- yeti appears to be quite a faithful port of sml to the jvm, while scala is perhaps best thought of as the bastard offspring of java and haskell.
- as a consequence, yeti is simpler and intended for programming in a functional style. while scala is much more complex and "object functional" (is that a recognised description these days?).
- the integration between java and scala appears to be somewhat cleaner than that between java and yeti (although both are fairly good).
- scala's type system is significantly more powerful. someone better educated than me can explain the difference in detail, but yeti seems to have just "simple" polymorphism, while scala has bounded types etc(1).
- scala also has a whole pile of tricks related to getting the type system to do things "implicitly" that is, well, i guess you either love it or hate it.
- yeti appears to be better than scala at getting optimised (ie not broken) tail calls to work.
- scala is baroque; yeti minimal.
- scala may be the future of java programming (which is good). yeti certainly isn't (which is a pity, imho).
- a lot of the above parallels the difference between ml and haskell (if you ignore the laziness part). these days, haskell is a bit of a monster; ml is significantly simpler (even ocaml).
(1) what happens with yeti when you have polymorphism and mutable containers? i have no idea. perhaps runtime type errors?
"Variable (and argument) bindings are not polymorphic (it would make typesystem unsound), and therefore their type changes:"
For more details about this type unsoundness in ML, see the long version of "Relaxing the value restriction":
* static typing
* discriminated unions
* first-class functions
* immutability by default
* pattern matching
These are all present in both Yeti and SML.
Yes, Yeti is not SML, but I think it's much closer to ML and Friends than it is to any other language family, even Haskell or Scala.
Scala was arguably more influenced by ML than Haskell. It's really more of an amalgamation of different languages than anything else.