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

David Fox ddssff at gmail.com
Thu Jun 25 11:31:24 EDT 2009

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.

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