#4355: Coud not deduce (Typeable a) from context (Typeable a, …)
---------------------------------+------------------------------------------
Reporter: maltem | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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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 }
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4355>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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