Unable to initialize a TriMat matrix from the SPRS library - syntax

Question

I am trying to use the sparse matrix library SPRS and am having trouble initializing a matrix. Why does this not work?
extern crate sprs;
use sprs::TriMat;
fn main() {
let mut matrix = TriMat::new((5, 5));
}
The error is
error[E0282]: type annotations needed
--> src/main.rs:5:22
|
5 | let mut matrix = TriMat::new((5, 5));
| ---------- ^^^^^^^^^^^ cannot infer type for `N`
| |
| consider giving `matrix` a type

Answer 1

You just need to tell it the type of the elements of the matrix. For example, if it's a matrix of i32 then:
let mut matrix: TriMat<i32> = TriMat::new((5, 5));
This can't be inferred from the new constructor because that only takes an argument for the shape of the matrix, which doesn't include elements of the element type.
If you actually start storing data in the matrix, then the type annotation mostly won't be necessary because it will be inferred from the data:
let mut matrix = TriMat::new((5, 5));
matrix.add_triplet(0, 0, 42); // 42 literal is `i32` by default.

Answer 2

TriMat is:
type TriMat<N> = TriMatI<N, usize>;
TriMatI is:
type TriMatI<N, I> = TriMatBase<Vec<I>, Vec<N>>;
TriMatBase is:
pub struct TriMatBase<IStorage, DStorage> { /* fields omitted */ }
TriMatBase::new is:
pub fn new(shape: (usize, usize)) -> TriMatI<N, I>
Putting those together, you are effectively calling
TriMatBase::<_, usize>::new((5, 5));
The first type parameter is undecidable based on the code you've provided. In many cases, you do something with the value which will allow the compiler to pin down the concrete type. Since you just construct it and throw it away, there's a theoretical infinite number of types this could end up being.
You need to specify the type using the turbofish:
TriMat::<usize>::new((5, 5));
Or an explicit type on the variable:
let matrix: TriMat<usize> = TriMat::new((5, 5));
Or write some more code that will force the compiler to know the concrete type.