What’s Up With All These Changes in Rails?

But HAML feels like such a vast, unambiguous, never-going-back improvement over ERB that I am honestly baffled Rails hasn't adopted it. It's at least as big a win as Sass and CoffeeScript, if not bigger.

I agree with Katz here. If anything, Rails has shown remarkable restraint in the face of a smorgasbord of shiny, new, and often superior, competing technologies.

The one point Yehuda's making I'd dispute is the impact of Arel.

I saw a very early version of Arel back when I was maintaining DataMapper. To be clear, Arel is beautiful code. It's ridiculously well done IMO from an OO stand-point. It's the code I wished I could write in DM.

I didn't use it (or something like it) in DM because I'd been there and seen the consequences. To give the point some context, you might expect NHibernate to impose a 10% overhead on c#. NHibernate is a very complex library, doing a lot more than most (any?) Ruby O/RM. In Ruby, once you get beyond the most trivial examples, you can easily see numbers like 50% overhead, and some queries that would be a cake-walk in .NET are impractical or even impossible in Ruby with constrained resources or service timeouts.

Ruby method dispatch and object instantiation is slow. Damn slow. Ridiculously slow.

The reason DM and now AR perform two-query-eager-loads to avoid JOINs has almost nothing to do with database performance. I doubt many Rubyists really understand or grok what that means. It has everything to do with the instantiation and even simple iteration of a cartesian product in Ruby being infeasibly slow.

Getting back to Arel: It's probably the very best of Rails OO. But that it's introduced some very serious performance regressions is no surprise at all. I said as much to NK and Yehuda when we discussed it during DataMapper 0.9 development.

With Ruby you have to make compromises. Especially in such a critical-loop portion of your stack.

On the other hand, if you're going to stay with AR, then it probably needed to happen sooner or later.

Best of all worlds would have been to drop AR altogether and promote a migration to Sequel for the official Rails O/RM. ;-)

I'd love to hear some practical limitations that applications using Ruby and NHibernate face when querying data.

Is this slowness a reason that so many no-sql engines seemed to have sprung at the same time as RoR?

For the few cases where that makes sense, ERb is great. But I have a hard time imagining it's common enough to make it the default.

Edit: Also, the post you linked to outlines several simple ways to mix text into your HAML (inlining HTML, using markdown filters, etc)