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
However, Foldable is a superclass of Traversable, so I get an error message:
Could not deduce (Foldable ((,) o)) from the context ()
arising from the superclasses of an instance declaration
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.
Some questions:
* Why is Foldable a superclass of Traversable?
* Is there a good choice of a Foldable instance of ((,) o)?
* Are there any other problems with the Traversable instance above (besides
foldability)?
- Conal
