cannam@48: ---
cannam@48: layout: page
cannam@48: title: Schema Language
cannam@48: ---
cannam@48: 
cannam@48: # Schema Language
cannam@48: 
cannam@48: Like Protocol Buffers and Thrift (but unlike JSON or MessagePack), Cap'n Proto messages are
cannam@48: strongly-typed and not self-describing. You must define your message structure in a special
cannam@48: language, then invoke the Cap'n Proto compiler (`capnp compile`) to generate source code to
cannam@48: manipulate that message type in your desired language.
cannam@48: 
cannam@48: For example:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: @0xdbb9ad1f14bf0b36;  # unique file ID, generated by `capnp id`
cannam@48: 
cannam@48: struct Person {
cannam@48:   name @0 :Text;
cannam@48:   birthdate @3 :Date;
cannam@48: 
cannam@48:   email @1 :Text;
cannam@48:   phones @2 :List(PhoneNumber);
cannam@48: 
cannam@48:   struct PhoneNumber {
cannam@48:     number @0 :Text;
cannam@48:     type @1 :Type;
cannam@48: 
cannam@48:     enum Type {
cannam@48:       mobile @0;
cannam@48:       home @1;
cannam@48:       work @2;
cannam@48:     }
cannam@48:   }
cannam@48: }
cannam@48: 
cannam@48: struct Date {
cannam@48:   year @0 :Int16;
cannam@48:   month @1 :UInt8;
cannam@48:   day @2 :UInt8;
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Some notes:
cannam@48: 
cannam@48: * Types come after names. The name is by far the most important thing to see, especially when
cannam@48:   quickly skimming, so we put it up front where it is most visible.  Sorry, C got it wrong.
cannam@48: * The `@N` annotations show how the protocol evolved over time, so that the system can make sure
cannam@48:   to maintain compatibility with older versions. Fields (and enumerants, and interface methods)
cannam@48:   must be numbered consecutively starting from zero in the order in which they were added. In this
cannam@48:   example, it looks like the `birthdate` field was added to the `Person` structure recently -- its
cannam@48:   number is higher than the `email` and `phones` fields. Unlike Protobufs, you cannot skip numbers
cannam@48:   when defining fields -- but there was never any reason to do so anyway.
cannam@48: 
cannam@48: ## Language Reference
cannam@48: 
cannam@48: ### Comments
cannam@48: 
cannam@48: Comments are indicated by hash signs and extend to the end of the line:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: # This is a comment.
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Comments meant as documentation should appear _after_ the declaration, either on the same line, or
cannam@48: on a subsequent line. Doc comments for aggregate definitions should appear on the line after the
cannam@48: opening brace.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: struct Date {
cannam@48:   # A standard Gregorian calendar date.
cannam@48: 
cannam@48:   year @0 :Int16;
cannam@48:   # The year.  Must include the century.
cannam@48:   # Negative value indicates BC.
cannam@48: 
cannam@48:   month @1 :UInt8;   # Month number, 1-12.
cannam@48:   day @2 :UInt8;     # Day number, 1-30.
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Placing the comment _after_ the declaration rather than before makes the code more readable,
cannam@48: especially when doc comments grow long. You almost always need to see the declaration before you
cannam@48: can start reading the comment.
cannam@48: 
cannam@48: ### Built-in Types
cannam@48: 
cannam@48: The following types are automatically defined:
cannam@48: 
cannam@48: * **Void:** `Void`
cannam@48: * **Boolean:** `Bool`
cannam@48: * **Integers:** `Int8`, `Int16`, `Int32`, `Int64`
cannam@48: * **Unsigned integers:** `UInt8`, `UInt16`, `UInt32`, `UInt64`
cannam@48: * **Floating-point:** `Float32`, `Float64`
cannam@48: * **Blobs:** `Text`, `Data`
cannam@48: * **Lists:** `List(T)`
cannam@48: 
cannam@48: Notes:
cannam@48: 
cannam@48: * The `Void` type has exactly one possible value, and thus can be encoded in zero bits. It is
cannam@48:   rarely used, but can be useful as a union member.
cannam@48: * `Text` is always UTF-8 encoded and NUL-terminated.
cannam@48: * `Data` is a completely arbitrary sequence of bytes.
cannam@48: * `List` is a parameterized type, where the parameter is the element type. For example,
cannam@48:   `List(Int32)`, `List(Person)`, and `List(List(Text))` are all valid.
