RFC-0050: FIDL Syntax Revamp

RFC-0050: FIDL Syntax Revamp
  • FIDL

We establish guiding principles for syntactic choices, and make a few syntax changes following these principles.

Date submitted (year-month-day)2020-02-26
Date reviewed (year-month-day)2019-06-04


We establish guiding principles for syntactic choices, and make a few syntax changes following these principles.


  • Placing types in second position, e.g. in method parameter names precede their respective types, in table declarations member names precede their respective types;
  • Changing types to separate layout from constraints to place layout related type information on the left hand side of the : separator with constraints information on the right hand side, e.g. array<T, 5> vs vector<T>:5 more clearly conveys that an array's size is layout impacting, whereas it is a constraint for vectors.
  • Introduction of anonymous layouts. For instance table { f1 uint8; f2 uint16; } can be used directly within a method parameter list.
  • Declaration of top-level types is done by using anonymous layouts with the help of the type introduction declarations of the form type Name = Layout;.
  • Lastly, for a protocol P, renaming P and request<P> to client_end:P and server_end:P respectively. Note that the protocol is a constraint of the client or server end, rather than the previous position which would incorrectly indicate a layout related concern.

Relation to other RFCs

This RFC was later amended by:


Introductory Examples

Algebraic Data Types

The syntax is versatile enough that representing algebraic data types (ADTs) can be done fluently, without requiring any more sugar. Consider for instance:

/// Describes simple algebraic expressions.
type Expression = flexible union {
    1: value int64;
    2: bin_op struct {
        op flexible enum {
            ADD = 1;
            MUL = 2;
            DIV = 3;
        left_exp Expression;
        right_exp Expression;
    3: un_op struct {
        op flexible enum {
            NEG = 1;
        exp Expression;

Pattern wise, we choose to use a union of struct: the union offers extensibility, and it is therefore not needed (and preferable) to use a more rigid variant. Should we need to change a variant, we can instead add a new one wholesale, and migrate to using this new variant. (In other places where evolvability is needed, e.g. the list of binary or unary operators, a flexible enum is chosen.)

Supporting ADTs requires much more than ergonomic syntax to describe data types. One of the key features expected, for instance, is easy construction and destruction (e.g. via pattern matching or visitor pattern).

This RFC does not introduce new functionality to FIDL, and limitations on recursive types would prevent the example to compile today. We plan to add support for generalized recursive types, and this extension will be the object of a future RFC.

Combining non-evolvable messages with evolvable messages more easily

For instance, expressing an "extensible struct" which has both struct elements (compact, inline, fast encoding/decoding), as well as the possibility to be extended:

type Something = struct {

    /// Provide extension point, initially empty.
    extension table {};

As an example, the fuchsia.test.breakpoints library needs to define an extensible event dubbed Invocation. These events all share common values, as well as specific payload for each variant of the event. This can now be more directly and succinctly expressed as:

type Invocation = table {
    1: target_moniker string:MAX_MONIKER_LENGTH;
    2: handler Handler;
    3: payload InvocationPayload;

type InvocationPayload = union {
    1: start_instance struct{};
    2: routing table {
        1: protocol RoutingProtocol;
        2: capability_id string:MAX_CAPABILITY_ID_LENGTH;
        3: source CapabilitySource;

Extensible method arguments

For instance, extensible method arguments:

protocol Peripheral {
    StartAdvertising(table {
        1: data AdvertisingData;
        2: scan_response AdvertisingData;
        3: mode_hint AdvertisingModeHint;
        4: connectable bool;
        5: handle server_end:AdvertisingHandle;
    }) -> () error PeripheralError;

Using a table for arguments is not a "best practice". It may be appropriate, but comes with its set of issues, e.g. 2N possibilities with N fields, possibly adding a lot of complexity on recipients.

Guiding principles

FIDL is primarily concerned with defining Application Binary Interface (ABI) concerns, and second with Application Programming Interface (API) concerns. This can result in a syntax that is more verbose than one may be accustomed to, or may be expecting when comparing to other programming languages. For instance, a unit variant of a union would be expressed as an empty struct as can be seen in the InvocationPayload example above. We could choose to introduce syntactic sugar to elide this type, but that would go against making ABI concerns front-and-center.

