaces of the current screen and the other screen are
-- swapped.

greedyView :: (Eq s, Eq i) => i -> StackSet i l a s sd -> StackSet i l a s sd
greedyView w ws
     | any wTag (hidden ws) = view w ws
     | (Just s) <- L.find (wTag . workspace) (visible ws)
                            = ws { current = (current ws) { workspace = workspace s }
                                 , visible = s { workspace = workspace (current ws) }
                                           : L.filter (not . wTag . workspace) (visible ws) }
     | otherwise = ws
   where wTag = (w == ) . tag

-- ---------------------------------------------------------------------
-- $xinerama

-- | Find the tag of the workspace visible on Xinerama screen 'sc'.
-- Nothing if screen is out of bounds.
lookupWorkspace :: Eq s => s -> StackSet i l a s sd -> Maybe i
lookupWorkspace sc w = listToMaybe [ tag i | Screen i s _ <- current w : visible w, s == sc ]

-- ---------------------------------------------------------------------
-- $stackOperations

-- |
-- The 'with' function takes a default value, a function, and a
-- StackSet. If the current stack is Nothing, 'with' returns the
-- default value. Otherwise, it applies the function to the stack,
-- returning the result. It is like 'maybe' for the focused workspace.
--
with :: b -> (Stack a -> b) -> StackSet i l a s sd -> b
with dflt f = maybe dflt f . stack . workspace . current

-- |
-- Apply a function, and a default value for Nothing, to modify the current stack.
--
modify :: Maybe (Stack a) -> (Stack a -> Maybe (Stack a)) -> StackSet i l a s sd -> StackSet i l a s sd
modify d f s = s { current = (current s)
                        { workspace = (workspace (current s)) { stack = with d f s }}}

-- |
-- Apply a function to modify the current stack if it isn't empty, and we don't
--  want to empty it.
--
modify' :: (Stack a -> Stack a) -> StackSet i l a s sd -> StackSet i l a s sd
modify' f = modify Nothing (Just . f)

-- |
-- /O(1)/. Extract the focused element of the current stack.
-- Return Just that element, or Nothing for an empty stack.
--
peek :: StackSet i l a s sd -> Maybe a
peek = with Nothing (return . focus)

-- |
-- /O(n)/. Flatten a Stack into a list.
--
integrate :: Stack a -> [a]
integrate (Stack x l r) = reverse l ++ x : r

-- |
-- /O(n)/ Flatten a possibly empty stack into a list.
integrate' :: Maybe (Stack a) -> [a]
integrate' = maybe [] integrate

-- |
-- /O(n)/. Texture a list.
--
differentiate :: [a] -> Maybe (Stack a)
differentiate []     = Nothing
differentiate (x:xs) = Just $ Stack x [] xs

-- |
-- /O(n)/. 'filter p s' returns the elements of 's' such that 'p' evaluates to
-- True.  Order is preserved, and focus moves as described for 'delete'.
--
filter :: (a -> Bool) -> Stack a -> Maybe (Stack a)
filter p (Stack f ls rs) = case L.filter p (f:rs) of
    f':rs' -> Just $ Stack f' (L.filter p ls) rs'    -- maybe move focus down
    []     -> case L.filter p ls of                  -- filter back up
                    f':ls' -> Just $ Stack f' ls' [] -- else up
                    []     -> Nothing

-- |
-- /O(s)/. Extract the stack on the current workspace, as a list.
-- The order of the stack is determined by the master window -- it will be
-- the head of the list. The implementation is given by the natural
-- integration of a one-hole list cursor, back to a list.
--
index :: StackSet i l a s sd -> [a]
index = with [] integrate

--  let is = t : r ++ reverse l in take (length is) (dropWhile (/= m) (cycle is))

-- |
-- /O(1), O(w) on the wrapping case/.
--
-- focusUp, focusDown. Move the window focus up or down the stack,
-- wrapping if we reach the end. The wrapping should model a 'cycle'
-- on the current stack. The 'master' window, and window order,
-- are unaffected by movement of focus.
--
-- swapUp, swapDown, swap the neighbour in the stack ordering, wrapping
-- if we reach the end. Again the wrapping model should 'cycle' on
-- the current stack.
--
focusUp, focusDown, swapUp, swapDown :: StackSet i l a s sd -> StackSet i l a s sd
focusUp   = modify' focusUp'
focusDown = modify' (reverseStack . focusUp' . reverseStack)

swapUp    = modify' swapUp'
swapDown  = modify' (reverseStack . swapUp' . reverseStack)

focusUp', swapUp' :: Stack a -> Stack a
focusUp' (Stack t (l:ls) rs) = Stack l ls (t:rs)
focusUp' (Stack t []     rs) = Stack x xs [] where (x:xs) = reverse (t:rs)

swapUp'  (Stack t (l:ls) rs) = Stack t ls (l:rs)
swapUp'  (Stack t []     rs) = Stack t (reverse rs) []