cannam@48: 
cannam@48: ### Structs
cannam@48: 
cannam@48: A struct has a set of named, typed fields, numbered consecutively starting from zero.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: struct Person {
cannam@48:   name @0 :Text;
cannam@48:   email @1 :Text;
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Fields can have default values:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: foo @0 :Int32 = 123;
cannam@48: bar @1 :Text = "blah";
cannam@48: baz @2 :List(Bool) = [ true, false, false, true ];
cannam@48: qux @3 :Person = (name = "Bob", email = "bob@example.com");
cannam@48: corge @4 :Void = void;
cannam@48: grault @5 :Data = 0x"a1 40 33";
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: ### Unions
cannam@48: 
cannam@48: A union is two or more fields of a struct which are stored in the same location. Only one of
cannam@48: these fields can be set at a time, and a separate tag is maintained to track which one is
cannam@48: currently set. Unlike in C, unions are not types, they are simply properties of fields, therefore
cannam@48: union declarations do not look like types.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: struct Person {
cannam@48:   # ...
cannam@48: 
cannam@48:   employment :union {
cannam@48:     unemployed @4 :Void;
cannam@48:     employer @5 :Company;
cannam@48:     school @6 :School;
cannam@48:     selfEmployed @7 :Void;
cannam@48:     # We assume that a person is only one of these.
cannam@48:   }
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Additionally, unions can be unnamed.  Each struct can contain no more than one unnamed union.  Use
cannam@48: unnamed unions in cases where you would struggle to think of an appropriate name for the union,
cannam@48: because the union represents the main body of the struct.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: struct Shape {
cannam@48:   area @0 :Float64;
cannam@48: 
cannam@48:   union {
cannam@48:     circle @1 :Float64;      # radius
cannam@48:     square @2 :Float64;      # width
cannam@48:   }
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Notes:
cannam@48: 
cannam@48: * Unions members are numbered in the same number space as fields of the containing struct.
cannam@48:   Remember that the purpose of the numbers is to indicate the evolution order of the
cannam@48:   struct. The system needs to know when the union fields were declared relative to the non-union
cannam@48:   fields.
cannam@48: 
cannam@48: * Notice that we used the "useless" `Void` type here. We don't have any extra information to store
cannam@48:   for the `unemployed` or `selfEmployed` cases, but we still want the union to distinguish these
cannam@48:   states from others.
cannam@48: 
cannam@48: * By default, when a struct is initialized, the lowest-numbered field in the union is "set".  If
cannam@48:   you do not want any field set by default, simply declare a field called "unset" and make it the
cannam@48:   lowest-numbered field.
cannam@48: 
cannam@48: * You can move an existing field into a new union without breaking compatibility with existing
cannam@48:   data, as long as all of the other fields in the union are new.  Since the existing field is
cannam@48:   necessarily the lowest-numbered in the union, it will be the union's default field.
cannam@48: 
cannam@48: **Wait, why aren't unions first-class types?**
cannam@48: 
cannam@48: Requiring unions to be declared inside a struct, rather than living as free-standing types, has
cannam@48: some important advantages:
cannam@48: 
cannam@48: * If unions were first-class types, then union members would clearly have to be numbered separately
cannam@48:   from the containing type's fields.  This means that the compiler, when deciding how to position
cannam@48:   the union in its containing struct, would have to conservatively assume that any kind of new
cannam@48:   field might be added to the union in the future.  To support this, all unions would have to
cannam@48:   be allocated as separate objects embedded by pointer, wasting space.
cannam@48: 
cannam@48: * A free-standing union would be a liability for protocol evolution, because no additional data
cannam@48:   can be attached to it later on.  Consider, for example, a type which represents a parser token.
cannam@48:   This type is naturally a union: it may be a keyword, identifier, numeric literal, quoted string,
cannam@48:   etc.  So the author defines it as a union, and the type is used widely.  Later on, the developer
cannam@48:   wants to attach information to the token indicating its line and column number in the source
cannam@48:   file.  Unfortunately, this is impossible without updating all users of the type, because the new
cannam@48:   information ought to apply to _all_ token instances, not just specific members of the union.  On
cannam@48:   the other hand, if unions must be embedded within structs, it is always possible to add new
cannam@48:   fields to the struct later on.
cannam@48: 
cannam@48: * When evolving a protocol it is common to discover that some existing field really should have
cannam@48:   been enclosed in a union, because new fields being added are mutually exclusive with it.  With
cannam@48:   Cap'n Proto's unions, it is actually possible to "retroactively unionize" such a field without
cannam@48:   changing its layout.  This allows you to continue being able to read old data without wasting
cannam@48:   space when writing new data.  This is only possible when unions are declared within their
cannam@48:   containing struct.
