[Haskell-cafe] Can you determine a constructor's arity using Data.Typeable and Data.Data?

[José Pedro Magalhães]

Hey David,

For instance:

arity :: (Data a) => a -> Int
> arity = length . gmapQ (const ())
>


Cheers,
Pedro

On Thu, Jun 25, 2009 at 17:31, David Fox <ddssff at gmail.com> wrote:

> Is it possible to determine the arity of a value's constructor?
> Suppose I have a value x of type
>
>   data A = B Int | C
>
> They typeOf function returns its TypeRep, which contains its type
> constructor, but I don't see how to decide whether that
> constructor's arity is 0 or 1.  If the type has field names
> I can look at those using Data.Data.conFields and count them,
> but if it doesn't I don't see how to do it.
>
> -david
>
>
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