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Hiding Internals When Returning Iterators in Rust

I wrote a function that returns an iterator that lists all Fibonacci numbers:

fn fib<'a>() -> Unfold<'a, int, (int, int)> {
  Unfold::new((1, 1), |st| {
    let (a, b) = *st;
    *st = (b, a + b);
    Some(a)
  })
}

Unfortunately, the return type provides many of the internal functions of this function, such as the type of internal state (int, int)

. What are my options for hiding these internals?

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1 answer


As A .B pointed out, the standard method is to wrap it in a struct:

pub struct Fibonacci {
    inner: Unfold<'static, i32, (i32, i32)>
}

impl Iterator for Fibonacci {
    type Item = i32;
    fn next(&mut self) -> Option<i32> {
        self.inner.next()
    }
}

pub fn fib() -> Fibonacci {
    Fibonacci {
        inner: Unfold::new((1, 1), |st| {
            let (a, b) = *st;
            *st = (b, a + b);
            Some(a)
        })
    }
}


By the way, I also fixed a few nits:

  • As Chris Morgan notes, it is clearer to use it 'static

    as a lifetime.

  • int

    (aka isize

    ) should not be used here since the Fibonacci sequence is independent of the pointer size. I changed it to a more idiomatic one i32

    .

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