Hello, List!
I got error:
Duplicate instance declarations:
instance [overlap ok] EnumTag a => Read a
-- Defined at /XXX/intero/intero2932Xpa-TEMP.hs:110:27
instance [overlap ok] StrTag a => Read a
-- Defined at /XXX/intero/intero2932Xpa-TEMP.hs:121:27 (intero)
For this code:
class (Show a, Enum a) => EnumTag a where
anyEnum :: a
instance {-# OVERLAPS #-} EnumTag a => Read a where
readPrec = RP.lift P.skipSpaces >> expectEnum
instance {-# OVERLAPS #-} EnumTag a => Eq a where
a == b | a == anyEnum || b == anyEnum = True
| otherwise = fromEnum a == fromEnum b
class StrTag a where
anyStr :: a
tagPrefix :: a -> String -- ^ should be constant
toStr :: String -> a
instance {-# OVERLAPS #-} StrTag a => Read a where
readPrec = parens $ do
RP.lift P.skipSpaces
(RP.lift $ expectShown anyStr) <++ RP.lift g
where g = do
Just s@(_:_) <- L.stripPrefix tagPrefix <$> expectTag
return $ toStr s
Why does it happen? `Read a` in 1st instance is valid only when a is
`EnumTag`, in 2nd one - is valid only when a is `StrTag`.
How can I fix this error and to create "default" instances for `EnumTag`
and to `StrTag`, so client code will "inherit" those functionality
(`Read`) simple, only with instantiation of `EnumTag` or `StrTag` ?
Sure, if I comment `instance ... StrTag a` then all work fine, but I need 2 specialized `Read`s (and `Eq`s too :)
===
Best regards, Paul
