Mark, On 2015-02-01 at 15:00:33 +0100, Mark Lentczner wrote: [...]
If we don't intend to make Foldable/Traversable a first-class citizen of Haskell to help increase its adoption, why have it in `base` in the first place?
"first-class citizen" better not equate to "you get it with the Prelude" nor "you can always import it unqualified".
If we use either of those as the bar, then we will end up with huge number of things in default scope, and we'll namespace it by prefix strings *alá PHP.*
That's bordering on slippery-slope argumentation and misrepresenting what the changes due to FTP actually are. FTP is mostly about replacing *existing* symbols in the Prelude with their generalised counterparts because they were *already* there (otherwise they wouldn't cause conflict in the first place). The few newly added symbols to the `Prelude` scope (specifically: Foldable(foldMap), Traversable(traverse), and Monoid(..) ) were done to satisfy the expectation to be able define instances as well as type-signatures without requiring additional imports, make the Prelude self-contained/closed. This particular design choice is not mandatory for the FTP.
Moreover, how do we justify (other than by historic accident) that the more generalised synonyms are stored away in modules … Do lists really require such a special privilege?
Yup, historic accident *and* lists do get special privilege. We could easily turn your question around: Why do Foldable and Traversable deserve to be not only importable unqualified, but by all programs as part of the Prelude?
Because they are more general and more powerful than their list-specialised counterparts. They can (almost) always be used in place of their more specialised versions. The converse, however, does not generally hold. Moreover, having them available by default in scope encourages users to think in terms of them and look for opportunities (i.e. recognize common shapes/patterns) to levarage the abstraction.
Why not rather place the more general versions into the default scope … This results in a more symmetric situation.
It might be more symmetric. But I don't see universal acclaim that Foldable and Traversable are a generalization to be preferred over lists in the majority of code. And the "why not" is because it is a big change to the Prelude with negative consequences judged significant by many of us.
Could you please spell out the significant negative consequences you're referring to? Otherwise it's too vague and denies me the chance to address your concern (and we are considering technical solutions to make the FTP more palatable to all concerned parties, but this requires being able exactly identify the pain points). -- hvr