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

[Simon Peyton-Jones]

Oh dear, that really is quite a strange error message. Something is definitely wrong. Can you please make a ticket for it, and include instructions on how to reproduce it?    I gather that it depends on other packages that themselves needed changes, so reproduction might not be entirely easy?

Simon

|  -----Original Message-----
|  From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
|  bounces at haskell.org] On Behalf Of Malte Sommerkorn
|  Sent: 29 September 2010 19:50
|  To: glasgow-haskell-users at haskell.org
|  Subject: GHC 7.0.1 rc1: Could not deduce (Typeable a) from the context (Typeable a,
|  …)
|  
|  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 }
|  
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