On Tue, 3 Jan 2012, Conal Elliott wrote:
I wanted a Traversable instance for pairing, so I defined one:
{-# LANGUAGE TupleSections #-} {-# OPTIONS_GHC -Wall -fno-warn-orphans #-}
import Data.Traversable (Traversable(..)) import Data.Foldable (Foldable(..)) import Control.Applicative ((<$>))
instance Traversable ((,) o) where sequenceA (o,fa) = (o,) <$> fa
What about using the Writer Monad/Functor?
The best I've thought of is the following:
instance Foldable ((,) o) where fold (_,m) = m
However, I don't like how it discards information.
I also didn't like that and thus asked the same question in the past. The answer is, that you can implement a 'fold' using 'traverse' and thus every Traversable type has also a natural Foldable instance. http://www.haskell.org/pipermail/haskell-cafe/2009-October/067535.html