How do I declare a generic function which can add together references to sparse vectors?

Question

I'm trying to use the sprs crate (version 0.6.3) to manipulate sparse vectors. I'd like to add two vectors together. I started off with an implementation of the Add trait, then simplified this to an implementation function. Finally, I've boiled down the problem to a simple generic function.

// This works: the scalar type `u64` is fixed here
fn adder(first: &CsVec<u64>, second: &CsVec<u64>) -> CsVec<u64> {
    first + second
}

// When I try to make the scalar type generic, it doesn't work
fn adder2<T>(first: &CsVec<T>, second: &CsVec<T>) -> CsVec<T>
where
    CsVec<T>: Add,
    T: Add + Debug,
{
    first + second
}

The first version compiles fine, but I'd like to know why the second version won't compile. I get this error message:

error[E0369]: binary operation `+` cannot be applied to type `&sprs::sparse::CsVecBase<std::vec::Vec<usize>, std::vec::Vec<T>>`
  --> libp3prime/src/lib/datacache.rs:62:5
   |
62 |     first + second
   |     ^^^^^^^^^^^^^^
   |
   = note: an implementation of `std::ops::Add` might be missing for `&sprs::sparse::CsVecBase<std::vec::Vec<usize>, std::vec::Vec<T>>`

I don't really understand the error message. I know that you can add two CsVecs together, since adder() compiles, so I am a bit lost.

The two vectors should add together.

@E_net4 OP mentioned [sprs crate where CsVec structure is defined](https://docs.rs/sprs/0.2.0/sprs/sparse/vec/struct.CsVec.html) — Alex Yu, Feb 15 '19 at 14:13
I believe the question does meet the MCVE standards. I did note that CsVec was part of the sprs crate (someone has kindly edited my question to add a link to the crate) and expected that anyone wanting to know the implementation of CsVec would look at the sprs documentation. Apart from that, I believed that all the information required was present. — garypen, Feb 15 '19 at 14:15
After answering, I found this question: [How do I require a generic type implement an operation like Add, Sub, Mul, or Div in a generic function?](https://stackoverflow.com/q/29184358/3650362) Is this close enough to flag as a duplicate? — trent, Feb 15 '19 at 14:28
@trentcl I did search and failed to find that. That is close enough to flag as duplicate. — garypen, Feb 15 '19 at 14:41

trent · Answer 1 · 2019-02-15T17:26:57.257

Be sure that the trait bounds defined on the function match the behavior used in the function.

first and second are not CsVec<T>, but &CsVec<T>. In Rust, &X is a different type from X. You need a trait bound that says that you can add two &CsVec<T>s and get a CsVec<T> as output:

fn adder2<'a, T>(first: &'a CsVec<T>, second: &'a CsVec<T>) -> CsVec<T>
where
    &'a CsVec<T>: Add<Output = CsVec<T>>,
{
    first + second
}

No bounds on T are needed in this example.

The 'a lifetime parameter in this case was passed in to the function. Sometimes it is useful to define a trait bound on a reference inside the function, for example, to use + on references to local variables. In that case you would want to use the higher-ranked trait bound for<'a> &'a CsVec<T>: Add<Output = CsVec<T>> instead. See the linked questions below for more information.

Lukas Kalbertodt points out that it may sometimes be more flexible to say "I just want to add two &CsVec<T>s, and I will return whatever type that operation gives me", which you can do by returning <&'a CsVec<T> as Add>::Output:

fn adder2<'a, T>(first: &'a CsVec<T>, second: &'a CsVec<T>) -> <&'a CsVec<T> as Add>::Output
where
    &'a CsVec<T>: Add,
{
    first + second
}

In this case the output type does not have to be exactly CsVec<T>, but when it is, it works the same way as the first version.

How do I declare a generic function which can add together references to sparse vectors?

1 Answers1

Related

Related