I can't really comment on source maps in Elm, but the approach in PureScript has been to generate clean, readable JS which is debuggable directly. Source maps are on the roadmap, but not really a priority right now. I haven't heard any complaints about the ability to debug compiled PureScript yet.
That's a nice approach for debugging! If the mapping is close enough, I don't mind reading the JS output. I'm curious about the general approach to compilation, though. It seems like a statically typed language could take advantage of the knowledge of types to generate an even more efficient version of the program that uses typed arrays and views (though, yes, it would require implementing a garbage collector unless relying on some kind of WeakMap in the JS engine).
I've heard of garbage collected languages compiling to LLVM which would allow Emscripten to assist you, but I've also heard that LLVM has a really hard time with GC languages.
Right now, the translation is very direct. My basic rule of thumb is - only perform those optimizations which the user opts into. Some things are standard though, like a few inlining rules and tail call elimination, but the plan is to provide a rewrite rules engine so that the developer can be as fine-grained as they like when it comes to optimizations.
