How can I define a Rust function type which returns its own type?

Question

I'm learning Rust, and still very much trying to get my head around it. Consider the following Go definition:

type FnType func(paramType) FnType

It's just a function that returns a function of the same type. Can something similar be implemented in Rust? And, ideally, can it be done generically, so that paramType is specified by the client?

Answer 1

I did some digging in the docs and took to the playground and I think I've been able to answer this myself, although it does require an intermediary type: an enum , to be specific.

fn main() {
    let mut state = State::Some(first);
    while let State::Some(s) = state {
        state = s(0)
    }
}

enum State<T> {
    Some(fn(T) -> State<T>),
    None,
}

fn first(_: i32) -> State<i32> {
    println!("First");
    State::Some(second)
}

fn second(_: i32) -> State<i32> {
    println!("Second");
    State::None
}

You can verify that it runs on the playground .

Answer 2

Cyclic types are unsupported in Rust:

type a = fn(String) -> a;

Yields the following error:

error: unsupported cyclic reference between types/traits detected [--explain E0391]
 --> <anon>:1:24
  |>
1 |> type a = fn(String) -> a;
  |>                        ^
note: the cycle begins when processing `a`...
note: ...which then again requires processing `a`, completing the cycle.

See on playground