Re: Proposal: Add missing Foldable/Traversable instances for Prelude types

On Sat, Jan 22, 2011 at 05:12:00PM +0100, Bas van Dijk wrote:

On 22 January 2011 05:16, Edward Kmett wrote:

...

I went with the default being to avoid prematurely forcing a bottom when possible, to avoid being called out by the strictness police, but I'm not terribly wedded to the behavior.

Just for reference, the Functor instance on pairs is strict in the pair:

instance Functor ((,) a) where fmap f (x,y) = (x, f y)

And this is necessary for the functor laws, because with fmap' f ~(x,y) = (x, f y) we obtain: fmap' id undefined = (undefined, undefined) /= undefined = id undefined Wolfram