Note that Monoid here is ONLY for the reason that Cow<str> doesn't satisfy Add. In fact, String doesn't satisfy it either. So I need to have a trait that allows me to call a function that concatenates my types. I already wrote how to concatenate Strings and Cow<str>s in my Monoid crate so I'm just re-using that.
It could be T: Concatenable for all I care, but the rest of the function needs to look this way, more or less.
I only have one type parameter, that's very common. I'm abstracting over String and Cow<str> because I want to bench the performance of both. (Cow<str> is always faster)
I'm not just talking about Monoid, I'm talking about the overall signature -- with the Fn and the abstractions it's very Haskelly. Such functions are not common in Rust. You find them (all the iterator adaptors, for one), and they can be "good Rust", but they're not common. Which is okay.
(I don't disagree that Rust is more complicated than Python, just addressing GP's comment whether or not this is "good" rust.)
It's actually very common to pass around hashmaps of functions in dynamic languages. If anything, since you don't even have to know how they're implemented it's easier. I realize that my function incurs a run time cost because I'm making an array of trait objects. When I'm writing JavaScript I don't know and I don't care.
It could be T: Concatenable for all I care, but the rest of the function needs to look this way, more or less.
I only have one type parameter, that's very common. I'm abstracting over String and Cow<str> because I want to bench the performance of both. (Cow<str> is always faster)