How to write a Rust function that takes an iterator?

Question

I'd like to write a function that accepts an iterator and returns the results of some operations on it. Specifically, I'm trying to iterate over the values of a HashMap:

use std::collections::HashMap;

fn find_min<'a>(vals: Iterator<Item=&'a u32>) -> Option<&'a u32> {
    vals.min()
}

fn main() {
    let mut map = HashMap::new();
    map.insert("zero", 0u32);
    map.insert("one", 1u32);
    println!("Min value {:?}", find_min(map.values()));
}

But alas:

error: the `min` method cannot be invoked on a trait object
 --> src/main.rs:4:10
  |
4 |     vals.min()
  |          ^^^

error[E0277]: the trait bound `std::iter::Iterator<Item=&'a u32> + 'static: std::marker::Sized` is not satisfied
 --> src/main.rs:3:17
  |
3 | fn find_min<'a>(vals: Iterator<Item = &'a u32>) -> Option<&'a u32> {
  |                 ^^^^ `std::iter::Iterator<Item=&'a u32> + 'static` does not have a constant size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `std::iter::Iterator<Item=&'a u32> + 'static`
  = note: all local variables must have a statically known size

error[E0308]: mismatched types
  --> src/main.rs:11:41
   |
11 |     println!("Min value {:?}", find_min(map.values()));
   |                                         ^^^^^^^^^^^^ expected trait std::iter::Iterator, found struct `std::collections::hash_map::Values`
   |
   = note: expected type `std::iter::Iterator<Item=&u32> + 'static`
              found type `std::collections::hash_map::Values<'_, &str, u32>`

I get the same error if I try to pass by reference; if I use a Box, I get lifetime errors.

Answer 1

You want to use generics here:

fn find_min<'a, I>(vals: I) -> Option<&'a u32>
where
    I: Iterator<Item = &'a u32>,
{
    vals.min()
}

Traits can be used in two ways: as bounds on type parameters and as trait objects. The book The Rust Programming Language has a chapter on traits and a chapter on trait objects that explain these two use cases.

Additionally, you often want to take something that implements IntoIterator as this can make the code calling your function nicer:

fn find_min<'a, I>(vals: I) -> Option<&'a u32>
where
    I: IntoIterator<Item = &'a u32>,
{
    vals.into_iter().min()
}

Answer 2

Since Rust 1.26 impl Trait are available. Here is a more compact version using impl Trait.

use std::collections::HashMap;

fn find_min<'a>(vals: impl Iterator<Item = &'a u32>) -> Option<&'a u32> {
    vals.min()
}

fn main() {
    let mut map = HashMap::new();
    map.insert("zero", 0u32);
    map.insert("one", 1u32);
    println!("Min value {:?}", find_min(map.values()));
}

playground

Answer 3

This behaviour is a little unintuitive from those with a Python background rather than, say, a C++ background, so let me clarify a little.

In Rust, values are conceptually stored inside the name that binds them. Thus, if you write

let mut x = Foo { t: 10 };
let mut y = x;
x.t = 999;

y.t will still be 10.

So when you write

let x: Iterator<Item=&'a u32>;

(or the same in the function parameter list), Rust needs to allocate enough space for any value of type Iterator<Item=&'a u32>. Even if this was possible, it wouldn't be efficient.

So what Rust does instead is offer you the option to

• Put the value on the heap, eg. with Box, which gives Python-style semantics. Then you can take generically with &mut Iterator<Item=&'a u32>.

• Specialize each function invocation for each possible type to satisfy the bound. This is more flexible, since a trait reference is a possible specialization, and gives the compiler more opportunities for specialization, but means you can't have dynamic dispatch (where the type can vary dependent on runtime parameters).