Time to add the Traversable laws to the Documentation

roconnor at theorem.ca roconnor at theorem.ca
Sat Aug 25 18:15:27 CEST 2012

On Sat, 25 Aug 2012, Ross Paterson wrote:

> On Fri, Aug 24, 2012 at 02:53:30AM +0100, roconnor at theorem.ca wrote:
>> On Fri, 24 Aug 2012, Ross Paterson wrote:
>>> 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).
>> Er right.  I'm not entirely sure how to address this issue.
> There doesn't seem to be any alternative to re-iterating the newtype
> definitions and the Functor and Applicative instances in the doc comment.

The only alternative I can think of would be to move the Idenity and 
Compose functors from transformers into base.  The consequences of this 
would be massive.  I don't really suggest doing this at this time.

>> 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".
> Noooo, please don't do that.  The transformations should match the
> functors, and the adjective "applicative" at least has a grain of
> relevant meaning.


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.''

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