[Haskell-cafe] Question about typeclass & constraints

[Tom Ellis]

On Tue, Apr 08, 2014 at 10:17:50AM +0200, jean-christophe mincke wrote:
> class PP m where
>     create :: a -> m a
> 
> data A a = A a
> instance PP A where
>     create a = A a
> 
> class CB a where
>     fb :: a -> a
> 
> data B m a = B (m a)
> instance (PP m) => PP (B m) where
>     create a =  let a' = fb a
>                 in B (create a')

Thanks for the clarified example.

The failure here is not a technicality.  There's a very good reason it can't
work this way: 'create' is required to be parametrically polymorphic in 'a'
but 'fb' is specialised to the *particular* 'a' in the typeclass constraint.
Thus you can't use 'fb' to implement 'create'!

> class PP' m a where
>     create' :: a -> m a
> 
> instance (PP m) => PP' m a where
>     create' = create
> 
> instance (PP m, CB a) => PP' (B m) a where
>     create' a = let a' = fb a
>                 in B (create a')

create' is not required to be parametric, hence you can use 'fb' in its
implementation.

Without knowing more about your specific use case it's hard to know what to
suggest.  Personally speaking though, I am not a fan of complicated
typeclass algebra, and prefer to create and pass dictionaries explicitly for
all but the most trivial cases.

Tom