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

[Daniel Fischer]

Am Donnerstag 17 September 2009 15:56:03 schrieb José Pedro Magalhães:
> Hey Andy,
>
> On Thu, Sep 17, 2009 at 15:40, Andy Gimblett <haskell at gimbo.org.uk> wrote:
> > Now, some of those algebraic data type types happen to be
> > enumerations; in this case, my idea is to list the constructors, with
> > the rule that each constructor's position in the list is the Int which
> > gets converted into that constructor.
> >
> > > class Enumerated a where
> > >     constructors :: [a]
> >
> > 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]

?