update in-place

data Foo = Foo { a :: Int, b :: String }

instance Show Foo where show f = show (a f) ++ " " ++ show (b f)

foo = Foo { a = 99, b = "green bottles\n" }

main = do print foo let bar = foo { a = 98 } print bar

The above code is from Haskell FAQ. I could not find the "data Foo = Foo }" notation in my Haskell, The Craft of Functional Programming book. What does it mean? Also

instance Show Foo where show f = show (a f) ++ " " ++ show (b f)

I do not understand show (a f) either. And same goes for "foo { a = 98 }" notation. Could you help me on this notation? Thanks