gnat 0.1.4

This crate provides type-level natural numbers, similar to typenum.

A type-level number is a type that represents a number. The Nat trait takes the role of the “type-level number type”, i.e. one accepts a type-level number using a generic parameter with bound Nat.

The use cases are the same as those of generic consts.

gnat differs from typenum in that its Nat trait is not a marker trait, but defines enough (internal) structure to be able to define and use operations on it, generically.

This crate is to the unstable generic_const_expr feature what typenum is to the already stable min_const_generics feature. For example, consider the case of concatenating arrays at compile time

// Ideal function, requires #![feature(generic_const_expr)]
const fn concat_arrays_gce<T, const M: usize, const N: usize>(
    a: [T; M],
    b: [T; N],
) -> [T; M + N] {
    todo!()
}
// typenum + generic-array implementation
use generic_array::{GenericArray, ArrayLength};
const fn concat_arrays_gar<T, M: ArrayLength, N: ArrayLength>(
    a: GenericArray<T, M>,
    b: GenericArray<T, N>,
) -> GenericArray<T, typenum::op!(M + N)>
where // ArrayLength is not enough, we also need to add a bound for `+`
    M: std::ops::Add<N, Output: ArrayLength>,
{
    todo!()
}

// gnat implementation
use gnat::{Nat, array::Arr};
const fn concat_arrays_nat<T, M: Nat, N: Nat>(
    a: Arr<T, M>,
    b: Arr<T, N>,
) -> Arr<T, gnat::eval!(M + N)> {
    a.concat_arr(b).retype()
}

It is also possible to implement custom operations without any extra bounds needed to use them. See the mod@expr module.