You indeed don't get Monad for free, we're in agreement. To pick a use case dear to Rust, it's definitely the best way to e.g. write data structures that are p

You indeed don't get Monad for free, we're in agreement. To pick a use case dear to Rust, it's definitely the best way to e.g. write data structures that are parameterized over say Rc, Arc, or some Gc. Again, I say "the best" with pedagogy and productivity in mind: the error messages are better and intent clearer with this mechanism over the associated type alternatives.