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

Hello, On Thu, Sep 17, 2009 at 16:05, Daniel Fischer wrote:

Am Donnerstag 17 September 2009 15:56:03 schrieb José Pedro Magalhães:

...

Hey Andy,

On Thu, Sep 17, 2009 at 15:40, Andy Gimblett 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]

?

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. Cheers, Pedro

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