[LinXitoW]

As someone that has done some Haskell, and does Java for a living, I think their metaphor is extremely accurate. It's not about purity, it's about how the functional parts were grafted onto the language, and therefore don"t "interop" with the classical ways very well.

In a good mixed paradim language like Rust, you can freely choose the correct paradigm for the problem, and can easily mix them. In Java, they are often at odds with each other. Best example is exceptions (esp. checked exceptions) and all Stream operations. They take a lambda which does NOT throw any exceptions. So, you need to either never use checked exceptions (which is impossible, because most libraries still do) or not use streaming, or create a horrendous hybrid.

[nobleach]

This is a very solid counterpoint to my argument. The no throw in a lambda is a fairly nasty wart and should have been handled in a more conformant way.

[yw3410]

To be fair, mixing higher order functions with effects (exceptions) is really hard without changing the type system completely. At that point you're moving into novel encoding territory.