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Attached is a module containing code from XMonadContrib that does not type-check anymore with ghc 7.0.1 rc1. I tried to strip the code somewhat down; at least it only depends on base and mtl. Here's the (somewhat irritating) error message:

tcBug.hs:50:30:
    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'

tcBug.hs:51:25:
    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 }

Trac metadata

Trac field	Value
Version	7.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
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Operating system
Architecture