mucell 0.2.0

A cell with the ability to mutate the value through an immutable reference when safe.

Comparison with `RefCell`

RefCell goes for completely runtime checking, having try_borrow, try_borrow_mut, borrow and borrow_mut all taking &self and using custom reference types everywhere.

MuCell (out of pity and the fact that “non-ascii idents are not fully supported” I did not name it ΜCell with the crate named µcell) makes much more use of true Rust borrow checking for a result that is more efficient and has no possibility of panicking.

However, its purpose is not the same as RefCell; it is designed specifically for cases where something only needs an immutable reference, but where being able to safely take a mutable reference can improve efficiency. Say, for example, where it’s beneficial to be able to cache the result of a calculation, but you don’t really want to need to do that.

The purpose of all of this is for an accessor for a T that can be made more efficient if it can have &mut self, but doesn’t strictly require it. For this reason, it’s often going to be paired with std::borrow::Cow, e.g. Cow<str> or Cow<[T]>, producing Borrowed if you are able to mutate the value or Owned of the same data if not.

Examples

This example covers most of the surface area of the library:

```rust

use mucell::MuCell;

let mut cell = MuCell::new(vec![1i, 2, 3]);

// You can borrow from the cell mutably at no cost. cell.borrow_mut().push(4);

// You can borrow immutably, too, and it’s very cheap. // (Rust’s standard borrow checking prevents you from doing // this while there’s a mutable reference taken out.) assert_eq!(&cell.borrow()[], &[1, 2, 3, 4][]);

// So long as there are no active borrows, // trymutate can be used to mutate the value. assert!(cell.trymutate(|x| x.push(5))); assert_eq!(&cell.borrow()[], &[1, 2, 3, 4, 5][]);

// But when there is an immutable borrow active, // trymutate says no. let b = cell.borrow(); assert!(!cell.trymutate(|_| unreachable!())); drop(b);

// We can have many immutable borrows at a time, too. { let a = cell.borrow(); let b = cell.borrow(); let c = cell.borrow(); assert_eq!(&a as *const _, &b as *const _); }

// Once they’re all cleared, trymutate is happy again. assert!(cell.trymutate(|x| x.push(6))); assert_eq!(&cell.borrow()[], &[1, 2, 3, 4, 5, 6][]); ```

Look at the examples in the repository for some slightly more practical (though still typically contrived) examples. Also see the mucell_ref_type! docs for an example of that part of the library.

Usage

Cargo all the way. http://crates.io/crates/mucell

This crate can be used with #![no_std] by enabling the no_std Cargo feature.

Author

Chris Morgan (chris-morgan) is the primary author and maintainer of this library.

License

This library is distributed under similar terms to Rust: dual licensed under the MIT license and the Apache license (version 2.0).