Data-Derive.hs

{-# LANGUAGE TemplateHaskell #-}

import Data.Monoid
import Data.DeriveTH

data Venn a =
  Venn
  { both    :: a -- number of elements in both t and p
  , pNotT   :: a -- number of elements in p but not t
  , tNotP   :: a -- number of elements in t but not p
  , neither :: a -- number of elements in neither t nor p
  } deriving (Show)

$( derive makeMonoid ''Venn )

main :: IO ()
main = print $ a <> b
  where
    a = Venn (Sum 1) (Sum 2) (Sum 3) (Sum 4)
    b = Venn (Sum 0) (Sum 1) (Sum 0) (Sum 1)

-- Success!
-- Venn {both = Sum {getSum = 1}, pNotT = Sum {getSum = 3}, tNotP = Sum {getSum = 3}, neither = Sum {getSum = 5}}

GHC-Generics.hs

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics (Generic)
import Data.Monoid

data Venn a =
  Venn
  { both    :: a -- number of elements in both t and p
  , pNotT   :: a -- number of elements in p but not t
  , tNotP   :: a -- number of elements in t but not p
  , neither :: a -- number of elements in neither t nor p
  } deriving (Show, Generic)

instance Monoid a => Monoid (Venn a)

main :: IO ()
main = print $ a <> b
  where
    a = Venn (Sum 1) (Sum 2) (Sum 3) (Sum 4)
    b = Venn (Sum 0) (Sum 1) (Sum 0) (Sum 1)

-- 
-- *** Exception: test.hs:14:10-36: No instance nor default method for class operation Data.Monoid.mappend
--