[jakewins]

This is probably naive but: How does this differ from something like “declare a type, implement it with methods that all throw NotImplementedException”?

As in, is this “just” a less boilerplate-heavy version of that, or is it more capable?

[7h3kk1d]

You can play with it at https://hazel.org/build/dev/ but programs don't "crash" when they're incomplete so "1 + 5 + ?" will evaluate to "6 + ?" in the editor. So your program can evaluate as far as possible with the holes. If you're using Java and throw NotImplementedException you lose all context to what did work.