Separating Layouts from Constraints

Align on the syntax


For types, i.e. anything that controls layout is before the colon, anything that controls constraint is after the colon. The layout describes how the bytes are laid out, vs how they are interpreted. The constraint restricts what can be represented given the layout, it is a validation step done during encoding/decoding.

This syntax provides a simplified way to consider ABI implications of a change, and in particular leads to two shorthand rules:

  1. If two types have a different layout, it is not possible to soft transition from one to the other, and vice versa 1, i.e. changing the left hand side breaks ABI
  2. Constraints can evolve, and as long as writers are more constrained than readers, things are compatible, i.e. it is possible to evolve the right hand side and preserve ABI

Here are example changes following this principle:

  • array<T>:N becomes array<T, N>
  • handle<K> becomes handle:K
  • vector<T>? becomes vector<T>:optional
  • Struct? becomes box<Struct>
  • Table? becomes Table:optional
  • Union? becomes Union:optional

Those changes are discussed in the design section of this RFC.

Binary wire format first

While many formats can represent FIDL messages, the FIDL Wire Format (or "FIDL Binary Wire Format") is the one which has preferential treatment, and is catered to first.

This means that syntax choices meant to align syntax consistency with ABI consistency should consider ABI under the binary wire format (and not, say, other formats like JSON).

As an example, names do not matter when it comes to types' ABI — names do matter for protocols and methods. While names might matter for a possible JSON format we choose to over rotate towards the binary ABI format when making syntax choices, and would not alter the syntax to advantage a textual representation if it hinders the understanding of ABI rules.

Fewest features

Wright's "form and function should be one" makes us strive for similar looking constructs to have similar looking meaning, and vice versa. As an example, all extensible data, which internally leverage envelopes, are always presented with ordinal:.

layout {
    ordinal: name type;

We strive to have the fewest features and rules, and aim to combine features to achieve use cases. In practice, when considering new features, we should first try to adapt or generalize other existing features rather than introduce new features. As an example, while special syntax can be designed for extensible method arguments (and returns) as discussed in RFC-0044: Extensible Method Arguments we prefer leveraging table and the normal syntax for those.

One could argue that we should even require anonymous struct layouts for methods requests and responses rather than the current syntactic sugar for arguments borrowed from most programming languages. However, a competing design consideration is to help library authors in aggregate achieve consistency: in enum layout declaration, we prefer syntactic sugar over explicitly choosing a wrapped type, as having a sensible default provides greater consistency for enums across FIDL libraries. This in turn provides a migration path to switch enums down the road, e.g. should a library define a general purpose ErrorStatus enum, it could be replaced later by another 'better' general purpose ErrorStatusV2.



Types follow the general form:

Name<Param1, Param2, ...>:<Constraint1, Constraint2, ...>

Empty type parameterization must omit < and >, i.e. uint32 (notuint32<>).

A type with no constraints must omit both the : separator, and <, >, i.e. uint32 (not uint32:<>, nor uint32:).

A type with a single constraint may omit < and >, i.e.vector<uint32>:5 and vector<uint32>:<5> are both allowed, and equivalent.

Built Ins

The following primitive types are supported:

  • Boolean bool
  • Signed integer int8, int16, int32, int64
  • Unsigned integer uint8, uint16, uint32, uint64
  • IEEE 754 Floating-point float32, float64

Fixed sized repeated values:

array<T, N>

Which can be thought of as a struct with N elements of type T.

Variable sized repeated values:


i.e. the size N can be omitted.

Variable sized UTF-8 strings:


i.e. the size N can be omitted.

References to kernel objects, i.e. handles:


Where the subtype S is one of bti, buffer, channel, debuglog, event, eventpair, exception, fifo, guest, interrupt, iommu, job, pager, pcidevice, pmt, port, process, profile, resource, socket, suspendtoken, thread, timer, vcpu, vmar, vmo.

Handles with rights introduced in RFC-0028: Handle Rights:

handle:<S, R>

Where the rights R are either a rights value, or a rights expression.

References to protocols objects, i.e. channel handles of targeted use:


i.e. client_end:fuchsia.media.AudioCore or server_end:fuchsia.ui.scenic.Session.