cannam@48: 
cannam@48: Cap'n Proto's unconventional approach to unions provides these advantages without any real down
cannam@48: side:  where you would conventionally define a free-standing union type, in Cap'n Proto you
cannam@48: may simply define a struct type that contains only that union (probably unnamed), and you have
cannam@48: achieved the same effect.  Thus, aside from being slightly unintuitive, it is strictly superior.
cannam@48: 
cannam@48: ### Groups
cannam@48: 
cannam@48: A group is a set of fields that are encapsulated in their own scope.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: struct Person {
cannam@48:   # ...
cannam@48: 
cannam@48:   # Note:  This is a terrible way to use groups, and meant
cannam@48:   #   only to demonstrate the syntax.
cannam@48:   address :group {
cannam@48:     houseNumber @8 :UInt32;
cannam@48:     street @9 :Text;
cannam@48:     city @10 :Text;
cannam@48:     country @11 :Text;
cannam@48:   }
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Interface-wise, the above group behaves as if you had defined a nested struct called `Address` and
cannam@48: then a field `address :Address`.  However, a group is _not_ a separate object from its containing
cannam@48: struct: the fields are numbered in the same space as the containing struct's fields, and are laid
cannam@48: out exactly the same as if they hadn't been grouped at all.  Essentially, a group is just a
cannam@48: namespace.
cannam@48: 
cannam@48: Groups on their own (as in the above example) are useless, almost as much so as the `Void` type.
cannam@48: They become interesting when used together with unions.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: struct Shape {
cannam@48:   area @0 :Float64;
cannam@48: 
cannam@48:   union {
cannam@48:     circle :group {
cannam@48:       radius @1 :Float64;
cannam@48:     }
cannam@48:     rectangle :group {
cannam@48:       width @2 :Float64;
cannam@48:       height @3 :Float64;
cannam@48:     }
cannam@48:   }
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: There are two main reason to use groups with unions:
cannam@48: 
cannam@48: 1. They are often more self-documenting.  Notice that `radius` is now a member of `circle`, so
cannam@48:    we don't need a comment to explain that the value of `circle` is its radius.
cannam@48: 2. You can add additional members later on, without breaking compatibility.  Notice how we upgraded
cannam@48:    `square` to `rectangle` above, adding a `height` field.  This definition is actually
cannam@48:    wire-compatible with the previous version of the `Shape` example from the "union" section
cannam@48:    (aside from the fact that `height` will always be zero when reading old data -- hey, it's not
cannam@48:    a perfect example).  In real-world use, it is common to realize after the fact that you need to
cannam@48:    add some information to a struct that only applies when one particular union field is set.
cannam@48:    Without the ability to upgrade to a group, you would have to define the new field separately,
cannam@48:    and have it waste space when not relevant.
cannam@48: 
cannam@48: Note that a named union is actually exactly equivalent to a named group containing an unnamed
cannam@48: union.
cannam@48: 
cannam@48: **Wait, weren't groups considered a misfeature in Protobufs?  Why did you do this again?**
cannam@48: 
cannam@48: They are useful in unions, which Protobufs did not have.  Meanwhile, you cannot have a "repeated
cannam@48: group" in Cap'n Proto, which was the case that got into the most trouble with Protobufs.
cannam@48: 
cannam@48: ### Dynamically-typed Fields
cannam@48: 
cannam@48: A struct may have a field with type `AnyPointer`.  This field's value can be of any pointer type --
cannam@48: i.e. any struct, interface, list, or blob.  This is essentially like a `void*` in C.
cannam@48: 
cannam@48: See also [generics](#generic-types).
cannam@48: 
cannam@48: ### Enums
cannam@48: 
cannam@48: An enum is a type with a small finite set of symbolic values.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: enum Rfc3092Variable {
cannam@48:   foo @0;
cannam@48:   bar @1;
cannam@48:   baz @2;
cannam@48:   qux @3;
cannam@48:   # ...
