Could someone comment on Haskell in comparison to Erlang, Clojure or Scala? I felt like when I started choosing between Ruby on Rails and Python/Django, they were so similar that choosing either one would be a fine choice. Now that I'm interested in functional languages too, it seems that comparing Haskell to Erlang is just not the same as Python to Ruby.
They're so different and yet so similar it's probably best taking them one topic at a time. I've written nontrival things in all of those except clojure, but I think I still have a pretty good idea what it's about from a bit of playing around, reading the notes of others, time spent in PLT Scheme etc. I hope this is helpful for some people looking to dive in--remember, all of this "IMO/IME". Type Systems: Haskell and…
HM is a type system first, and that means is a logic system. HM is a logic that is deliberately kept relatively weak in order to allow full inference. Scala is not HM and never has been - it's much, much more sophisticated and so Scala uses a form of type inference not based on HM. Haskell '98 is HM plus a tiny bit (and yes that tiny bit does mean that the occasional type annotation is required). GHC Haskell is HM plus a ton more and so there are a lot more GHC Haskell programs that need some type annotation. The people behind GHC Haskell have done a good job of carefully adding type system features that don't totally break the model, though, so you rarely have to annotate much unless you love playing in advanced type land.
I would hesitate before saying GHC Haskell is more sophisticated "by far". It's not at all obvious that that is so. I'd want a type theorist to spend some time with the two core logics to determine which one subsumes what parts of the other before making that call.