The downside is that you would have to put () (Rust's version of “nil”) in a bunch of functions to fulfil the requirements of the callback's signature since oth

The downside is that you would have to put () (Rust's version of “nil”) in a bunch of functions to fulfil the requirements of the callback's signature since otherwise the type inferred from the function would be the value of the last expression Wouldn't co/contra-variance solve this entirely? It works just fine in Scala for example scala> def runTwice(f: () => Unit) = { | f() | f() | } runTwice: (f: () => Unit)Unit scala> runTwice{ () => | println("moo") | 1 | } moo moo Note how it expects a function that returns Unit, i'm passing in a function that returns an Int (1), but the compiler is perfectly happy.