#6015: "No instance" when using PolyKinds/DataKinds/FunDeps/Undecidable
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Reporter: atnnn | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: |
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Consider this GHCi session:
{{{
> value (Proxy :: Proxy (Just True))
No instance for (Value (Maybe Bool) (Just Bool 'True) t0)
> :i Value
instance Value Bool 'True Bool
instance Value k a t => Value (Maybe k) (Just k a) (Maybe t)
> value (Proxy :: Proxy (Just True)) :: Maybe Bool
Just True
}}}
It would be helpful if GHC could use the instance (which seems suitable)
or emit a better error message.
{{{
{-# LANGUAGE DataKinds, MultiParamTypeClasses, FunctionalDependencies,
PolyKinds, UndecidableInstances, ScopedTypeVariables #-}
data Proxy a = Proxy
class Value a t | a -> t where value :: Proxy a -> t
instance Value True Bool where value _ = True
instance Value a t => Value (Just a) (Maybe t)
where value _ = Just (value (Proxy :: Proxy a))
-- main = print (value (Proxy :: Proxy (Just True)))
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6015>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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