[Haskell-cafe] Functional Dependencies conflicts

Limestraël limestrael at gmail.com
Sat Apr 17 16:01:23 EDT 2010

Yes! Sorry, I forgot a bit:
Binary types are automatically made instances of Binarizable/Unbinarizable
(that's my line 16):

instance (Binary a) => Binarizable a a where
  toBinary = id

instance (Binary a, Monad m) => Unbinarizable a a m where
  fromBinary = return

To me, the functional dependency in:
class (Binary b) => Binarizable a b | a -> b
meant that for each a, there only one type b that can match.

That's what I want: for every Binary type 'a', the matching Binary is also
And for GameObject, the sole matching type is String.
In other words, GameObject implies String.
I would have undestood the error if GameObject was also an instance of
Binary (then the two instances would match), but it's not the case...

Is my FunDep wrong?

I done this especially because I didn't wanted to declare each type one by
one instance of Binarizable,
Haskell type system normally enables me to automatically define a Binary as
an instance of Binarizable.

2010/4/17 Daniel Fischer <daniel.is.fischer at web.de>

> Am Samstag 17 April 2010 19:14:02 schrieb Limestraël:
> > Hello,
> >
> > Well, here comes the trouble:
> > GameStructs.hs:16:9:
> >     Functional dependencies conflict between instance declarations:
> >       instance (Binary a) => Binarizable a a
> >         -- Defined at MagBots/GameStructs.hs:16:9-37
> >       instance Binarizable GameObject String
> >         -- Defined at MagBots/GameStructs.hs:38:9-37
> >
> > GameStructs.hs:19:9:
> >     Functional dependencies conflict between instance declarations:
> >       instance (Binary a, Monad m) => Unbinarizable a a m
> >         -- Defined at MagBots/GameStructs.hs:19:9-50
> >       instance (MonadReader [GameObject] m) =>
> >                Unbinarizable GameObject String m
> >         -- Defined at MagBots/GameStructs.hs:41:9-73
> >
> > I don't see why the functional dependencies conflict, since GameObject
> > is not an instance of Binary...
> Somebody somewhere might write such an instance.
> But more fundamentally:
> GHC doesn't look at the constraints for instance selection, so your
> instance in line 16 looks like
> instance Binarizable a a where ..., oh, and by the way, a must be an
> instance of Binary, otherwise please refuse to compile
> to the compiler. The FunDep then says in each instance (and you'd need at
> least OverlappingInstances to declare more) a and b are the same type.
> instance Binarizable GameObject String violates that FunDep.
> (Analogous for Unbinarizable.)
> I think removing the
> instance Binary a => ...
> and declaring an instance for the types you need manually is the easiest
> solution.
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