I am trying to traverse all members of a data structure in haskell using Data.Traversable, which is documented at the following urls:
http://hackage.haskell.org/package/base-4.6.0.1/docs/Data-Traversable.html http://www.haskell.org/haskellwiki/Foldable_and_Traversable
So far I have come up with the following code which is as far as I know missing a proper implementation of the Tr.Traversable instancing.
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
}
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)
instance Functor Data where
fmap = Tr.fmapDefault
instance Fl.Foldable Data where
foldMap = Tr.foldMapDefault
instance Tr.Traversable Data where
traverse f = Data f -- Try to show a Test entry inside the Data structure
--
-- traverse :: Applicative f => (a -> f b) -> t a -> f (t b)
--
main = do
putStrLn $ show exampleData
Tr.traverse (putStrLn show) exampleData
I am trying to print all the items in exampleData, member by member and using show. Am I on the right track and how should the traversable instancing be implemented?
I would just use the language extensions to derive it for me:
If you want to know how the compiler automatically implements it, you could compile it with the
-ddump-deriv
flag (cleaned up a bit):