[Haskell-beginners] Overlapping instances problem

Baa aquagnu at gmail.com
Fri Dec 1 11:57:56 UTC 2017

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

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