Specifically, it is not legal to reference a protocol by itself: protocol declarations do not introduce a type, only what can be thought of as a kind of client or server ends. This is discussed at greater length in the Transport generalization section.


In addition to the built in layouts, we have five layouts which can be configured to introduce new types:

  • enum
  • bits
  • struct
  • table
  • union

Finite layout

Both enum and bits layout are expressed in similar ways:

layout : WrappedType {
    MEMBER = expression;

Where the : WrappedType is optional[^2], and defaults to uint32 if omitted.

An example enum:

enum {
    OTHER = 1;
    AUDIO = 2;
    VIDEO = 3;

An example bits:

bits : uint64 {
    TOTAL_BYTES = 0x1;
    USED_BYTES  = 0x2;
    TOTAL_NODES = 0x4;

Flexible layouts

Both table and union layouts are expressed in similar ways:

layout {
    ordinal: member_name type;

Here, the ordinal: can be thought of as syntactic sugar to describe an envelope<type>.

For tables, members are often referred to as fields. For unions, members are often referred to as variants. Additionally, members may be reserved:

layout {
    ordinal: reserved;

Rigid layouts

The only rigid layout struct is expressed in a way that is close to flexible layouts, without the flexible notation:

layout {
    member_name type;

For structs, members are often referred to as fields.


A layout may be preceded by attributes for that layout:

[MaxBytes = "64"] struct {
    x uint32;
    y uint32;

This makes it possible to unambiguously attach attributes to both the member of a layout, and the type of that member:

table {
    [OnMember = "origin"]
    1: origin [OnLayout] struct {
        x uint32;
        y uint32;

In the case of the introduction of a new type that is a layout, there are two possible placements for attributes on the newly introduced type:

  • On the new type: [Attr] type MyStruct = struct { ... }.
  • On the layout: type MyStruct = [Attr] struct { ... }.

fidlc will consider these equivalent, and raise an error if attributes are specified in both places.

Regardless of which placement is used to specify the attributes, the attributes are conceptually attached to the layout itself rather than the type stanza as a whole. An example of a practical application of this is that in any IR the preference would be to lower attributes on the type stanza down to the layout rather than hoist the attributes on the layout up to the type stanza.

Naming context and use of layouts

Layouts themselves do not carry names, in a way all layouts are "anonymous". Instead, it is a specific use of a layout which determines the name it will have in the target language.

For instance, the most common use of layouts is to introduce a new top-level type:

library fuchsia.mem;

type Buffer = struct {
    vmo handle:vmo;
    size uint64;

Here, the struct layout is used in a "new type" declaration within the top-level library.

An example use in an anonymous context was covered in the introductory notes to express extensible method arguments:

library fuchsia.bluetooth.le;

protocol Peripheral {
    StartAdvertising(table {
        1: data AdvertisingData;
        2: scan_response AdvertisingData;
        3: mode_hint AdvertisingModeHint;
        4: connectable bool;
        5: handle server_end:AdvertisingHandle;
    }) -> () error PeripheralError;

Here, the table layout is used within the request of the StartAdvertising method, in the Peripheral protocol declaration.

We refer to the list of names, from least specific to most specific, which identifies the use of a layout as its "naming context". In the two examples above, we have respectively fuchsia.mem/Buffer and fuchsia.bluetooth.le/Peripheral, StartAdvertising, request as the two naming contexts.

In the JSON IR, layout declarations will include their naming context, i.e. the hierarchical list of names described above.

Naming contexts

Within a library some.library, a type Name = declaration introduces a naming context for some.library/Name.

A use within a request (respectively a response) of a Method within Protocol introduces a naming context of some.library/Protocol, Method, request/response

A use within a layout adds the field name (or variant name) to the naming context. For instance:

type Outer = struct {
    inner struct {

The first outer struct layout's naming context is some.library/Outer, and the second inner struct layout's naming context is some.library/Outer, inner.

Generated flattened name

Many target languages can represent naming context hierarchically. In C++ for instance, a type can be defined within an enclosing type. However, some target languages do not have this ability, and we must therefore consider name clashing caused by flattening naming contexts.

