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haskelltypesgeneric-programming

Grouping data types by constructor in Haskell

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]]
Solution

  • 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]]