recordtest.hs

{-# LANGUAGE RecordWildCards #-}

data Foo = Bar { a :: Int, b :: Int, c :: Int, d :: Int }
         | Baz { x :: Int, y :: Int, z :: Int, w :: Int }
  deriving (Show)


bar = Bar {a=1, b=2, c=3, d=4}
baz = Baz {x=10, y=20, z=30, w=40}


-- With record wildcards
--
sum2 :: Foo -> Int
sum2 Bar{..} = a + b
sum2 Baz{..} = x + y

-- without record wildcards, I need to know the number of arguments:
sum2' :: Foo -> Int
sum2' f@(Bar _ _ _ _) = a f + b f
sum2' f@(Baz _ _ _ _) = x f + y f


-- I can mediate this problem probably with Pattern synonyms...