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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)))

Trac metadata

Trac field	Value
Version	7.4.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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