Struct yoke::Yoke

source · [−]
pub struct Yoke<Y: for<'a> Yokeable<'a>, C> { /* private fields */ }
Expand description

A Cow-like borrowed object “yoked” to its backing data.

This allows things like zero copy deserialized data to carry around shared references to their backing buffer, by “erasing” their static lifetime and turning it into a dynamically managed one.

Y (the Yokeable) is the object containing the references, and will typically be of the form Foo<'static>. The 'static is not the actual lifetime of the data, rather it is a convenient way to mark the erased lifetime and make it dynamic.

C is the “cart”, which Y may contain references to. After the yoke is constructed, the cart serves little purpose except to guarantee that Y’s references remain valid for as long as the yoke remains in memory (by calling the destructor at the appropriate moment).

The primary constructor for Yoke is Yoke::attach_to_cart(). Several variants of that constructor are provided to serve numerous types of call sites and Yoke signatures.

The key behind this type is Yoke::get(), where calling .get() on a type like Yoke<Cow<'static, str>, _> will get you a short-lived &'a Cow<'a, str>, restricted to the lifetime of the borrow used during .get(). This is entirely safe since the Cow borrows from the cart type C, which cannot be interfered with as long as the Yoke is borrowed by .get (). .get() protects access by essentially reifying the erased lifetime to a safe local one when necessary.

Furthermore, there are various .map_project() methods that allow turning a Yoke into another Yoke containing a different type that may contain elements of the original yoked value. See the Yoke::map_project() docs for more details.

In general, C is a concrete type, but it is also possible for it to be a trait object; for more information, see IsCovariant.

Example

For example, we can use this to store zero-copy deserialized data in a cache:


fn load_object(filename: &str) -> Yoke<Cow<'static, str>, Rc<[u8]>> {
    let rc: Rc<[u8]> = load_from_cache(filename);
    Yoke::<Cow<'static, str>, Rc<[u8]>>::attach_to_cart(rc, |data: &[u8]| {
        // essentially forcing a #[serde(borrow)]
        Cow::Borrowed(bincode::deserialize(data).unwrap())
    })
}

let yoke = load_object("filename.bincode");
assert_eq!(&**yoke.get(), "hello");
assert!(matches!(yoke.get(), &Cow::Borrowed(_)));

Implementations

Construct a Yoke by yokeing an object to a cart in a closure.

See also Yoke::try_attach_to_cart() to return a Result from the closure.

Call sites for this function may not compile pre-1.61; if this still happens, use Yoke::attach_to_cart_badly() and file a bug.

Examples

fn load_object(filename: &str) -> Yoke<Cow<'static, str>, Rc<[u8]>> {
    let rc: Rc<[u8]> = load_from_cache(filename);
    Yoke::<Cow<'static, str>, Rc<[u8]>>::attach_to_cart(rc, |data: &[u8]| {
        // essentially forcing a #[serde(borrow)]
        Cow::Borrowed(bincode::deserialize(data).unwrap())
    })
}

let yoke: Yoke<Cow<str>, _> = load_object("filename.bincode");
assert_eq!(&**yoke.get(), "hello");
assert!(matches!(yoke.get(), &Cow::Borrowed(_)));

Construct a Yoke by yokeing an object to a cart. If an error occurs in the deserializer function, the error is passed up to the caller.

Call sites for this function may not compile pre-1.61; if this still happens, use Yoke::try_attach_to_cart_badly() and file a bug.

👎 Deprecated

Use Yoke::attach_to_cart().

This was needed because the pre-1.61 compiler couldn’t always handle the FnOnce trait bound.

👎 Deprecated

Use Yoke::try_attach_to_cart().

This was needed because the pre-1.61 compiler couldn’t always handle the FnOnce trait bound.

Obtain a valid reference to the yokeable data

This essentially transforms the lifetime of the internal yokeable data to be valid. For example, if you’re working with a Yoke<Cow<'static, T>, C>, this will return an &'a Cow<'a, T>

Example

// load_object() defined in the example at the top of this page
let yoke: Yoke<Cow<str>, _> = load_object("filename.bincode");
assert_eq!(yoke.get(), "hello");

Get a reference to the backing cart.

This can be useful when building caches, etc. However, if you plan to store the cart separately from the yoke, read the note of caution below in Yoke::into_backing_cart.

Get the backing cart by value, dropping the yokeable object.

Caution: Calling this method could cause information saved in the yokeable object but not the cart to be lost. Use this method only if the yokeable object cannot contain its own information.

Example

Good example: the yokeable object is only a reference, so no information can be lost.

use yoke::Yoke;

let local_data = "foo".to_string();
let yoke = Yoke::<&'static str, Box<String>>::attach_to_zero_copy_cart(Box::new(local_data));
assert_eq!(*yoke.get(), "foo");

// Get back the cart
let cart = yoke.into_backing_cart();
assert_eq!(&*cart, "foo");

Bad example: information specified in .with_mut() is lost.

use std::borrow::Cow;
use yoke::Yoke;

let local_data = "foo".to_string();
let mut yoke =
    Yoke::<Cow<'static, str>, Box<String>>::attach_to_zero_copy_cart(Box::new(local_data));
assert_eq!(yoke.get(), "foo");

// Override data in the cart
yoke.with_mut(|cow| {
    let mut_str = cow.to_mut();
    mut_str.clear();
    mut_str.push_str("bar");
});
assert_eq!(yoke.get(), "bar");

// Get back the cart
let cart = yoke.into_backing_cart();
assert_eq!(&*cart, "foo"); // WHOOPS!