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Like fields, enumerants must be numbered sequentially starting from zero. In languages where
cannam@48: enums have numeric values, these numbers will be used, but in general Cap'n Proto enums should not
cannam@48: be considered numeric.
cannam@48: 
cannam@48: ### Interfaces
cannam@48: 
cannam@48: An interface has a collection of methods, each of which takes some parameters and return some
cannam@48: results.  Like struct fields, methods are numbered.  Interfaces support inheritance, including
cannam@48: multiple inheritance.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: interface Node {
cannam@48:   isDirectory @0 () -> (result :Bool);
cannam@48: }
cannam@48: 
cannam@48: interface Directory extends(Node) {
cannam@48:   list @0 () -> (list :List(Entry));
cannam@48:   struct Entry {
cannam@48:     name @0 :Text;
cannam@48:     node @1 :Node;
cannam@48:   }
cannam@48: 
cannam@48:   create @1 (name :Text) -> (file :File);
cannam@48:   mkdir @2 (name :Text) -> (directory :Directory);
cannam@48:   open @3 (name :Text) -> (node :Node);
cannam@48:   delete @4 (name :Text);
cannam@48:   link @5 (name :Text, node :Node);
cannam@48: }
cannam@48: 
cannam@48: interface File extends(Node) {
cannam@48:   size @0 () -> (size :UInt64);
cannam@48:   read @1 (startAt :UInt64 = 0, amount :UInt64 = 0xffffffffffffffff)
cannam@48:        -> (data :Data);
cannam@48:   # Default params = read entire file.
cannam@48: 
cannam@48:   write @2 (startAt :UInt64, data :Data);
cannam@48:   truncate @3 (size :UInt64);
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Notice something interesting here: `Node`, `Directory`, and `File` are interfaces, but several
cannam@48: methods take these types as parameters or return them as results.  `Directory.Entry` is a struct,
cannam@48: but it contains a `Node`, which is an interface.  Structs (and primitive types) are passed over RPC
cannam@48: by value, but interfaces are passed by reference. So when `Directory.list` is called remotely, the
cannam@48: content of a `List(Entry)` (including the text of each `name`) is transmitted back, but for the
cannam@48: `node` field, only a reference to some remote `Node` object is sent.
cannam@48: 
cannam@48: When an address of an object is transmitted, the RPC system automatically manages making sure that
cannam@48: the recipient gets permission to call the addressed object -- because if the recipient wasn't
cannam@48: meant to have access, the sender shouldn't have sent the reference in the first place. This makes
cannam@48: it very easy to develop secure protocols with Cap'n Proto -- you almost don't need to think about
cannam@48: access control at all. This feature is what makes Cap'n Proto a "capability-based" RPC system -- a
cannam@48: reference to an object inherently represents a "capability" to access it.
cannam@48: 
cannam@48: ### Generic Types
cannam@48: 
cannam@48: A struct or interface type may be parameterized, making it "generic". For example, this is useful
cannam@48: for defining type-safe containers:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: struct Map(Key, Value) {
cannam@48:   entries @0 :List(Entry);
cannam@48:   struct Entry {
cannam@48:     key @0 :Key;
cannam@48:     value @1 :Value;
cannam@48:   }
cannam@48: }
cannam@48: 
cannam@48: struct People {
cannam@48:   byName @0 :Map(Text, Person);
cannam@48:   # Maps names to Person instances.
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Cap'n Proto generics work very similarly to Java generics or C++ templates. Some notes:
cannam@48: 
cannam@48: * Only pointer types (structs, lists, blobs, and interfaces) can be used as generic parameters,
cannam@48:   much like in Java. This is a pragmatic limitation: allowing parameters to have non-pointer types
cannam@48:   would mean that different parameterizations of a struct could have completely different layouts,
cannam@48:   which would excessively complicate the Cap'n Proto implementation.
cannam@48: 
cannam@48: * A type declaration nested inside a generic type may use the type parameters of the outer type,
cannam@48:   as you can see in the example above. This differs from Java, but matches C++. If you want to
cannam@48:   refer to a nested type from outside the outer type, you must specify the parameters on the outer
cannam@48:   type, not the inner. For example, `Map(Text, Person).Entry` is a valid type;
cannam@48:   `Map.Entry(Text, Person)` is NOT valid. (Of course, an inner type may declare additional generic
cannam@48:   parameters.)
cannam@48: 
cannam@48: * If you refer to a generic type but omit its parameters (e.g. declare a field of type `Map` rather
cannam@48:   than `Map(T, U)`), it is as if you specified `AnyPointer` for each parameter. Note that such
cannam@48:   a type is wire-compatible with any specific parameterization, so long as you interpret the
cannam@48:   `AnyPointer`s as the correct type at runtime.
