Type signature of a Rust HashMap of a function

Question

I create a HashMap which maps strings to functions of type Vec -> Expression, where Expression is a type I have defined. The code

Answer 1

The relevant parts of error message are Box and Box. The first is a boxed Fn trait object. The second is a boxed function plus. Note that it isn't a boxed pointer to a function, which would be Box with no {plus} part. It is the unique and unnameable type of the function plus itself.

That is you cannot write real type of this HashMap, as the type it contains is unnameable. It's not a big deal though, you can only put plus function into it.

The following code gives compilation error

let functions: HashMap<_, _> =
    vec![("+", Box::new(plus)), 
         ("-", Box::new(minus))].into_iter().collect();
                        ^^^^^ expected fn item, found a different fn item

This works, but it is useless

let functions: HashMap<_, _> =
    vec![("+", Box::new(plus)), 
         ("-", Box::new(plus))].into_iter().collect();

One possible solution is to convert first element of a vector into the required type.

type BoxedFn = Box) -> Expression>;

let functions: HashMap<&str, BoxedFn> =
    vec![("+", Box::new(plus) as BoxedFn),
         ("_", Box::new(minus))].into_iter().collect();

Another one is type ascription of intermediate variable.

type BoxedFn = Box) -> Expression>;

let v: Vec<(_, BoxedFn)> = vec![("+", Box::new(plus)), ("_", Box::new(minus))];
let functions: HashMap<&str, BoxedFn> = v.into_iter().collect();