Trait naan::Equiv

source ·

pub trait Equiv {
    type To;
}

Expand description

An Equiv type is one that is conceptually the same as some different type.

This is used to allow types to implement typeclasses for other types in a still strict way. For examples see functor::FunctorSurrogate, monad::MonadSurrogate

Example - Iterators

In the following example, iter map and filter are all conceptually “an iterator over usize”

use std::iter::{Filter, Map};

let vec: Vec<usize> = vec![1, 2, 3, 4, 5];
let iter: std::vec::IntoIter<usize> = vec.into_iter();
let map: Map<std::vec::IntoIter<usize>, _> = iter.map(|n| n + 1);
let filter: Filter<Map<std::vec::IntoIter<usize>, _>, _> = map.filter(|n| n % 2 == 0);

this would imply:

<std::vec::IntoIter<usize> as Equiv>::To == std::vec::IntoIter<usize>
<Map<std::vec::IntoIter<usize>, _> as Equiv>::To == std::vec::IntoIter<usize>
<Filter<Map<std::vec::IntoIter<usize>, _>, _> as Equiv>::To == std::vec::IntoIter<usize>

Other examples:

Result<A, Infallible> is Equiv::To Result<A, !> (and vice versa)
IO<Lazy> is Equiv::To IO<Lazy::T> (IO<Suspend<_, usize>> == IO<usize>)