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Rust library that provides `Opt<T>` as an alternative to `Option<T>`. This is advantageous when: 1. the generic type, `T`, is expensive to allocate, and 2. mutation between `None` and `Some(...)` is frequent.

Rust 100.00%

rust-library option-type data-structure

another-option's Introduction

`another-option`

This package provides Opt<T> as an alternative to Option<T>. Why would you want another option? Opt provides advantages when:

the generic type, T, is expensive to allocate, and
mutation between None and Some(...) is frequent.

Examples

Since Rust's built-in Option<T> is an enum, it will drop its Some(...) value when None is assigned.

let mut option: Option<String> = Some(String::with_capacity(1024));
option = None; // drops the string
option = Some(String::with_capacity(1024)); // allocation

Since Opt<T> always owns the value, even when empty, the value can be reused without drops or allocations:

use crate::another_option::Opt;
let mut opt: Opt<String> = Opt::some(String::with_capacity(1024));
opt.map_in_place(|v| v.push_str("value"));
opt.set_none(); // does *not* drop the string
opt.set_some();
assert_eq!(opt.unwrap(), String::from("value"));

another-option's People

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another-option's Issues

Some consequences of how equality is defined

Given the current code, some unexpected behavior is possible. Consider this example:

let opt_1 = Opt::none(1);
let opt_2 = Opt::none(2);
assert_eq!(opt_1, opt_2); // First, the two variables are equal.
opt_1.set_some(); // Next, the same function ...
opt_2.set_some(); // ... is applied to both.
assert_ne!(opt_1, opt_2); // Now, the two are no longer equal.

The example shows two values that start off being equal. After having the same function applied (set_some()), they are no longer equal. This seems problematic in some sense, even though the definition of equality seems reasonable:

From the source code:

impl<T> PartialEq for Opt<T>
where
    T: std::cmp::PartialEq,
{
    fn eq(&self, other: &Self) -> bool {
        if self.empty && other.empty {
            true
        } else if !self.empty && !other.empty {
            self.value == other.value
        } else {
            false
        }
    }
}

I need to think about this more.

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