-- | reverse a stack: up becomes down and down becomes up.
reverseStack :: Stack a -> Stack a
reverseStack (Stack t ls rs) = Stack t rs ls

--
-- | /O(1) on current window, O(n) in general/. Focus the window 'w',
-- and set its workspace as current.
--
focusWindow :: (Eq s, Eq a, Eq i) => a -> StackSet i l a s sd -> StackSet i l a s sd
focusWindow w s | Just w == peek s = s
                | otherwise        = maybe s id $ do
                    n <- findIndex w s
                    return $ until ((Just w ==) . peek) focusUp (view n s)

-- | Get a list of all screens in the StackSet.
screens :: StackSet i l a s sd -> [Screen i l a s sd]
screens s = current s : visible s

-- | Get a list of all workspaces in the StackSet.
workspaces :: StackSet i l a s sd -> [Workspace i l a]
workspaces s = workspace (current s) : map workspace (visible s) ++ hidden s

-- | Get a list of all windows in the StackSet in no particular order
allWindows :: Eq a => StackSet i l a s sd -> [a]
allWindows = L.nub . concatMap (integrate' . stack) . workspaces

-- | Is the given tag present in the StackSet?
tagMember :: Eq i => i -> StackSet i l a s sd -> Bool
tagMember t = elem t . map tag . workspaces

-- | Rename a given tag if present in the StackSet.
renameTag :: Eq i => i -> i -> StackSet i l a s sd -> StackSet i l a s sd
renameTag o n = mapWorkspace rename
    where rename w = if tag w == o then w { tag = n } else w

-- | Ensure that a given set of tags is present.
ensureTags :: Eq i => l -> [i] -> StackSet i l a s sd -> StackSet i l a s sd
ensureTags l allt st = et allt (map tag (workspaces st) \\ allt) st
    where et [] _ s = s
          et (i:is) rn s | i `tagMember` s = et is rn s
          et (i:is) [] s = et is [] (s { hidden = Workspace i l Nothing : hidden s })
          et (i:is) (r:rs) s = et is rs $ renameTag r i s

-- | Map a function on all the workspaces in the StackSet.
mapWorkspace :: (Workspace i l a -> Workspace i l a) -> StackSet i l a s sd -> StackSet i l a s sd
mapWorkspace f s = s { current = updScr (current s)
                     , visible = map updScr (visible s)
                     , hidden  = map f (hidden s) }
    where updScr scr = scr { workspace = f (workspace scr) }

-- | Map a function on all the layouts in the StackSet.
mapLayout :: (l -> l') -> StackSet i l a s sd -> StackSet i l' a s sd
mapLayout f (StackSet v vs hs m) = StackSet (fScreen v) (map fScreen vs) (map fWorkspace hs) m
 where
    fScreen (Screen ws s sd) = Screen (fWorkspace ws) s sd
    fWorkspace (Workspace t l s) = Workspace t (f l) s

-- | /O(n)/. Is a window in the StackSet.
member :: Eq a => a -> StackSet i l a s sd -> Bool
member a s = maybe False (const True) (findIndex a s)

-- | /O(1) on current window, O(n) in general/.
-- Return Just the workspace index of the given window, or Nothing
-- if the window is not in the StackSet.
findIndex :: Eq a => a -> StackSet i l a s sd -> Maybe i
findIndex a s = listToMaybe
    [ tag w | w <- workspaces s, has a (stack w) ]
    where has _ Nothing         = False
          has x (Just (Stack t l r)) = x `elem` (t : l ++ r)

-- ---------------------------------------------------------------------
-- $modifyStackset

-- |
-- /O(n)/. (Complexity due to duplicate check). Insert a new element into
-- the stack, above the currently focused element.
--
-- The new element is given focus, and is set as the master window.
-- The previously focused element is moved down.  The previously
-- 'master' element is forgotten. (Thus, 'insert' will cause a retiling).
--
-- If the element is already in the stackset, the original stackset is
-- returned unmodified.
--
-- Semantics in Huet's paper is that insert doesn't move the cursor.
-- However, we choose to insert above, and move the focus.
--
insertUp :: Eq a => a -> StackSet i l a s sd -> StackSet i l a s sd
insertUp a s = if member a s then s else insert
  where insert = modify (Just $ Stack a [] []) (\(Stack t l r) -> Just $ Stack a l (t:r)) s