cannam@48: 
cannam@48: * Relatedly, it is safe to cast an generic interface of a specific parameterization to a generic
cannam@48:   interface where all parameters are `AnyPointer` and vice versa, as long as the `AnyPointer`s are
cannam@48:   treated as the correct type at runtime. This means that e.g. you can implement a server in a
cannam@48:   generic way that is correct for all parameterizations but call it from clients using a specific
cannam@48:   parameterization.
cannam@48: 
cannam@48: * The encoding of a generic type is exactly the same as the encoding of a type produced by
cannam@48:   substituting the type parameters manually. For example, `Map(Text, Person)` is encoded exactly
cannam@48:   the same as:
cannam@48: 
cannam@48:   <div>{% highlight capnp %}
cannam@48:   struct PersonMap {
cannam@48:     # Encoded the same as Map(Text, Person).
cannam@48:     entries @0 :List(Entry);
cannam@48:     struct Entry {
cannam@48:       key @0 :Text;
cannam@48:       value @1 :Person;
cannam@48:     }
cannam@48:   }
cannam@48:   {% endhighlight %}
cannam@48:   </div>
cannam@48: 
cannam@48:   Therefore, it is possible to upgrade non-generic types to generic types while retaining
cannam@48:   backwards-compatibility.
cannam@48: 
cannam@48: * Similarly, a generic interface's protocol is exactly the same as the interface obtained by
cannam@48:   manually substituting the generic parameters.
cannam@48: 
cannam@48: ### Generic Methods
cannam@48: 
cannam@48: Interface methods may also have "implicit" generic parameters that apply to a particular method
cannam@48: call. This commonly applies to "factory" methods. For example:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: interface Assignable(T) {
cannam@48:   # A generic interface, with non-generic methods.
cannam@48:   get @0 () -> (value :T);
cannam@48:   set @1 (value :T) -> ();
cannam@48: }
cannam@48: 
cannam@48: interface AssignableFactory {
cannam@48:   newAssignable @0 [T] (initialValue :T)
cannam@48:       -> (assignable :Assignable(T));
cannam@48:   # A generic method.
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Here, the method `newAssignable()` is generic. The return type of the method depends on the input
cannam@48: type.
cannam@48: 
cannam@48: Ideally, calls to a generic method should not have to explicitly specify the method's type
cannam@48: parameters, because they should be inferred from the types of the method's regular parameters.
cannam@48: However, this may not always be possible; it depends on the programming language and API details.
cannam@48: 
cannam@48: Note that if a method's generic parameter is used only in its returns, not its parameters, then
cannam@48: this implies that the returned value is appropriate for any parameterization. For example:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: newUnsetAssignable @1 [T] () -> (assignable :Assignable(T));
cannam@48: # Create a new assignable. `get()` on the returned object will
cannam@48: # throw an exception until `set()` has been called at least once.
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Because of the way this method is designed, the returned `Assignable` is initially valid for any
cannam@48: `T`. Effectively, it doesn't take on a type until the first time `set()` is called, and then `T`
cannam@48: retroactively becomes the type of value passed to `set()`.
cannam@48: 
cannam@48: In contrast, if it's the case that the returned type is unknown, then you should NOT declare it
cannam@48: as generic. Instead, use `AnyPointer`, or omit a type's parameters (since they default to
cannam@48: `AnyPointer`). For example:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: getNamedAssignable @2 (name :Text) -> (assignable :Assignable);
cannam@48: # Get the `Assignable` with the given name. It is the
cannam@48: # responsibility of the caller to keep track of the type of each
cannam@48: # named `Assignable` and cast the returned object appropriately.
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Here, we omitted the parameters to `Assignable` in the return type, because the returned object
cannam@48: has a specific type parameterization but it is not locally knowable.
cannam@48: 
cannam@48: ### Constants
cannam@48: 
cannam@48: You can define constants in Cap'n Proto.  These don't affect what is sent on the wire, but they
cannam@48: will be included in the generated code, and can be [evaluated using the `capnp`
cannam@48: tool](capnp-tool.html#evaluating-constants).
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: const pi :Float32 = 3.14159;
cannam@48: const bob :Person = (name = "Bob", email = "bob@example.com");
cannam@48: const secret :Data = 0x"9f98739c2b53835e 6720a00907abd42f";
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Additionally, you may refer to a constant inside another value (e.g. another constant, or a default
cannam@48: value of a field).
