Can a trait guarantee certain type properties such

2019-08-07 09:29发布

问题:

Imagine I have functions like this:

fn min_max_difference(row: &Vec<u32>) -> u32 {
    let mut min_elem: u32 = row[0];
    let mut max_elem: u32 = min_elem;

    for &element in row.iter().skip(1) {
        if element < min_elem {
            min_elem = element;
        } else if element > max_elem {
            max_elem = element;
        }
    }

    result = max_elem - min_elem;
}

fn execute_row_operation(row: &Vec<u32>, operation: Fn(&Vec<u32>) -> u32) -> Option<(u32, u32)> {
    let mut result = None;

    if row.len() > 0 {
        result = operation(row);
    }

    result
}

Note that the if block in execute_row_operation guarantees that the Vec<u32> I am passing to the operation function is non-empty. In general, I want "operations" to be functions which only accept non-empty rows. I would like it if I could do something like this:

fn min_max_difference<T: &Vec<u32> + NonEmpty>(row: T) -> u32 {
    //snip
}

This would allow the compiler to disallow passing references to empty vectors to a function like min_max_difference which expects this.

But traits as I understand them specify what methods a type has, rather than what properties a type has. In my head, I am imagining a trait for a type T that is composed of boolean predicates with type: Fn<T> -> bool, and such a trait is "implemented" for a type if it all those predicates evaluate to true.

Can something like this be achieved?

回答1:

Can a trait guarantee certain type properties

Yes, that is what they are for. In many cases, these properties are that a set of functions exist (e.g. PartialEq::eq) and that a set of behaviors are present (e.g. symmetric and transitive equality, required by PartialEq).

Traits can also have no methods, such as Eq. These only add a set of behaviors (e.g. reflexive equality). These types of traits are often referred to as marker traits.

such as a vector is non-empty?

However, you aren't asking for what you really want. You actually want a way to implement a trait for certain values of a type. This is not possible in Rust.

At best, you can introduce a newtype. This might be sufficient for your needs, but you could also implement your own marker traits for that newtype, if useful:

struct NonEmptyVec<T>(Vec<T>);

impl<T> NonEmptyVec<T> {
    fn new(v: Vec<T>) -> Result<Self, Vec<T>> {
        if v.is_empty() {
            Err(v)
        } else {
            Ok(NonEmptyVec(v))
        }
    }
}

fn do_a_thing<T>(items: NonEmptyVec<T>) {}

fn main() {
    let mut a = Vec::new();
    // do_a_thing(a); // expected struct `NonEmptyVec`, found struct `std::vec::Vec`
    a.push(42);
    let b = NonEmptyVec::new(a).expect("nope");
    do_a_thing(b);
}
T: &Vec<u32> + NonEmpty

This isn't valid because Vec is a type and NonEmpty would presumably be a trait — you can't use types as trait bounds.


Historical note:

Way back in the long ago, as I understand it, Rust actually did support what you wanted under the name typestate. See What is typestate? and Typestate Is Dead, Long Live Typestate!.

An example of emulating it:

struct MyVec<T, S>
where
    S: VecState,
{
    vec: Vec<T>,
    state: S,
}

trait VecState {}

struct Empty;
struct NonEmpty;

impl VecState for Empty {}
impl VecState for NonEmpty {}

impl<T> MyVec<T, Empty> {
    fn new() -> Self {
        MyVec {
            vec: Vec::new(),
            state: Empty,
        }
    }

    fn push(mut self, value: T) -> MyVec<T, NonEmpty> {
        self.vec.push(value);
        MyVec {
            vec: self.vec,
            state: NonEmpty,
        }
    }
}

fn do_a_thing<T>(items: MyVec<T, NonEmpty>) {}

fn main() {
    let a = MyVec::new();
    // do_a_thing(a); // expected struct `NonEmpty`, found struct `Empty`
    let b = a.push(42);
    do_a_thing(b);
}


标签: rust traits