Consider for instance the naming context some.library/Protocol, Method, request. This may be flattened to some.library/MethodRequestOfProtocool in Go. If some other definition happens to use the naming context some.library/MethodRequestOfProtocool then the Go bindings are faced with a conundrum: one of the two declarations must be renamed. Worst, should a library with one declaration (no name clash) evolve into a library with the two declarations (with a name clash), then the Go bindings must be consistent with what was generated before in order to avoid a source breaking change.

Our experience has shown that these decisions are best left to the core FIDL compiler, rather than delegated down the toolchain to FIDL bindings. We will therefore compute and guarantee a stable flattened name.

In the JSON IR, naming contexts will include a generated flattened name which the compiler guarantees is unique in global scope, i.e. the frontend compiler is responsible for generating flattened names, and verifying that flattened names do not clash with other declarations (be it other flattened names, or top-level declarations).

Take the example before, should a library author add a declaration type MethodRequestOfProtocool = ... which clashes with the generated flattened name of another declaration, compilation will fail.

Use of naming contexts by bindings

Bindings can be split in roughly two categories:

  1. Ability to represent naming context scoping in the target language, e.g. bindings for the C++ language;
  2. Inability to represent naming context and fallback to the use of the generated flattened nuse flattened name, e.g. bindings for the Go language.

That's an improvement over the situation today because we'll at least be consistent between bindings, and have compiler help on the frontend. Today, we have to generate some of the names late in the game (in the backend), which is a hazardous and error prone approach.

For instance, consider the definition:

type BinOp = union {
    add struct {
        left uint32;
        right uint32;

In C++ bindings, we could end up:

class BinOp {
    class Add {

The accessor to the variant add would be:


which does not clash with the class definition.

Or in Go, with the use of flattened names:

type BinOp struct { ... };
type BinOpAdd struct { ... };

Should the library author later decide to introduce a top-level declaration named BinOpAdd, this would be caught by the frontend compiler and reported as an error. The library author is put in control to think through the ramifications of this change, and would have the option to decide to break source compatibility for the introduction of this new declaration. Again, this is an improvement over the current situation where such source compatibility breakages are discovered later, and farther from where the decision was made.

Type Aliasing, and New Type

In RFC-0052: Type Aliasing and New Types we evolved type aliasing and new type declarations.

Aliases are declared as:

alias NewName = AliasedType;

i.e. unchanged from syntax proposed in RFC-0052.

New types are declared as:

type NewType = WrappedType;

i.e. the syntax for new types is the same whether the wrapped type is another existing type (wrapping) or some layout (new top-level type). This differs from the initially proposed syntax in RFC-0052.


Certain types are inherently capable of being optional: vectors, strings, envelopes, and layouts using such constructs i.e. table which is a vector (of envelopes) and a union which is a tag plus an envelope. As a result, whether these types are optional or not is a constraint, and can be evolved into (becoming nullable, by relaxing the constraint), or evolved out of (becoming required, by tightening the constraint).

On the other hand, types such as int8 or struct layout are not inherently capable of being optional. In order to have optionality, one needs to introduce an indirection, for instance via an indirect reference in the struct case. As a result, unlike types which are inherently optional, no evolutionary path is possible.

To distinguish between these two cases, and following the principle of keeping ABI concerns "on the left" and evolvable concerns "on the right" have:

Naturally optional Not naturally optional
string:optional box<struct>

Naming wise, we prefer the terms "optional", "required", "present", "absent". (We should avoid "nullable", "not nullable", "null fields".) In line with that naming preference, we choose box<T> rather than pointer<T>. A box is an optional by default structure, i.e. box<struct> in the new syntax is equivalent to struct? in the old syntax, and box<struct>:optional is redundant and may trigger a warning from the compiler or linter. This is to better match the use case we expect: users generally box structs to get optionality rather than to add indirection.


Constants are declared as:

const NAME type = expression;

Constraint ordering

When parameterizing a type based on layouts and constraints, the ordering of these arguments is fixed for a given type. This RFC defines the following orders for constraints (no type has multiple layout arguments yet):

  • Handles: subtype, rights, optionality.
  • Protocol client/server_end: protocol, optionality.
  • Vector: size, optionality.
  • Unions: optionality.

As a guiding principle, optionality always comes last, and, for handles, subtype before rights.