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: const foo :Int32 = 123;
cannam@48: const bar :Text = "Hello";
cannam@48: const baz :SomeStruct = (id = .foo, message = .bar);
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Note that when substituting a constant into another value, the constant's name must be qualified
cannam@48: with its scope.  E.g. if a constant `qux` is declared nested in a type `Corge`, it would need to
cannam@48: be referenced as `Corge.qux` rather than just `qux`, even when used within the `Corge` scope.
cannam@48: Constants declared at the top-level scope are prefixed just with `.`.  This rule helps to make it
cannam@48: clear that the name refers to a user-defined constant, rather than a literal value (like `true` or
cannam@48: `inf`) or an enum value.
cannam@48: 
cannam@48: ### Nesting, Scope, and Aliases
cannam@48: 
cannam@48: You can nest constant, alias, and type definitions inside structs and interfaces (but not enums).
cannam@48: This has no effect on any definition involved except to define the scope of its name. So in Java
cannam@48: terms, inner classes are always "static". To name a nested type from another scope, separate the
cannam@48: path with `.`s.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: struct Foo {
cannam@48:   struct Bar {
cannam@48:     #...
cannam@48:   }
cannam@48:   bar @0 :Bar;
cannam@48: }
cannam@48: 
cannam@48: struct Baz {
cannam@48:   bar @0 :Foo.Bar;
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: If typing long scopes becomes cumbersome, you can use `using` to declare an alias.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: struct Qux {
cannam@48:   using Foo.Bar;
cannam@48:   bar @0 :Bar;
cannam@48: }
cannam@48: 
cannam@48: struct Corge {
cannam@48:   using T = Foo.Bar;
cannam@48:   bar @0 :T;
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: ### Imports
cannam@48: 
cannam@48: An `import` expression names the scope of some other file:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: struct Foo {
cannam@48:   # Use type "Baz" defined in bar.capnp.
cannam@48:   baz @0 :import "bar.capnp".Baz;
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Of course, typically it's more readable to define an alias:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: using Bar = import "bar.capnp";
cannam@48: 
cannam@48: struct Foo {
cannam@48:   # Use type "Baz" defined in bar.capnp.
cannam@48:   baz @0 :Bar.Baz;
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: Or even:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: using import "bar.capnp".Baz;
cannam@48: 
cannam@48: struct Foo {
cannam@48:   baz @0 :Baz;
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: The above imports specify relative paths.  If the path begins with a `/`, it is absolute -- in
cannam@48: this case, the `capnp` tool searches for the file in each of the search path directories specified
cannam@48: with `-I`.
cannam@48: 
cannam@48: ### Annotations
cannam@48: 
cannam@48: Sometimes you want to attach extra information to parts of your protocol that isn't part of the
cannam@48: Cap'n Proto language.  This information might control details of a particular code generator, or
cannam@48: you might even read it at run time to assist in some kind of dynamic message processing.  For
cannam@48: example, you might create a field annotation which means "hide from the public", and when you send
cannam@48: a message to an external user, you might invoke some code first that iterates over your message and
cannam@48: removes all of these hidden fields.
cannam@48: 
cannam@48: You may declare annotations and use them like so:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: # Declare an annotation 'foo' which applies to struct and enum types.
cannam@48: annotation foo(struct, enum) :Text;
cannam@48: 
cannam@48: # Apply 'foo' to to MyType.
cannam@48: struct MyType $foo("bar") {
cannam@48:   # ...
cannam@48: }
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: The possible targets for an annotation are: `file`, `struct`, `field`, `union`, `enum`, `enumerant`,
cannam@48: `interface`, `method`, `parameter`, `annotation`, `const`.  You may also specify `*` to cover them
cannam@48: all.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: # 'baz' can annotate anything!
cannam@48: annotation baz(*) :Int32;
cannam@48: 
cannam@48: $baz(1);  # Annotate the file.
cannam@48: 
cannam@48: struct MyStruct $baz(2) {
cannam@48:   myField @0 :Text = "default" $baz(3);
cannam@48:   myUnion :union $baz(4) {
cannam@48:     # ...
cannam@48:   }
cannam@48: }
cannam@48: 
cannam@48: enum MyEnum $baz(5) {
cannam@48:   myEnumerant @0 $baz(6);
cannam@48: }
cannam@48: 
cannam@48: interface MyInterface $baz(7) {
cannam@48:   myMethod @0 (myParam :Text $baz(9)) -> () $baz(8);
cannam@48: }
cannam@48: 
cannam@48: annotation myAnnotation(struct) :Int32 $baz(10);
cannam@48: const myConst :Int32 = 123 $baz(11);
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: `Void` annotations can omit the value.  Struct-typed annotations are also allowed.  Tip:  If
cannam@48: you want an annotation to have a default value, declare it as a struct with a single field with
cannam@48: a default value.