-- insertDown :: a -> StackSet i l a s sd -> StackSet i l a s sd
-- insertDown a = modify (Stack a [] []) $ \(Stack t l r) -> Stack a (t:l) r
-- Old semantics, from Huet.
-- >    w { down = a : down w }

-- |
-- /O(1) on current window, O(n) in general/. Delete window 'w' if it exists.
-- There are 4 cases to consider:
--
--   * delete on an Nothing workspace leaves it Nothing
--   * otherwise, try to move focus to the down
--   * otherwise, try to move focus to the up
--   * otherwise, you've got an empty workspace, becomes Nothing
--
-- Behaviour with respect to the master:
--
--   * deleting the master window resets it to the newly focused window
--   * otherwise, delete doesn't affect the master.
--
delete :: (Ord a, Eq s) => a -> StackSet i l a s sd -> StackSet i l a s sd
delete w = sink w . delete' w

-- | Only temporarily remove the window from the stack, thereby not destroying special
-- information saved in the Stackset
delete' :: (Eq a, Eq s) => a -> StackSet i l a s sd -> StackSet i l a s sd
delete' w s = s { current = removeFromScreen        (current s)
                , visible = map removeFromScreen    (visible s)
                , hidden  = map removeFromWorkspace (hidden  s) }
    where removeFromWorkspace ws = ws { stack = stack ws >>= filter (/=w) }
          removeFromScreen scr   = scr { workspace = removeFromWorkspace (workspace scr) }

------------------------------------------------------------------------

-- | Given a window, and its preferred rectangle, set it as floating
-- A floating window should already be managed by the StackSet.
float :: Ord a => a -> RationalRect -> StackSet i l a s sd -> StackSet i l a s sd
float w r s = s { floating = M.insert w r (floating s) }

-- | Clear the floating status of a window
sink :: Ord a => a -> StackSet i l a s sd -> StackSet i l a s sd
sink w s = s { floating = M.delete w (floating s) }

------------------------------------------------------------------------
-- $settingMW

-- | /O(s)/. Set the master window to the focused window.
-- The old master window is swapped in the tiling order with the focused window.
-- Focus stays with the item moved.
swapMaster :: StackSet i l a s sd -> StackSet i l a s sd
swapMaster = modify' $ \c -> case c of
    Stack _ [] _  -> c    -- already master.
    Stack t ls rs -> Stack t [] (xs ++ x : rs) where (x:xs) = reverse ls

-- natural! keep focus, move current to the top, move top to current.

-- | /O(s)/. Set focus to the master window.
focusMaster :: StackSet i l a s sd -> StackSet i l a s sd
focusMaster = modify' $ \c -> case c of
    Stack _ [] _  -> c
    Stack t ls rs -> Stack x [] (xs ++ t : rs) where (x:xs) = reverse ls

--
-- ---------------------------------------------------------------------
-- $composite

-- | /O(w)/. shift. Move the focused element of the current stack to stack
-- 'n', leaving it as the focused element on that stack. The item is
-- inserted above the currently focused element on that workspace.
-- The actual focused workspace doesn't change. If there is no
-- element on the current stack, the original stackSet is returned.
--
shift :: (Ord a, Eq s, Eq i) => i -> StackSet i l a s sd -> StackSet i l a s sd
shift n s | n `tagMember` s && n /= curtag = maybe s go (peek s)
          | otherwise                      = s
    where go w = view curtag . insertUp w . view n . delete' w $ s
          curtag = tag (workspace (current s))

-- | /O(n)/. shiftWin. Searches for the specified window 'w' on all workspaces
-- of the stackSet and moves it to stack 'n', leaving it as the focused
-- element on that stack. The item is inserted above the currently
-- focused element on that workspace.
-- The actual focused workspace doesn't change. If the window is not
-- found in the stackSet, the original stackSet is returned.
-- TODO how does this duplicate 'shift's behaviour?
shiftWin :: (Ord a, Eq a, Eq s, Eq i) => i -> a -> StackSet i l a s sd -> StackSet i l a s sd
shiftWin n w s | from == Nothing                     = s -- not found
               | n `tagMember` s && (Just n) /= from = go
               | otherwise                           = s
    where from   = findIndex w s

          go     = on n (insertUp w) . on (fromJust from) (delete' w) $ s
          curtag = tag (workspace (current s))
          on i f = view curtag . f . view i