As an example, consider this struct with all possible constraints defined on its members:

type Foo = struct {
  h1 zx.handle,
  h2 zx.handle:optional,
  h3 zx.handle:VMO,
  h4 zx.handle:<VMO,optional>,
  h5 zx.handle:<VMO,zx.READ>,
  h6 zx.handle:<VMO,zx.READ,optional>,
  p1 client_end:MyProtocol,
  p2 client_end:<MyProtocol,optional>,
  r1 server_end:P,
  r2 server_end:<MyProtocol,optional>,
  s1 MyStruct,
  s2 box<MyStruct>,
  u1 MyUnion,
  u2 MyUnion:optional,
  v1 vector<bool>,
  v2 vector<bool>:optional,
  v3 vector<bool>:16,
  v4 vector<bool>:<16,optional>,

Future Direction

In addition to changes to the syntax to features which currently exist, we look and set the direction for features which are expected to see the light of day in the near future. Here, the focus is on intended expressivity and it's syntactic rendering (not on the precise semantics, which warrants separate RFCs). For instance, while we describe transport generalization, we do not discuss various thorny design issues (e.g. extent of configurability, representation in JSON IR).

This section is also expected to be read as directional, and not as a future specification. As new features are introduced, their corresponding syntax will be evaluated along with the precise workings of those features.

Contextual name resolution


const A_OR_B MyBits = MyBits.A | MyBits.B;

Would be simplified to:

const A_OR_B MyBits = A | B;


zx.handle:<zx.VMO, zx.rights.READ_ONLY>

Would be simplified to:

zx.handle:<VMO, READ_ONLY>


Declaration site constraints

type CircleCoordinates = struct {
    x int32;
    y int32;
}:x^2 + y^2 < 100;

Use site constraints

type Small = struct {
    content fuchsia.mem.Buffer:vmo.size < 1024;

Standalone constraints

constraint Circular : Coordinates {
    x^2 + y^2 < 100

Constraints on envelopes

The syntax of tables and extensible unions hides the use of envelopes:

  • A table is a vector<envelope<...>>, and
  • A union is a struct { tag uint64; variant envelope<...>; }.

Right now, the ordinal: which appears in table and union declarations are the only places where envelopes exist, and it's useful to think of this syntax as the "sugared" introduction of an envelope. Essentially, we can de-sugar as follows:

Desugaring tables and flexible unions
table ExampleTable {
    1: name string;
    2: size uint32;
table ExampleTable {
    @1 name envelope;
    @2 size envelope;
union ExampleUnion {
    1: name string;
    2: size uint32;
union ExampleUnion {
    @1 name envelope;
    @2 size envelope;

Should we want to constrain the envelope, say to require an element, we would place this constraint on the ordinal ordinal:C such as:

Desugaring tables and flexible unions
table ExampleTable {
    1:C1 name string:C2;
    2:C size uint32;
table ExampleTable {
    @1 name envelope:C1;
    @2 size envelope:C;
union ExampleUnion {
    1:C1 name string:C2;
    2:C size uint32;
union ExampleUnion {
    @1 name envelope:C1;
    @2 size envelope:C;


FIDL's type system is already one which has the concept of constraints. We have vector<uint8>:8 to mean that a vector has at most 8 elements, or string:optional to relax the optionality constraint and allow the string to be optional.

Various needs are pushing towards both more expressive constraints, and an opinionated view of how these constraints are unified and handled.

For instance, fuchsia.mem/Buffer notes "This size must not be greater than the physical size of the VMO." Work is ongoing to introduce RFC-0028: Handle Rights, i.e. constraining handles. Or idea of requiring table fields, i.e. constraining the presence on otherwise optional envelopes.

Right now, there is no way to describe runtime properties of the values or entities being manipulated. While a string value has a size, it is not possible to name this. While a handle has rights associated with it, it is not possible to name these either.

To properly solve the expressivity problem associated with constrained types, we must first bridge the runtime aspects of values, with the limited view which FIDL has of these values. We plan to introduce **properties **which can be thought of as virtual fields attached to values. Properties have no impact on the wire format, they are purely a language level construct, and appear in the JSON IR for bindings to give runtime meaning to them. Properties exist for the sole purpose of expressing constraints over them. Each and every property would need to be known to bindings, in a similar fashion that built ins are known to bindings.

