HI Pedro,

> Would this help?
>
>> {-# LANGUAGE ScopedTypeVariables  #-}
>> {-# LANGUAGE FlexibleContexts     #-}
>>
>> module Data.Generics.Builders (empty) where
>>
>> import Data.Data
>> import Data.Generics.Aliases (extB)
>>
>> -- | Construct the empty value for a datatype. For algebraic datatypes,
>> the
>> -- leftmost constructor is chosen.
>> empty :: forall a. Data a => a
>> empty = general
>>       `extB` char
>>       `extB` int
>>       `extB` integer
>>       `extB` float
>>       `extB` double where
>>   -- Generic case
>>   general :: Data a => a
>>   general = fromConstrB empty (indexConstr (dataTypeOf general) 1)
>>
>>   -- Base cases
>>   char    = '\NUL'
>>   int     = 0      :: Int
>>   integer = 0      :: Integer
>>   float   = 0.0    :: Float
>>   double  = 0.0    :: Double

Yep, that should do it!