We can do functional programming on Java. We use all the design patterns for that.

At the very end, everything is just some noisy, hairy, side-effectfull, gotofull machinery code.

The beauty of Haskell is that it allows you to limit the things you need to reason about. If I see a function with the type "(a, b) -> a" I don't need to read a man page to see where I should use it or not. I know what it can do by its type. In C I can not do this. What can I say about a function "int foo(char* bar)"? Does it allocate memory? Does it asks a number for the user on stdin? Or does it returns the length of a zero-ending char sequence? In fact it can do anything, and I can't forbid that. I can't guarantee that my function has good behaviour. You need to trust the man page.