Add basic information about associated types

src/doc/trpl/SUMMARY.md

@@ -22,6 +22,7 @@
     * [More Strings](more-strings.md)
     * [Patterns](patterns.md)
     * [Method Syntax](method-syntax.md)
+    * [Associated Types](associated-types.md)
     * [Closures](closures.md)
     * [Iterators](iterators.md)
     * [Generics](generics.md)

src/doc/trpl/associated-types.md

@@ -0,0 +1,202 @@
+% Associated Types
+
+Associated types are a powerful part of Rust's type system. They're related to
+the idea of a 'type family', in other words, grouping multiple types together. That
+description is a bit abstract, so let's dive right into an example. If you want
+to write a `Graph` trait, you have two types to be generic over: the node type
+and the edge type. So you might write a trait, `Graph<N, E>`, that looks like
+this:
+
+```rust
+trait Graph<N, E> {
+    fn has_edge(&self, &N, &N) -> bool;
+    fn edges(&self, &N) -> Vec<E>;
+    // etc
+}
+```
+
+While this sort of works, it ends up being awkward. For example, any function
+that wants to take a `Graph` as a parameter now _also_ needs to be generic over
+the `N`ode and `E`dge types too:
+
+```rust,ignore
+fn distance<N, E, G: Graph<N, E>>(graph: &G, start: &N, end: &N) -> u32 { ... }
+```
+
+Our distance calculation works regardless of our `Edge` type, so the `E` stuff in
+this signature is just a distraction.
+
+What we really want to say is that a certain `E`dge and `N`ode type come together
+to form each kind of `Graph`. We can do that with associated types:
+
+```rust
+trait Graph {
+    type N;
+    type E;
+
+    fn has_edge(&self, &Self::N, &Self::N) -> bool;
+    fn edges(&self, &Self::N) -> Vec<Self::E>;
+    // etc
+}
+```
+
+Now, our clients can be abstract over a given `Graph`:
+
+```rust,ignore
+fn distance<G: Graph>(graph: &G, start: &G::N, end: &G::N) -> uint { ... }
+```
+
+No need to deal with the `E`dge type here!
+
+Let's go over all this in more detail.
+
+## Defining associated types
+
+Let's build that `Graph` trait. Here's the definition:
+
+```rust
+trait Graph {
+    type N;
+    type E;
+
+    fn has_edge(&self, &Self::N, &Self::N) -> bool;
+    fn edges(&self, &Self::N) -> Vec<Self::E>;
+}
+```
+
+Simple enough. Associated types use the `type` keyword, and go inside the body
+of the trait, with the functions.
+
+These `type` declarations can have all the same thing as functions do. For example,
+if we wanted our `N` type to implement `Display`, so we can print the nodes out,
+we could do this:
+
+```rust
+use std::fmt;
+
+trait Graph {
+    type N: fmt::Display;
+    type E;
+
+    fn has_edge(&self, &Self::N, &Self::N) -> bool;
+    fn edges(&self, &Self::N) -> Vec<Self::E>;
+}
+```
+
+## Implementing associated types
+
+Just like any trait, traits that use associated types use the `impl` keyword to
+provide implementations. Here's a simple implementation of Graph:
+
+```rust
+# trait Graph {
+#     type N;
+#     type E;
+#     fn has_edge(&self, &Self::N, &Self::N) -> bool;
+#     fn edges(&self, &Self::N) -> Vec<Self::E>;
+# }
+struct Node;
+
+struct Edge;
+
+struct MyGraph;
+
+impl Graph for MyGraph {
+    type N = Node;
+    type E = Edge;
+
+    fn has_edge(&self, n1: &Node, n2: &Node) -> bool {
+        true
+    }
+
+    fn edges(&self, n: &Node) -> Vec<Edge> {
+        Vec::new()
+    }
+}
+```
+
+This silly implementation always returns `true` and an empty `Vec<Edge>`, but it
+gives you an idea of how to implement this kind of thing. We first need three
+`struct`s, one for the graph, one for the node, and one for the edge. If it made
+more sense to use a different type, that would work as well, we're just going to
+use `struct`s for all three here.
+
+Next is the `impl` line, which is just like implementing any other trait.
+
+From here, we use `=` to define our associated types. The name the trait uses
+goes on the left of the `=`, and the concrete type we're `impl`ementing this
+for goes on the right. Finally, we use the concrete types in our function
+declarations.
+
+## Trait objects with associated types
+
+There’s one more bit of syntax we should talk about: trait objects. If you
+try to create a trait object from an associated type, like this:
+
+```rust,ignore
+# trait Graph {
+#     type N;
+#     type E;
+#     fn has_edge(&self, &Self::N, &Self::N) -> bool;
+#     fn edges(&self, &Self::N) -> Vec<Self::E>;
+# }
+# struct Node;
+# struct Edge;
+# struct MyGraph;
+# impl Graph for MyGraph {
+#     type N = Node;
+#     type E = Edge;
+#     fn has_edge(&self, n1: &Node, n2: &Node) -> bool {
+#         true
+#     }
+#     fn edges(&self, n: &Node) -> Vec<Edge> {
+#         Vec::new()
+#     }
+# }
+let graph = MyGraph;
+let obj = Box::new(graph) as Box<Graph>;
+```
+
+You’ll get two errors:
+
+```text
+error: the value of the associated type `E` (from the trait `main::Graph`) must
+be specified [E0191]
+let obj = Box::new(graph) as Box<Graph>;
+          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+24:44 error: the value of the associated type `N` (from the trait
+`main::Graph`) must be specified [E0191]
+let obj = Box::new(graph) as Box<Graph>;
+          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+```
+
+We can’t create a trait object like this, becuase we don’t know the associated
+types. Instead, we can write this:
+
+```rust
+# trait Graph {
+#     type N;
+#     type E;
+#     fn has_edge(&self, &Self::N, &Self::N) -> bool;
+#     fn edges(&self, &Self::N) -> Vec<Self::E>;
+# }
+# struct Node;
+# struct Edge;
+# struct MyGraph;
+# impl Graph for MyGraph {
+#     type N = Node;
+#     type E = Edge;
+#     fn has_edge(&self, n1: &Node, n2: &Node) -> bool {
+#         true
+#     }
+#     fn edges(&self, n: &Node) -> Vec<Edge> {
+#         Vec::new()
+#     }
+# }
+let graph = MyGraph;
+let obj = Box::new(graph) as Box<Graph<N=Node, E=Edge>>;
+```
+
+The `N=Node` syntax allows us to provide a concrete type, `Node`, for the `N`
+type parameter. Same with `E=Edge`. If we didn’t proide this constraint, we
+couldn’t be sure which `impl` to match this trait object to.