Unsafe function for replacing the cart with another

This can be used for type-erasing the cart, for example.

Safety
  • f() must not panic
  • References from the yokeable Y should still be valid for the lifetime of the returned cart type C.

Typically, this means implementing f as something which wraps the inner cart type C. Yoke only really cares about destructors for its carts so it’s fine to erase other information about the cart, as long as the backing data will still be destroyed at the same time.

Mutate the stored Yokeable data.

See Yokeable::transform_mut() for why this operation is safe.

Example

This can be used to partially mutate the stored data, provided no new borrowed data is introduced.


// also implements Yokeable
struct Bar<'a> {
    numbers: Cow<'a, [u8]>,
    string: Cow<'a, str>,
    owned: Vec<u8>,
}

// `load_object()` deserializes an object from a file
let mut bar: Yoke<Bar, _> = load_object("filename.bincode");
assert_eq!(bar.get().string, "hello");
assert!(matches!(bar.get().string, Cow::Borrowed(_)));
assert_eq!(&*bar.get().numbers, &[0x68, 0x65, 0x6c, 0x6c, 0x6f]);
assert!(matches!(bar.get().numbers, Cow::Borrowed(_)));
assert_eq!(&*bar.get().owned, &[]);

bar.with_mut(|bar| {
    bar.string.to_mut().push_str(" world");
    bar.owned.extend_from_slice(&[1, 4, 1, 5, 9]);
});

assert_eq!(bar.get().string, "hello world");
assert!(matches!(bar.get().string, Cow::Owned(_)));
assert_eq!(&*bar.get().owned, &[1, 4, 1, 5, 9]);
// Unchanged and still Cow::Borrowed
assert_eq!(&*bar.get().numbers, &[0x68, 0x65, 0x6c, 0x6c, 0x6f]);
assert!(matches!(bar.get().numbers, Cow::Borrowed(_)));

Helper function allowing one to wrap the cart type C in an Option<T>.

Construct a new Yoke from static data. There will be no references to cart here since Yokeables are 'static, this is good for e.g. constructing fully owned Yokes with no internal borrowing.

This is similar to Yoke::new_owned() but it does not allow you to mix the Yoke with borrowed data. This is primarily useful for using Yoke in generic scenarios.

Example

let owned: Cow<str> = "hello".to_owned().into();
// this yoke can be intermingled with actually-borrowed Yokes
let yoke: Yoke<Cow<str>, ()> = Yoke::new_always_owned(owned);

assert_eq!(yoke.get(), "hello");

Obtain the yokeable out of a Yoke<Y, ()>

For most Yoke types this would be unsafe but it’s fine for Yoke<Y, ()> since there are no actual internal references

Construct a new Yoke from static data. There will be no references to cart here since Yokeables are 'static, this is good for e.g. constructing fully owned Yokes with no internal borrowing.

This can be paired with [Yoke:: wrap_cart_in_option()] to mix owned and borrowed data.

If you do not wish to pair this with borrowed data, Yoke::new_always_owned() can be used to get a Yoke API on always-owned data.

Example

let owned: Cow<str> = "hello".to_owned().into();
// this yoke can be intermingled with actually-borrowed Yokes
let yoke: Yoke<Cow<str>, Option<Rc<[u8]>>> = Yoke::new_owned(owned);

assert_eq!(yoke.get(), "hello");

Obtain the yokeable out of a Yoke<Y, Option<C>> if possible.

If the cart is None, this returns Some, but if the cart is Some, this returns self as an error.

Allows one to “project” a yoke to perform a transformation on the data, potentially looking at a subfield, and producing a new yoke. This will move cart, and the provided transformation is only allowed to use data known to be borrowed from the cart.

This takes an additional PhantomData<&()> parameter as a workaround to the issue described in #86702. This parameter should just be ignored in the function.

To capture data and pass it to the closure, use Yoke::map_project_with_capture(). See #1061.

This can be used, for example, to transform data from one format to another:

fn slice(y: Yoke<&'static str, Rc<[u8]>>) -> Yoke<&'static [u8], Rc<[u8]>> {
    y.map_project(move |yk, _| yk.as_bytes())
}

This can also be used to create a yoke for a subfield

// also safely implements Yokeable<'a>
struct Bar<'a> {
    string_1: &'a str,
    string_2: &'a str,
}

fn map_project_string_1(bar: Yoke<Bar<'static>, Rc<[u8]>>) -> Yoke<&'static str, Rc<[u8]>> {
    bar.map_project(|bar, _| bar.string_1)
}

This is similar to Yoke::map_project, however it does not move Self and instead clones the cart (only if the cart is a CloneableCart)

This is a bit more efficient than cloning the Yoke and then calling Yoke::map_project because then it will not clone fields that are going to be discarded.

