Re: [Haskell-beginners] Overlapping instances problem

1 Dec 2017


      Thanks, I got it. So I workarounded the problem with newtypes (all
"default" instances are for newtypes, my types are wrapping in those
newtypes).

===
Best regards, Paul
...
What happens if type 'a' is both an instance EnumTag and StrTag at
the same time?  Then which instance does it choose?  You may know
that can't happen in your code, but there's no guarantee that someone
using your library or that some import won't bring such a type into
scope.
Because of this ambiguity, during type checking haskell ignores class
contexts and merely looks at the instance head (Read a), and says hey
there are two instances 'Read a', they are overlapping.
As to what to do about it, I'm not sure.  But I don't think I would be
trying to get different read instances based on whatever typeclasses
happen to be in scope for that type.
On Fri, Dec 1, 2017 at 6:57 AM, Baa  wrote:
...
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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