I would just use the language extensions to derive it for me:
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE DeriveTraversable #-}
{-# LANGUAGE DeriveFoldable #-}
import qualified Data.Traversable as Tr
import qualified Data.Foldable as Fl
import Control.Monad
import Control.Applicative
data Test = Test { desc :: String
, value :: Int }
data Data t = Data { foo :: t
, bar :: t } deriving (Functor, Tr.Traversable, Fl.Foldable)
exampleData = Data { foo = Test "Foo" 1
, bar = Test "Bar" 2
}
instance Show Test where
show f = (desc f) ++ ": " ++ (show $ value f)
instance (Show a) => Show (Data a) where
show f = show (foo f)
main = do
putStrLn $ show exampleData
Tr.traverse (putStrLn . show) exampleData
> runhaskell test_traversable.hs
Foo: 1
Foo: 1
Bar: 2
()
If you want to know how the compiler automatically implements it, you could compile it with the -ddump-deriv
flag (cleaned up a bit):
instance Functor Data where
fmap f (Data foo' bar') = Data (f foo') (f bar')
instance Tr.Traversable Data where
tr.traverse f (Data foo' bar') = Data <$> (f foo') <*> (f bar')
instance Fl.Foldable Data where
Fl.foldr f z (Data foo' bar') = f foo' (f bar' z)