It's interesting to compare this to CAL - an earlier Haskell-like language on the JVM that was a project of some of my former colleagues at Crystal Decisions/Business Objects back in the 00's: http://openquark.org/Welcome.html

I've worked quite a bit with CAL over the years and it has been used for some serious work. One of my big frustrations with it though has been the clunkiness of the Java interop, especially since this is one of the reasons you might choose to use a language on the JVM.

Looking at the Frege docs, I like the look of the way this has been handled - rather than using an FFI-style approach, things like Int, String etc map directly to Java types and Java classes can be used like ADTs, which would make life way, way easier.

Given the nature of the language calling Frege from Java is always going to be more complicated but it looks like they've done a nice "fluent" Java API for calling into the runtime.

Would be interesting to see how this performs relative to CAL, in terms of both speed and space.