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

Oh, that make sense! 2009/6/25 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 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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