Testing Haskell traversable with a simple example

Question

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?

Answer 1

The typical Traversable implementation here would be

traverse f (Data foo bar) = Data <$> f foo <*> f bar

Answer 2

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)