Add tutorial chapter on interfaces

doc/tutorial/iface.md

+# Interfaces
+
+Interfaces are Rust's take on value polymorphism—the thing that
+object-oriented languages tend to solve with methods and inheritance.
+For example, writing a function that can operate on multiple types of
+collections.
+
+NOTE: This feature is very new, and will need a few extensions to be
+applicable to more advanced use cases.
+
+## Declaration
+
+An interface consists of a set of methods. A method is a function that
+can be applied to a `self` value and a number of arguments, using the
+dot notation: `self.foo(arg1, arg2)`.
+
+For example, we could declare the interface `to_str` for things that
+can be converted to a string, with a single method of the same name:
+
+    iface to_str {
+        fn to_str() -> str;
+    }
+
+## Implementation
+
+To actually implement an interface for a given type, the `impl` form
+is used. This defines implementations of `to_str` for the `int` and
+`str` types.
+
+    # iface to_str { fn to_str() -> str; }
+    impl of to_str for int {
+        fn to_str() -> str { int::to_str(self, 10u) }
+    }
+    impl of to_str for str {
+        fn to_str() -> str { self }
+    }
+
+Given these, we may call `1.to_str()` to get `"1"`, or
+`"foo".to_str()` to get `"foo"` again. This is basically a form of
+static overloading—when the Rust compiler sees the `to_str` method
+call, it looks for an implementation that matches the type with a
+method that matches the name, and simply calls that.
+
+## Scoping
+
+Implementations are not globally visible. Resolving a method to an
+implementation requires that implementation to be in scope. You can
+import and export implementations using the name of the interface they
+implement (multiple implementations with the same name can be in scope
+without problems). Or you can give them an explicit name if you
+prefer, using this syntax:
+
+    # iface to_str { fn to_str() -> str; }
+    impl nil_to_str of to_str for () {
+        fn to_str() -> str { "()" }
+    }
+
+## Bounded type parameters
+
+The useful thing about value polymorphism is that it does not have to
+be static. If object-oriented languages only let you call a method on
+an object when they knew exactly which sub-type it had, that would not
+get you very far. To be able to call methods on types that aren't
+known at compile time, it is possible to specify 'bounds' for type
+parameters.
+
+    # iface to_str { fn to_str() -> str; }
+    fn comma_sep<T: to_str>(elts: [T]) -> str {
+        let result = "", first = true;
+        for elt in elts {
+            if first { first = false; }
+            else { result += ", "; }
+            result += elt.to_str();
+        }
+        ret result;
+    }
+
+The syntax for this is similar to the syntax for specifying that a
+parameter type has to be copyable (which is, in principle, another
+kind of bound). By declaring `T` as conforming to the `to_str`
+interface, it becomes possible to call methods from that interface on
+values of that type inside the function. It will also cause a
+compile-time error when anyone tries to call `comma_sep` on an array
+whose element type does not have a `to_str` implementation in scope.
+
+## Polymorphic interfaces
+
+Interfaces may contain type parameters. This defines an interface for
+generalized sequence types:
+
+    iface seq<T> {
+        fn len() -> uint;
+        fn iter(block(T));
+    }
+    impl <T> of seq<T> for [T] {
+        fn len() -> uint { vec::len(self) }
+        fn iter(b: block(T)) {
+            for elt in self { b(elt); }
+        }
+    }
+
+Note that the implementation has to explicitly declare the its
+parameter `T` before using it to specify its interface type. This is
+needed because it could also, for example, specify an implementation
+of `seq<int>`—the `of` clause *refers* to a type, rather than defining
+one.
+
+## Casting to an interface type
+
+The above allows us to define functions that polymorphically act on
+values of *an* unknown type that conforms to a given interface.
+However, consider this function:
+
+    # iface drawable { fn draw(); }
+    fn draw_all<T: drawable>(shapes: [T]) {
+        for shape in shapes { shape.draw(); }
+    }
+
+You can call that on an array of circles, or an array of squares
+(assuming those have suitable `drawable` interfaces defined), but not
+on an array containing both circles and squares.
+
+When this is needed, an interface name can be used as a type, causing
+the function to be written simply like this:
+
+    # iface drawable { fn draw(); }
+    fn draw_all(shapes: [drawable]) {
+        for shape in shapes { shape.draw(); }
+    }
+
+There is no type parameter anymore (since there isn't a single type
+that we're calling the function on). Instead, the `drawable` type is
+used to refer to a type that is a reference-counted box containing a
+value for which a `drawable` implementation exists, combined with
+information on where to find the methods for this implementation. This
+is very similar to the 'vtables' used in most object-oriented
+languages.
+
+To construct such a value, you use the `as` operator to cast a value
+to an interface type:
+
+    # type circle = int; type rectangle = int;
+    # iface drawable { fn draw(); }
+    # impl of drawable for int { fn draw() {} }
+    # fn new_circle() -> int { 1 }
+    # fn new_rectangle() -> int { 2 }
+    # fn draw_all(shapes: [drawable]) {}
+    let c: circle = new_circle();
+    let r: rectangle = new_rectangle();
+    draw_all([c as drawable, r as drawable]);
+
+This will store the value into a box, along with information about the
+implementation (which is looked up in the scope of the cast). The
+`drawable` type simply refers to such boxes, and calling methods on it
+always works, no matter what implementations are in scope.
+
+Note that the allocation of a box is somewhat more expensive than
+simply using a type parameter and passing in the value as-is, and much
+more expensive than statically resolved method calls.
+
+## Interface-less implementations
+
+If you only intend to use an implementation for static overloading,
+and there is no interface available that it conforms to, you are free
+to leave off the `of` clause.
+
+    # type currency = ();
+    # fn mk_currency(x: int, s: str) {}
+    impl int_util for int {
+        fn times(b: block(int)) {
+            let i = 0;
+            while i < self { b(i); i += 1; }
+        }
+        fn dollars() -> currency {
+            mk_currency(self, "USD")
+        }
+    }
+
+This allows cutesy things like `send_payment(10.dollars())`. And the
+nice thing is that it's fully scoped, so the uneasy feeling that
+anybody with experience in object-oriented languages (with the
+possible exception of Rubyists) gets at the sight of such things is
+not justified. It's harmless!

doc/tutorial/order

@@ -7,6 +7,7 @@ data
 args
 generic
 mod
+iface
 ffi
 task
 test