[Haskell-cafe] [Haskell-beginners] Multi-parameter type classes and ambiguous type variables...
Sumit Sahrawat, Maths & Computing, IIT (BHU)
sumit.sahrawat.apm13 at iitbhu.ac.in
Sat Mar 21 07:37:52 UTC 2015
This might be better answered at the haskell-cafe. Sending to cafe.
On 21 March 2015 at 03:09, Stuart Hungerford <stuart.hungerford at gmail.com>
> As a learning exercise I'm modelling some algebraic structures as
> Haskell typeclasses. Some of these are multi-parameter type classes.
> Here's a very simplified version of the type class relationships:
> class MM a where
> one :: a
> class AM a where
> zero :: a
> class (AM a, MM a) => SR a
> class (AM a) => AG a where
> inv :: a -> a
> class (SR a) => R a where
> neg :: a -> a
> class (R r, AG g) => M r g where
> sca :: r -> g -> g
> check :: (Eq g, M r g) => g -> Bool
> check x = sca one x == x
> The problem I have is that GHC is finding the "r" type variable in the
> "check" function ambiguous. Given my still limited Haskell knowledge
> I'm not surprised this is happening. What I would like to know is how
> experienced Haskellers handle this situation in practice: is there an
> idiomatic way of disambiguating "r" or is it a sign of poor type class
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> Beginners at haskell.org
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