[Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)

[Andy Gimblett]

On 17 Sep 2009, at 15:21, José Pedro Magalhães wrote:

> > > E.g. here's a type Bar with three constructors:
> > > > data Bar = X | Y | Z deriving (Show)
> > > > instance Enumerated Bar where
> > > >     constructors = [X, Y, Z]
> > >
> > > (This is certainly ugly.  Any suggestions?)
> > >
> > |constructors| is expressible in SYB:
> Wow.
>
> What about
>
> data Bar = X | Y | Z deriving (Show, Eq, Ord, Enum, Bounded)
>
> instance Enumerated Bar where
>    constructors = [minBound .. maxBound]
>
> ?
>
> Oh yes, that will certainly work for this very simple datatype.  
> However, one cannot automatically derive instances of |Bounded| for  
> datatypes with non-nullary constructors.

That would be OK in this instance, I think; I'm already dealing with  
some of those cases by hand, but there were enough purely nullary ones  
that this seemed worth doing.

I don't know if that will work any better with Foo/convert,  
though... :-)

Thanks though Daniel - it's good to meet Enum and Bounded.

-Andy