On Tue, 9 Apr 2013, Petr Pudlák wrote:
Dear haskellers,
I propose a correction to the documentation for Traversable. Currently it says:
Minimal complete definition: traverse or sequenceA.
This is not (completely) correct, it should be:
Minimal complete definition: - traverse or - sequenceA with fmap.
What happened to me: I defined a Traversable instance with just sequenceA and used fmapDefault and foldMapDefault to define instances of Functor and Foldable. This resulted in an infinite loop. This is because traverse is defined using sequenceA and fmap, but fmapDefault is defined using traverse. So it's not enough to define sequenceA and use the default implementations for the rest, one also has to define fmap explicitly.
That's right. I think I also stumbled about it, but became curious how fmap can be implemented in terms of only sequenceA. Then I saw that fmapDefault is implemented in terms of 'traverse', where the default implementation of 'traverse' calls fmap and sequenceA. Thus, if I want to define Functor, Foldable and Traversable instances with minimal effort, then I implement only traversable.