moving Traversable and Foldable to Prelude (Was: Control.Monad proposal: Add whenJust)

[Henning Thielemann]

On Sun, 19 May 2013, Anthony Cowley wrote:

> I think this issue may be too big to rely on mailing list +1s. Is there any precedent for having a web-based
> poll of some sort? We often get more engagement in debates on IRC and /r/haskell than the mailing list, so
> let's not let the choice of forum drive the result.

I also consider the proposal to be a pretty big thing. I am not sure that 
all people who are affected are aware of this discussion, especially since 
it happens under the subject "add whenJust". Changing the Prelude was only 
done for 'catch' as far as I remember. Moving Traversable and Foldable to 
Prelude is for me a precedent. It has more implications: I think it must 
also go into the Haskell 201x specification. It would mean that 
Traversable and Foldable are immediately in scope in every module. And 
what is the long-term goal? Shall Prelude contain all classes, that 
generalize Prelude's types? Applicative? Category? Monoid? MonadPlus? 
Semigroup? If you think this out, you may want not to do the first step in 
this direction.

The reason for the proposal is really only to save writing some "import" 
statements for people who prefer the unqualified import style, right? I 
still think it is much simpler to achieve this by putting more effort in 
alternative Prelude projects.