Time to add the Traversable laws to the Documentation

Ross Paterson ross at soi.city.ac.uk
Fri Aug 24 02:53:59 CEST 2012

On Fri, Aug 24, 2012 at 12:08:04AM +0100, roconnor at 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).
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.

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