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: annotation qux(struct, field) :Void;
cannam@48: 
cannam@48: struct MyStruct $qux {
cannam@48:   string @0 :Text $qux;
cannam@48:   number @1 :Int32 $qux;
cannam@48: }
cannam@48: 
cannam@48: annotation corge(file) :MyStruct;
cannam@48: 
cannam@48: $corge(string = "hello", number = 123);
cannam@48: 
cannam@48: struct Grault {
cannam@48:   value @0 :Int32 = 123;
cannam@48: }
cannam@48: 
cannam@48: annotation grault(file) :Grault;
cannam@48: 
cannam@48: $grault();  # value defaults to 123
cannam@48: $grault(value = 456);
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: ### Unique IDs
cannam@48: 
cannam@48: A Cap'n Proto file must have a unique 64-bit ID, and each type and annotation defined therein may
cannam@48: also have an ID.  Use `capnp id` to generate a new ID randomly.  ID specifications begin with `@`:
cannam@48: 
cannam@48: {% highlight capnp %}
cannam@48: # file ID
cannam@48: @0xdbb9ad1f14bf0b36;
cannam@48: 
cannam@48: struct Foo @0x8db435604d0d3723 {
cannam@48:   # ...
cannam@48: }
cannam@48: 
cannam@48: enum Bar @0xb400f69b5334aab3 {
cannam@48:   # ...
cannam@48: }
cannam@48: 
cannam@48: interface Baz @0xf7141baba3c12691 {
cannam@48:   # ...
cannam@48: }
cannam@48: 
cannam@48: annotation qux @0xf8a1bedf44c89f00 (field) :Text;
cannam@48: {% endhighlight %}
cannam@48: 
cannam@48: If you omit the ID for a type or annotation, one will be assigned automatically.  This default
cannam@48: ID is derived by taking the first 8 bytes of the MD5 hash of the parent scope's ID concatenated
cannam@48: with the declaration's name (where the "parent scope" is the file for top-level declarations, or
cannam@48: the outer type for nested declarations).  You can see the automatically-generated IDs by "compiling"
cannam@48: your file with the `-ocapnp` flag, which echos the schema back to the terminal annotated with
cannam@48: extra information, e.g. `capnp compile -ocapnp myschema.capnp`.  In general, you would only specify
cannam@48: an explicit ID for a declaration if that declaration has been renamed or moved and you want the ID
cannam@48: to stay the same for backwards-compatibility.
cannam@48: 
cannam@48: IDs exist to provide a relatively short yet unambiguous way to refer to a type or annotation from
cannam@48: another context.  They may be used for representing schemas, for tagging dynamically-typed fields,
cannam@48: etc.  Most languages prefer instead to define a symbolic global namespace e.g. full of "packages",
cannam@48: but this would have some important disadvantages in the context of Cap'n Proto:
cannam@48: 
cannam@48: * Programmers often feel the need to change symbolic names and organization in order to make their
cannam@48:   code cleaner, but the renamed code should still work with existing encoded data.
cannam@48: * It's easy for symbolic names to collide, and these collisions could be hard to detect in a large
cannam@48:   distributed system with many different binaries using different versions of protocols.
cannam@48: * Fully-qualified type names may be large and waste space when transmitted on the wire.
cannam@48: 
cannam@48: Note that IDs are 64-bit (actually, 63-bit, as the first bit is always 1).  Random collisions
cannam@48: are possible, but unlikely -- there would have to be on the order of a billion types before this
cannam@48: becomes a real concern.  Collisions from misuse (e.g. copying an example without changing the ID)
cannam@48: are much more likely.
cannam@48: 
cannam@48: ## Evolving Your Protocol
cannam@48: 
cannam@48: A protocol can be changed in the following ways without breaking backwards-compatibility, and
cannam@48: without changing the [canonical](encoding.html#canonicalization) encoding of a message:
cannam@48: 
cannam@48: * New types, constants, and aliases can be added anywhere, since they obviously don't affect the
cannam@48:   encoding of any existing type.
cannam@48: 
cannam@48: * New fields, enumerants, and methods may be added to structs, enums, and interfaces, respectively,
cannam@48:   as long as each new member's number is larger than all previous members.  Similarly, new fields
cannam@48:   may be added to existing groups and unions.
