|
| 1 | +% Associated Types |
| 2 | + |
| 3 | +Associated types are a powerful part of Rust's type system. They're related to |
| 4 | +the idea of a 'type family', in other words, grouping multiple types together. That |
| 5 | +description is a bit abstract, so let's dive right into an example. If you want |
| 6 | +to write a `Graph` trait, you have two types to be generic over: the node type |
| 7 | +and the edge type. So you might write a trait, `Graph<N, E>`, that looks like |
| 8 | +this: |
| 9 | + |
| 10 | +```rust |
| 11 | +trait Graph<N, E> { |
| 12 | + fn has_edge(&self, &N, &N) -> bool; |
| 13 | + fn edges(&self, &N) -> Vec<E>; |
| 14 | + // etc |
| 15 | +} |
| 16 | +``` |
| 17 | + |
| 18 | +While this sort of works, it ends up being awkward. For example, any function |
| 19 | +that wants to take a `Graph` as a parameter now _also_ needs to be generic over |
| 20 | +the `N`ode and `E`dge types too: |
| 21 | + |
| 22 | +```rust,ignore |
| 23 | +fn distance<N, E, G: Graph<N, E>>(graph: &G, start: &N, end: &N) -> u32 { ... } |
| 24 | +``` |
| 25 | + |
| 26 | +Our distance calculation works regardless of our `Edge` type, so the `E` stuff in |
| 27 | +this signature is just a distraction. |
| 28 | + |
| 29 | +What we really want to say is that a certain `E`dge and `N`ode type come together |
| 30 | +to form each kind of `Graph`. We can do that with associated types: |
| 31 | + |
| 32 | +```rust |
| 33 | +trait Graph { |
| 34 | + type N; |
| 35 | + type E; |
| 36 | + |
| 37 | + fn has_edge(&self, &Self::N, &Self::N) -> bool; |
| 38 | + fn edges(&self, &Self::N) -> Vec<Self::E>; |
| 39 | + // etc |
| 40 | +} |
| 41 | +``` |
| 42 | + |
| 43 | +Now, our clients can be abstract over a given `Graph`: |
| 44 | + |
| 45 | +```rust,ignore |
| 46 | +fn distance<G: Graph>(graph: &G, start: &G::N, end: &G::N) -> uint { ... } |
| 47 | +``` |
| 48 | + |
| 49 | +No need to deal with the `E`dge type here! |
| 50 | + |
| 51 | +Let's go over all this in more detail. |
| 52 | + |
| 53 | +## Defining associated types |
| 54 | + |
| 55 | +Let's build that `Graph` trait. Here's the definition: |
| 56 | + |
| 57 | +```rust |
| 58 | +trait Graph { |
| 59 | + type N; |
| 60 | + type E; |
| 61 | + |
| 62 | + fn has_edge(&self, &Self::N, &Self::N) -> bool; |
| 63 | + fn edges(&self, &Self::N) -> Vec<Self::E>; |
| 64 | +} |
| 65 | +``` |
| 66 | + |
| 67 | +Simple enough. Associated types use the `type` keyword, and go inside the body |
| 68 | +of the trait, with the functions. |
| 69 | + |
| 70 | +These `type` declarations can have all the same thing as functions do. For example, |
| 71 | +if we wanted our `N` type to implement `Display`, so we can print the nodes out, |
| 72 | +we could do this: |
| 73 | + |
| 74 | +```rust |
| 75 | +use std::fmt; |
| 76 | + |
| 77 | +trait Graph { |
| 78 | + type N: fmt::Display; |
| 79 | + type E; |
| 80 | + |
| 81 | + fn has_edge(&self, &Self::N, &Self::N) -> bool; |
| 82 | + fn edges(&self, &Self::N) -> Vec<Self::E>; |
| 83 | +} |
| 84 | +``` |
| 85 | + |
| 86 | +## Implementing associated types |
| 87 | + |
| 88 | +Just like any trait, traits that use associated types use the `impl` keyword to |
| 89 | +provide implementations. Here's a simple implementation of Graph: |
| 90 | + |
| 91 | +```rust |
| 92 | +# trait Graph { |
| 93 | +# type N; |
| 94 | +# type E; |
| 95 | +# fn has_edge(&self, &Self::N, &Self::N) -> bool; |
| 96 | +# fn edges(&self, &Self::N) -> Vec<Self::E>; |
| 97 | +# } |
| 98 | +struct Node; |
| 99 | + |
| 100 | +struct Edge; |
| 101 | + |
| 102 | +struct MyGraph; |
| 103 | + |
| 104 | +impl Graph for MyGraph { |
| 105 | + type N = Node; |
| 106 | + type E = Edge; |
| 107 | + |
| 108 | + fn has_edge(&self, n1: &Node, n2: &Node) -> bool { |
