On Fri, 24 Aug 2012, Ross Paterson wrote:
On Fri, Aug 24, 2012 at 12:08:04AM +0100, roconnor@theorem.ca wrote:
With such wide spread agreement going back at least 6 years, I think it is time to add the following 2 laws to the documentation for Data.Traversable.Traversable.
(1) traverse Identity == Identity (2) traverse (Compose . fmap g . f) == Compose . fmap (traverse g) . traverse f
Sounds good (except that Identity and Compose aren't defined in base).
Er right. I'm not entirely sure how to address this issue.
I guess you're assuming that the naturality property, i.e.
(3) traverse (t . f) = t . traverse f
for any Applicative transformation t, is automatic. Even so it would be useful to state it.
Yes, I understand from Voigtlander's "Free theorems involving type constructor classes: functional pearl" that for fast and loose reasoning it is automatic. I agree that it would be good to state. If you want to follow the terminology from "Essence of the Iterator Pattern" of using "idiomatic" for adjectives, you would call t an "Idiomatic transformation". -- Russell O'Connor http://r6.ca/ ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''