This is similar to Yoke::map_project, but it works around it not being able to use FnOnce by using an explicit capture input. See #1061.

See the docs of Yoke::map_project for how this works.

This is similar to Yoke::map_project_cloned, however it works around it not being able to use FnOnce by using an explicit capture input. See #1061.

See the docs of Yoke::map_project_cloned for how this works.

A version of Yoke::map_project that takes a capture and bubbles up an error from the callback function.

A version of Yoke::map_project_cloned that takes a capture and bubbles up an error from the callback function.

Allows type-erasing the cart in a Yoke<Y, Rc<C>>.

The yoke only carries around a cart type C for its destructor, since it needs to be able to guarantee that its internal references are valid for the lifetime of the Yoke. As such, the actual type of the Cart is not very useful unless you wish to extract data out of it via Yoke::backing_cart(). Erasing the cart allows for one to mix Yokes obtained from different sources.

In case the cart type C is not already an Rc<T>, you can use Yoke::wrap_cart_in_rc() to wrap it.

Example
use std::rc::Rc;
use yoke::erased::ErasedRcCart;
use yoke::Yoke;

let buffer1: Rc<String> = Rc::new("   foo bar baz  ".into());
let buffer2: Box<String> = Box::new("  baz quux  ".into());

let yoke1 = Yoke::<&'static str, _>::attach_to_cart(buffer1, |rc| rc.trim());
let yoke2 = Yoke::<&'static str, _>::attach_to_cart(buffer2, |b| b.trim());

let erased1: Yoke<_, ErasedRcCart> = yoke1.erase_rc_cart();
// Wrap the Box in an Rc to make it compatible
let erased2: Yoke<_, ErasedRcCart> = yoke2.wrap_cart_in_rc().erase_rc_cart();

// Now erased1 and erased2 have the same type!

Available with the "alloc" feature enabled.

Allows type-erasing the cart in a Yoke<Y, Box<C>>.

The yoke only carries around a cart type C for its destructor, since it needs to be able to guarantee that its internal references are valid for the lifetime of the Yoke. As such, the actual type of the Cart is not very useful unless you wish to extract data out of it via Yoke::backing_cart(). Erasing the cart allows for one to mix Yokes obtained from different sources.

In case the cart type C is not already Box<T>, you can use Yoke::wrap_cart_in_box() to wrap it.

Example
use std::rc::Rc;
use yoke::erased::ErasedBoxCart;
use yoke::Yoke;

let buffer1: Rc<String> = Rc::new("   foo bar baz  ".into());
let buffer2: Box<String> = Box::new("  baz quux  ".into());

let yoke1 = Yoke::<&'static str, _>::attach_to_cart(buffer1, |rc| rc.trim());
let yoke2 = Yoke::<&'static str, _>::attach_to_cart(buffer2, |b| b.trim());

// Wrap the Rc in an Box to make it compatible
let erased1: Yoke<_, ErasedBoxCart> = yoke1.wrap_cart_in_box().erase_box_cart();
let erased2: Yoke<_, ErasedBoxCart> = yoke2.erase_box_cart();

// Now erased1 and erased2 have the same type!

Available with the "alloc" feature enabled.

Helper function allowing one to wrap the cart type C in a Box<T>. Can be paired with Yoke::erase_box_cart()

Available with the "alloc" feature enabled.

Helper function allowing one to wrap the cart type C in an Rc<T>. Can be paired with Yoke::erase_rc_cart(), or generally used to make the Yoke cloneable.

Available with the "alloc" feature enabled.

Helper function allowing one to wrap the cart type C in an EitherCart.

This function wraps the cart into the A variant. To wrap it into the B variant, use Self::wrap_cart_in_either_b().

For an example, see EitherCart.

Helper function allowing one to wrap the cart type C in an EitherCart.

This function wraps the cart into the B variant. To wrap it into the A variant, use Self::wrap_cart_in_either_a().

For an example, see EitherCart.

Construct a Yoke<Y, C> from a cart implementing StableDeref by zero-copy cloning the cart to Y and then yokeing that object to the cart.

The type Y must implement ZeroFrom<C::Target>. This trait is auto-implemented on many common types and can be custom implemented or derived in order to make it easier to construct a Yoke.

Example

Attach to a cart:

use std::borrow::Cow;
use yoke::Yoke;

let yoke = Yoke::<Cow<'static, str>, String>::attach_to_zero_copy_cart("demo".to_string());

assert_eq!("demo", yoke.get());

Trait Implementations

Clone requires that the cart type C derefs to the same address after it is cloned. This works for Rc, Arc, and &’a T.

For other cart types, clone .backing_cart() and re-use .attach_to_cart(); however, doing so may lose mutations performed via .with_mut().

Cloning a Yoke is often a cheap operation requiring no heap allocations, in much the same way that cloning an Rc is a cheap operation. However, if the yokeable contains owned data (e.g., from .with_mut()), that data will need to be cloned.

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.

Creates owned data from borrowed data, usually by cloning. Read more

Uses borrowed data to replace owned data, usually by cloning. Read more

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.