| 109 | + true |
| 110 | + } |
| 111 | + |
| 112 | + fn edges(&self, n: &Node) -> Vec<Edge> { |
| 113 | + Vec::new() |
| 114 | + } |
| 115 | +} |
| 116 | +``` |
| 117 | + |
| 118 | +This silly implementation always returns `true` and an empty `Vec<Edge>`, but it |
| 119 | +gives you an idea of how to implement this kind of thing. We first need three |
| 120 | +`struct`s, one for the graph, one for the node, and one for the edge. If it made |
| 121 | +more sense to use a different type, that would work as well, we're just going to |
| 122 | +use `struct`s for all three here. |
| 123 | + |
| 124 | +Next is the `impl` line, which is just like implementing any other trait. |
| 125 | + |
| 126 | +From here, we use `=` to define our associated types. The name the trait uses |
| 127 | +goes on the left of the `=`, and the concrete type we're `impl`ementing this |
| 128 | +for goes on the right. Finally, we use the concrete types in our function |
| 129 | +declarations. |
| 130 | + |
| 131 | +## Trait objects with associated types |
| 132 | + |
| 133 | +There’s one more bit of syntax we should talk about: trait objects. If you |
| 134 | +try to create a trait object from an associated type, like this: |
| 135 | + |
| 136 | +```rust,ignore |
| 137 | +# trait Graph { |
| 138 | +# type N; |
| 139 | +# type E; |
| 140 | +# fn has_edge(&self, &Self::N, &Self::N) -> bool; |
| 141 | +# fn edges(&self, &Self::N) -> Vec<Self::E>; |
| 142 | +# } |
| 143 | +# struct Node; |
| 144 | +# struct Edge; |
| 145 | +# struct MyGraph; |
| 146 | +# impl Graph for MyGraph { |
| 147 | +# type N = Node; |
| 148 | +# type E = Edge; |
| 149 | +# fn has_edge(&self, n1: &Node, n2: &Node) -> bool { |
| 150 | +# true |
| 151 | +# } |
| 152 | +# fn edges(&self, n: &Node) -> Vec<Edge> { |
| 153 | +# Vec::new() |
| 154 | +# } |
| 155 | +# } |
| 156 | +let graph = MyGraph; |
| 157 | +let obj = Box::new(graph) as Box<Graph>; |
| 158 | +``` |
| 159 | + |
| 160 | +You’ll get two errors: |
| 161 | + |
| 162 | +```text |
| 163 | +error: the value of the associated type `E` (from the trait `main::Graph`) must |
| 164 | +be specified [E0191] |
| 165 | +let obj = Box::new(graph) as Box<Graph>; |
| 166 | + ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| 167 | +24:44 error: the value of the associated type `N` (from the trait |
| 168 | +`main::Graph`) must be specified [E0191] |
| 169 | +let obj = Box::new(graph) as Box<Graph>; |
| 170 | + ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| 171 | +``` |
| 172 | + |
| 173 | +We can’t create a trait object like this, becuase we don’t know the associated |
| 174 | +types. Instead, we can write this: |
| 175 | + |
| 176 | +```rust |
| 177 | +# trait Graph { |
| 178 | +# type N; |
| 179 | +# type E; |
| 180 | +# fn has_edge(&self, &Self::N, &Self::N) -> bool; |
| 181 | +# fn edges(&self, &Self::N) -> Vec<Self::E>; |
| 182 | +# } |
| 183 | +# struct Node; |
| 184 | +# struct Edge; |
| 185 | +# struct MyGraph; |
| 186 | +# impl Graph for MyGraph { |
| 187 | +# type N = Node; |
| 188 | +# type E = Edge; |
| 189 | +# fn has_edge(&self, n1: &Node, n2: &Node) -> bool { |
| 190 | +# true |
| 191 | +# } |
| 192 | +# fn edges(&self, n: &Node) -> Vec<Edge> { |
| 193 | +# Vec::new() |
| 194 | +# } |
| 195 | +# } |
| 196 | +let graph = MyGraph; |
| 197 | +let obj = Box::new(graph) as Box<Graph<N=Node, E=Edge>>; |
| 198 | +``` |
| 199 | + |
| 200 | +The `N=Node` syntax allows us to provide a concrete type, `Node`, for the `N` |
| 201 | +type parameter. Same with `E=Edge`. If we didn’t proide this constraint, we |
| 202 | +couldn’t be sure which `impl` to match this trait object to. |
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