1. Heist allows you to define your own HTML/XML tags in the host language (Haskell in this case). This means you're only dealing with (an extended) XML document when doing layout and design so all the normal XML tools still work.
2. Some popular template engines try to separate logic and design but end up letting you cheat a little. Any time you want/have to cheat and put logic in the template, it really was a shortcoming of the template engine. In Heist, you can't cheat but you never want to.
3. The reason #1 and #2 work is because Heist's "recursively applied splices" is just the right abstraction. My HTML templates end up looking just as pretty as well factored Haskell code. Heist makes the perfectionist in me happy.
In short, I would say you're right here: '"Separates view and business logic" and "enables DRY design" are valuable goals, but most template languages have them.' But just because most template languages have these goals doesn't mean they've achieved their goals. Heist, in my experience and opinion, does achieve these goals.
There are two things that compiled Heist loses: the ability to bind new splices on the fly at runtime and splice recursion/composability.
I haven't checked or read the doc thoroughly, but if it's what I think it means - all we get is hierarchal splices. Which is still a lot, but it's not quite as magical.
We still keep some of the magic by allowing you to run the old "interpreted" style splices at load time. These don't have access to dynamic data, but they do have recursion/composability. This combination retains most of the power while allowing a huge speed increase. It just means that to take advantage of both you have to structure things in a certain way.
At this point it seems to me that this structure also ends up being a desirable one for organizational reasons. But the jury is still out as far as whether there will still be reason to want more. We're aware that there might be good reasons to support this extra power and I have a pretty good idea of how it would be implemented. But I want to get more people using it in the real world before we address that issue.
1. Heist allows you to define your own HTML/XML tags in the host language (Haskell in this case). This means you're only dealing with (an extended) XML document when doing layout and design so all the normal XML tools still work.
2. Some popular template engines try to separate logic and design but end up letting you cheat a little. Any time you want/have to cheat and put logic in the template, it really was a shortcoming of the template engine. In Heist, you can't cheat but you never want to.
3. The reason #1 and #2 work is because Heist's "recursively applied splices" is just the right abstraction. My HTML templates end up looking just as pretty as well factored Haskell code. Heist makes the perfectionist in me happy.
In short, I would say you're right here: '"Separates view and business logic" and "enables DRY design" are valuable goals, but most template languages have them.' But just because most template languages have these goals doesn't mean they've achieved their goals. Heist, in my experience and opinion, does achieve these goals.