Dafny is quite different from Scala in that it is a formal language that can be compiled to a number of different targets such as Go or Python or C#. This allows an algorithm to be formally verified while still producing executable code.

You could add Scala as a compilation target or you could just use the Java output and call formally verified Java functions from Scala. Even if you do get an implementation that produces Scala, don't expect the full power of idiomatic Scala to be available in the code you formally verify. To verify code, you have to write the code in Dafny with associated assertions to be proven. Since there are multiple compilation targets multiple formal constraints on what can usefully be verified, the data types available will not match the data types that you would use natively from Scala.

It's similar in spirit, but in Dafny one can express much more complicated and complex invariants which get checked at build time -- compared to eiffel where pre/post conditions are checked at runtime (in dev builds mostly).