cannam@48: 
cannam@48: * New parameters may be added to a method.  The new parameters must be added to the end of the
cannam@48:   parameter list and must have default values.
cannam@48: 
cannam@48: * Members can be re-arranged in the source code, so long as their numbers stay the same.
cannam@48: 
cannam@48: * Any symbolic name can be changed, as long as the type ID / ordinal numbers stay the same.  Note
cannam@48:   that type declarations have an implicit ID generated based on their name and parent's ID, but
cannam@48:   you can use `capnp compile -ocapnp myschema.capnp` to find out what that number is, and then
cannam@48:   declare it explicitly after your rename.
cannam@48: 
cannam@48: * Type definitions can be moved to different scopes, as long as the type ID is declared
cannam@48:   explicitly.
cannam@48: 
cannam@48: * A field can be moved into a group or a union, as long as the group/union and all other fields
cannam@48:   within it are new.  In other words, a field can be replaced with a group or union containing an
cannam@48:   equivalent field and some new fields.
cannam@48: 
cannam@48: * A non-generic type can be made [generic](#generic-types), and new generic parameters may be
cannam@48:   added to an existing generic type. Other types used inside the body of the newly-generic type can
cannam@48:   be replaced with the new generic parameter so long as all existing users of the type are updated
cannam@48:   to bind that generic parameter to the type it replaced. For example:
cannam@48: 
cannam@48:   <div>{% highlight capnp %}
cannam@48:   struct Map {
cannam@48:     entries @0 :List(Entry);
cannam@48:     struct Entry {
cannam@48:       key @0 :Text;
cannam@48:       value @1 :Text;
cannam@48:     }
cannam@48:   }
cannam@48:   {% endhighlight %}
cannam@48:   </div>
cannam@48: 
cannam@48:   Can change to:
cannam@48: 
cannam@48:   <div>{% highlight capnp %}
cannam@48:   struct Map(Key, Value) {
cannam@48:     entries @0 :List(Entry);
cannam@48:     struct Entry {
cannam@48:       key @0 :Key;
cannam@48:       value @1 :Value;
cannam@48:     }
cannam@48:   }
cannam@48:   {% endhighlight %}
cannam@48:   </div>
cannam@48: 
cannam@48:   As long as all existing uses of `Map` are replaced with `Map(Text, Text)` (and any uses of
cannam@48:   `Map.Entry` are replaced with `Map(Text, Text).Entry`).
cannam@48: 
cannam@48:   (This rule applies analogously to generic methods.)
cannam@48: 
cannam@48: The following changes are backwards-compatible but may change the canonical encoding of a message.
cannam@48: Apps that rely on canonicalization (such as some cryptographic protocols) should avoid changes in
cannam@48: this list, but most apps can safely use them:
cannam@48: 
cannam@48: * A field of type `List(T)`, where `T` is a primitive type, blob, or list, may be changed to type
cannam@48:   `List(U)`, where `U` is a struct type whose `@0` field is of type `T`.  This rule is useful when
cannam@48:   you realize too late that you need to attach some extra data to each element of your list.
cannam@48:   Without this rule, you would be stuck defining parallel lists, which are ugly and error-prone.
cannam@48:   As a special exception to this rule, `List(Bool)` may **not** be upgraded to a list of structs,
cannam@48:   because implementing this for bit lists has proven unreasonably expensive.
cannam@48: 
cannam@48: Any change not listed above should be assumed NOT to be safe.  In particular:
cannam@48: 
cannam@48: * You cannot change a field, method, or enumerant's number.
cannam@48: * You cannot change a field or method parameter's type or default value.
cannam@48: * You cannot change a type's ID.
cannam@48: * You cannot change the name of a type that doesn't have an explicit ID, as the implicit ID is
cannam@48:   generated based in part on the type name.
cannam@48: * You cannot move a type to a different scope or file unless it has an explicit ID, as the implicit
cannam@48:   ID is based in part on the scope's ID.
cannam@48: * You cannot move an existing field into or out of an existing union, nor can you form a new union
cannam@48:   containing more than one existing field.
cannam@48: 
cannam@48: Also, these rules only apply to the Cap'n Proto native encoding.  It is sometimes useful to
cannam@48: transcode Cap'n Proto types to other formats, like JSON, which may have different rules (e.g.,
cannam@48: field names cannot change in JSON).