GHC 7.0.1 rc1: Could not deduce (Typeable a) from the context (Typeable a, …)

[Malte Sommerkorn]

While compile failures were easy to fix for syb, X11 and xmonad, here's a more
mysterious encounter with xmonad-contrib. The module in question is

    http://code.haskell.org/XMonadContrib/XMonad/Layout/MultiToggle.hs

and the rest of this mail is the error:

XMonad/Layout/MultiToggle.hs:194:30:
    Could not deduce (Typeable a)
      from the context (Typeable a, Show ts, HList ts a, LayoutClass l a)
      arising from a use of `fromMessage'
    Possible fix:
      add (Typeable a) to the context of the instance declaration
    In a stmt of a pattern guard for
                 an equation for `handleMessage':
        Just (Toggle t) <- fromMessage m
    In an equation for `handleMessage':
        handleMessage mt m
          | Just (Toggle t) <- fromMessage m,
            i@(Just _) <- find (transformers mt) t
          = case currLayout mt of {
              EL l det
                -> do { l' <- fromMaybe l
                            `fmap`
                              handleMessage l (SomeMessage ReleaseResources);
                        .... }
                where
                    cur = (i == currIndex mt) }
          | otherwise
          = case currLayout mt of {
              EL l det
                -> fmap (fmap (\ x -> mt {currLayout = EL x det}))
                 $ handleMessage l m }
    In the instance declaration for `LayoutClass (MultiToggle ts l) a'

XMonad/Layout/MultiToggle.hs:195:25:
    Could not deduce (HList ts a)
      from the context (Typeable a,
                        Show ts,
                        HList ts a,
                        LayoutClass l a,
                        Transformer t a)
      arising from a use of `find'
    Possible fix:
      add (HList ts a) to the context of
        the data constructor `Toggle'
        or the instance declaration
    In a stmt of a pattern guard for
                 an equation for `handleMessage':
        i@(Just _) <- find (transformers mt) t
    In a stmt of a pattern guard for
                 an equation for `handleMessage':
        Just (Toggle t) <- fromMessage m
    In an equation for `handleMessage':
        handleMessage mt m
          | Just (Toggle t) <- fromMessage m,
            i@(Just _) <- find (transformers mt) t
          = case currLayout mt of {
              EL l det
                -> do { l' <- fromMaybe l
                            `fmap`
                              handleMessage l (SomeMessage ReleaseResources);
                        .... }
                where
                    cur = (i == currIndex mt) }
          | otherwise
          = case currLayout mt of {
              EL l det
                -> fmap (fmap (\ x -> mt {currLayout = EL x det}))
                 $ handleMessage l m }