Re: Distinguishing case class members

On Sun, Jan 10, 2016 at 8:12 AM, Matt Farmer <[email protected]> wrote:

> I’m concerned that the rarity of needing to do this may make it
> unreasonable to maintain such a feature. I’ve been doing Scala since 2011
> and have maybe needed to do that once? How frequently do you find yourself
> wanting this?
>

Yeah, agreed -- in about five years of full-time Scala, and very heavy use
of case classes, I don't think I've ever wanted this particular feature.
It seems like a rare edge case to me, and not a necessarily desireable one.

And to Seth's point -- I think we have a real problem, as a community, that
there's never been a clear, universally-accepted statement of what case
classes are *for*.

-- Do they exist for the "case" concept, making unapply cheap and easy?
-- Are they just the sum of the functionality, a way to get apply, equals
and hashCode for free?
-- Or are they a signal that this class is immutable, and that all that
functionality makes sense *because* this is an immutable class described by
its parameters?

Personally, I subscribe to the third of those; from what he's said, I
suspect Seth is coming from a similar viewpoint.  I think of the
functionality of case classes as directly derived from the notion that this
object is precisely described by its immutable parameters -- that's the
mental model, and the driver of how it works.  When I see a case class, I
*expect* it to be immutable, and I *expect* all of its parameters to factor
into equals.  It's possible to violate those expectations, but that's a
recipe for bugs.

Sadly, the horse has *long* since bolted the barn on this one: "impure"
case classes are, in practice, pretty common.  But I don't think they
should be driving our thinking.  I agree with what I think Seth is saying
here: with 20/20 hindsight, it's actually a bit of a language design bug
that case classes are more flexible than they ought to be, because that
impedes reasoning about them easily.  We shouldn't encourage that...

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