Two questions on Reducer

[Ben Hutchison]

1. I noticed there isn't a Reducer for Set, although one exists in the source Haskell library. Send a PR?

[http://hackage.haskell.org/package/monoids-0.2.0/docs/src/Data-Monoid-Reducer.html]

2. The Reducer typeclass seem a little inconsistent to others, eg Monoid/Monad, in that it's declared sealed, and a constructor provided. Any reason for this beyond "historical accident"?

[https://github.com/scalaz/scalaz/blob/series/7.2.x/core/src/main/scala/scalaz/Reducer.scala#L154]

-Ben

[Stephen Compall]

On Mon, 2015-03-30 at 09:18 +1100, Ben Hutchison wrote:

1. I noticed there isn't a Reducer for Set, although one exists in the source Haskell library. Send a PR?

[http://hackage.haskell.org/package/monoids-0.2.0/docs/src/Data-Monoid-Reducer.html]

Sure.

2. The Reducer typeclass seem a little inconsistent to others, eg Monoid/Monad, in that it's declared sealed, and a constructor provided. Any reason for this beyond "historical accident"?

[https://github.com/scalaz/scalaz/blob/series/7.2.x/core/src/main/scala/scalaz/Reducer.scala#L154]

It's kind of in a middle area; I hesitate to call it a "typeclass" at all, despite the presence and use of implicits for it. It has straightforward laws, but seems more amenable to first-class manipulation and resistant to coherence complaints than typeclasses in general.

-- Stephen Compall "^aCollection allSatisfy: [:each|aCondition]": less is better

[Ken Scambler]

A bit off topic, but what makes something amenable for typeclass vs first-class usage?  I hear some grumbling in Haskell about typeclasses being a special separate thing with worse tools of abstraction, but on the Scala side where the instances are tangible objects, I hear stern warnings to clearly demarcate the two usages.

I find myself a bit short of intuition on the matter.

Cheers,

Ken

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