Thanks a lot for your answer, it was exactly what I was looking for. Just for the record, based on your solution I can now easily code a function to check if a Data value belongs to an enumerated algebraic type (as I defined it in my first mail). {-# LANGUAGE DeriveDataTypeable, ScopedTypeVariables #-} import Data.Generics newtype Arity a = Arity Int deriving (Show, Eq) consArity :: Data a => Constr -> Arity a consArity = gunfold (\(Arity n) -> Arity (n+1)) (\_ -> Arity 0) belongs2EnumAlg :: forall a . Data a => a -> Bool belongs2EnumAlg a = case (dataTypeRep.dataTypeOf) a of AlgRep cons -> all (\c -> consArity c == ((Arity 0) :: Arity a )) cons _ -> False -- tests data Colors = Blue | Green | Red deriving (Data, Typeable) test1 = belongs2EnumAlg 'a' -- False test2 = belongs2EnumAlg Red -- True test3 = belongs2EnumAlg "a" -- False On Tue, May 6, 2008 at 7:42 PM, Edward Kmett wrote:

On Tue, May 6, 2008 at 12:34 PM, Alfonso Acosta wrote:

| So, the question is. Is there a way to figure out the arity of data | constructors using Data.Generics ?

| I'm totally new to generics, but (tell me if I'm wrong) it seems that | Constr doesn't hold any information about the data-constructor | arguments. Why is it so?

Hmrmm,

Playing around with it, I was able to abuse gunfold and the reader comonad to answer the problem :

fst $ (gunfold (\(i,_) -> (i+1,undefined)) (\r -> (0,r)) (toConstr "Hello") :: (Int,String))

returns 2, the arity of (:), the outermost constructor in "Hello"

A longer version which does not depend on undefined would be to take and define a functor that discarded its contents like:

...

module Args where

...

import Data.Generics

...

newtype Args a = Args { runArgs :: Int } deriving (Read,Show)

...

tick :: Args (b -> r) -> Args r tick (Args i) = Args (i + 1)

...

tock = const (Args 0)

...

argsInCons = runArgs $ (gunfold tick tock (toConstr "Hello") :: (Args String)

Basically all I do is rely on the fact that gunfold takes the 'tick' argument and calls it repeatedly for each argument after a 'tock' base case.

The use of the reader comonad or functor is to give gunfold a 'functor-like' argument to meet its type signature.

-Edward Kmett _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe