Haskell: Get data constructor name as string

Question

Let us say we have

data D = X Int | Y Int Int | Z String

I wish to have a function getDConst

getDConst :: D -> String

that returns either "X", "Y", or "Z", according to the data constructor used for its input. Is there a generic way to write this without having to do case on every data constructor? (I am ok with solutions relying on Data.Typeable or something similar)

Answer 1

Found the solution myself, but leaving this question to help others:

import Data.Data
data D = X Int | Y Int Int deriving (Data,Typeable)

let result = show $ toConstr (X 3) -- result contains what we wanted

Answer 2

If you don't want to use Typeable, you can also do this with Show.

getDConst :: D -> String
getDConst = head . words . show

Show will not output all the fields, because it is lazy. You can test it runing this code in ghci:

Prelude> data D = D [Int] deriving (Show)
Prelude> getDConst $ D [1..]
"D"