Functional Programming in Rust

Rust has mutability by default and some types built in, and no null either. So why do you need functional programming if you've got Rust? I would go as far as to say it does not work. There just comes a point where trying to do too much with higher order functions can lead to error messages like "I cannot deal with these types anymore" And so the solution was literally take some of my higher order functions and rewrite them to be exactly the same except macros. And then they didn't blow up quite so much. The language is very trendy right now, but we'll see how it goes in the future.