1// This file is part of ICU4X. For terms of use, please see the file
2// called LICENSE at the top level of the ICU4X source tree
3// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
4
5//! Types to enable polymorphic carts.
6
7use crate::CloneableCart;
8
9use core::ops::Deref;
10use stable_deref_trait::StableDeref;
11
12/// A cart that can be one type or the other. Enables ergonomic polymorphic carts.
13///
14/// `EitherCart` enables yokes originating from different data sources and therefore
15/// having different cart types to be merged into the same yoke type, but still being
16/// able to recover the original cart type if necessary.
17///
18/// All relevant Cart traits are implemented for `EitherCart`, and carts can be
19/// safely wrapped in an `EitherCart`.
20///
21/// Also see [`Yoke::erase_box_cart()`](crate::Yoke::erase_box_cart).
22///
23/// # Examples
24///
25/// ```
26/// use std::rc::Rc;
27/// use yoke::either::EitherCart;
28/// use yoke::Yoke;
29///
30/// let y1: Yoke<&'static str, Rc<str>> =
31/// Yoke::attach_to_zero_copy_cart("reference counted hello world".into());
32///
33/// let y2: Yoke<&'static str, &str> = Yoke::attach_to_zero_copy_cart("borrowed hello world");
34///
35/// type CombinedYoke<'a> = Yoke<&'static str, EitherCart<Rc<str>, &'a str>>;
36///
37/// // Both yokes can be combined into a single yoke type despite different carts
38/// let y3: CombinedYoke = y1.wrap_cart_in_either_a();
39/// let y4: CombinedYoke = y2.wrap_cart_in_either_b();
40///
41/// assert_eq!(*y3.get(), "reference counted hello world");
42/// assert_eq!(*y4.get(), "borrowed hello world");
43///
44/// // The resulting yoke is cloneable if both cart types implement CloneableCart
45/// let y5 = y4.clone();
46/// assert_eq!(*y5.get(), "borrowed hello world");
47/// ```
48#[derive(Clone, PartialEq, Eq, Debug)]
49#[allow(clippy::exhaustive_enums)] // stable
50pub enum EitherCart<C0, C1> {
51 A(C0),
52 B(C1),
53}
54
55impl<C0, C1, T> Deref for EitherCart<C0, C1>
56where
57 C0: Deref<Target = T>,
58 C1: Deref<Target = T>,
59{
60 type Target = T;
61 fn deref(&self) -> &T {
62 use EitherCart::*;
63 match self {
64 A(a: &C0) => a.deref(),
65 B(b: &C1) => b.deref(),
66 }
67 }
68}
69
70// Safety: Safe because both sub-types implement the trait.
71unsafe impl<C0, C1, T> StableDeref for EitherCart<C0, C1>
72where
73 C0: StableDeref,
74 C1: StableDeref,
75 C0: Deref<Target = T>,
76 C1: Deref<Target = T>,
77{
78}
79
80// Safety: Safe because both sub-types implement the trait.
81unsafe impl<C0, C1> CloneableCart for EitherCart<C0, C1>
82where
83 C0: CloneableCart,
84 C1: CloneableCart,
85{
86}
87