The main thing that sets Haskell apart is its type systems, there is none better. In addition to the strong type system Haskell is purely functional which mean

The main thing that sets Haskell apart is its type systems, there is none better. In addition to the strong type system Haskell is purely functional which means the compiler can reason pretty strongly about an applications side-effects. I'm not sure Erlang or Scala can really compare on either of these things (though I am not an expert on either of those languages). The last big thing I can think of is that Haskell is "lazy" which means it only evaluates values as they are needed, this aspect has its upsides but also many downsides. Laziness can help one write clear concise code but you must be aware of it or things can get a little sticky.