Grouping data types by constructor in Haskell

Question

Given this data type

data Val = X Int | Y Bool | Z Double deriving (Eq, Show)

and a list such as

let vals = [X 1, Z 2.7, Y True, X 2, Z 3.14, Y True]

how to group elements in vals into this list,

[[X 1,X 2],[Y True,Y True],[Z 2.7, Z 3.14]]

Answer 1

To add to @RamonSnir's answer, the function for grouping a data type by constructors can be also constructed automatically using the "Scrap your boilerplate" framework:

{-# LANGUAGE DeriveDataTypeable #-}

import Data.Data
import Data.Function (on)
import Data.List (groupBy, sort)

data Val = X Int | Y Bool | Z Double
  deriving (Eq, Ord, Show, Typeable, Data)

vals :: [Val]
vals = [X 1, Z 2.7, Y True, X 2, Z 3.14, Y True]

main :: IO ()
main = print $ groupBy (on (==) toConstr) $ sort vals

The two important parts are:

• derive Typeable and Data, and
• use toConstr to get the representation of the constructor used in a particular value.

Answer 2

I've the following:

data Val = X Int | Y Bool | Z Double deriving (Eq, Ord, Show)

vals :: [Val]
vals = [X 1, Z 2.7, Y True, X 2, Z 3.14, Y True]

valCtorEq :: Val -> Val -> Bool
valCtorEq (X _) (X _) = True
valCtorEq (Y _) (Y _) = True
valCtorEq (Z _) (Z _) = True
valCtorEq _ _ = False

And then:

*Main Data.List> groupBy valCtorEq $ sort vals
[[X 1,X 2],[Y True,Y True],[Z 2.7,Z 3.14]]