On Tue, Jun 23, 2009 at 03:07:04PM +0300, Yitzchak Gale wrote:
The other amendment is to fix the documentation for the functions fmapDefault and foldMapDefault in Data.Traversable. Contrary to what is stated, these cannot be used as methods in superclasses, since the superclasses must already exist before these functions can be defined. Instead, I have paraphrased what is stated in the documentation above: that the corresponding superclass methods should be equivalent to these.
The intention is that if you only want to define Traversable, you can fill in the required superclass instances by defining instance Functor F where fmap = fmapDefault instance Foldable F where foldMap = foldMapDefault instance Traversable F where traverse = ... That should work fine. However the fmap definition won't work if you defined sequenceA instead of traverse, so the documentation of fmapDefault needs to be corrected to say that.