Continuing the example above, a string value may have a uint32 size property, a handle may have a zx.rights rights property.

For instance:

layout name {
    properties {
        size uint32;

Transport generalization

Declaring a new transport would at least require defining a new name, specifying constraints for the messages the transport supports (e.g. "no handles", "no tables"), and specifying constraints for the protocol (e.g. only "fire-and-forget methods", "no events").

The envisaged syntax resembles a configuration expressed in untyped FIDL Text:

transport ipc = {
    methods: {
        fire_and_forget: true,
        request_response: true,
    allowed_resources: [handle],

And then used as:

protocol SomeProtocol over zx.ipc {

Handle generalization

Right now, handles are a purely Fuchsia specific concept: they are directly tied to the Zircon kernel, map to zx_handle_t (or equivalent in other languages than C), and their kinds are only the objects exposed by the kernel such as port, vmo, fifo, etc.

When considering other cases (e.g. in process communication), one desirable extension point is to be able to define handles in FIDL directly, rather than have that be a part of the language definition.

As an example, defining zircon handles:

library zx;

resource handle : uint32 {
    properties {
        subtype handle_subtype;
        rights rights;

type handle_subtype = enum {
    PROCESS = 1;
    THREAD = 2;
    VMO = 3;
    CHANNEL = 4;

type rights = bits {
    READ = ...;
    WRIE = ...;

Which would allow handle or handle:VMO (or in another library zx.handle:zx.handle.VMO).

An experimental implementation exists, and will be used to break the cyclic dependency between Zircon and FIDL (until this change, Zircon's API was described in FIDL, but FIDL was partly defined in terms of Zircon's API).

Implementation Strategy

A temporary "version declaration" will be added to the top of all .fidl files to be used by fidlc to detect whether a .fidl file is in the prior or new syntax.

This token will be immediately preceding the library statement:

// Copyright notice...


library fidl.test;

An explicit marker is preferred in order to simplify the role of fidlc in detecting the syntax and to improve readability. An example of a challenge from detecting syntax is the case where interpreting as either syntax leads to compilation errors. These scenarios would require a heuristic to decide between the old and new syntax, which could lead to surprising results.

Further, this token is added to all files in the prior syntax rather than in the new syntax (e.g. new_syntax;") in order to socialize the aspect of the upcoming migration - readers of FIDL files will get a sense that the syntax is about to change and can seek additional context through other channels (e.g. documentation, mailing lists).

A new fidlconv host tool will be added that can take FIDL files in the old format and convert them to files in the new format, referred to as .fidl_new for the purposes of this section. Though this tool is separate from fidlc, it will need to leverage the compiler's internal representation to perform this conversion correctly. For example, a type Foo will need to be converted to client_end:Foo only if it is a protocol - to determine whether the case fidlconv will leverage fidlc to compile the FIDL library first.

The FIDL frontend compiler fidlc as well as accompanying tools like the formatter and linter will be extended to support either syntax based on the marker defined above.

With this added functionality, the build pipeline will be extended as follows:

Visualization: build pipeline strategy

That is:

  • A fidlconv tool will convert FIDL files in the old syntax to the new syntax.
  • The fidlc compiler will output the .json by compiling the old syntax.
  • Separately, the fidlc compiler will output the .json IR by compiling the new syntax.
  • The fidlfmt formatter will format the generated new library files .fidl_new.

For testing and verification:

  • The two json IR will be compared, and verified to match (except for span information).
  • Idempotency of the formatting of new libraries files will be verified to check both the output of the fidlc compiler, and of the fidlfmt formatter with the new syntax.

As part of this implementation, the FIDL team will also move the coding tables backend to be a standalone binary (in the same vein as other backends), and will obsolete and delete the C bindings backend by generating the last uses, and checking them in the fuchsia.git tree repository.


This RFC is all about ergonomics.

We are willing to trade a short term productivity loss to developers familiar with the current syntax as they retrain to use this modified syntax as we strongly believe the many more developers who will be using FIDL in the future will greatly benefit.

Documentation and Examples

This will require changing:

Backwards Compatibility

This change is not backwards compatible. See the implementation section for the transition plan.


This change has no impact on performance.


This change has no impact on security.


See the implementation section for the transition plan, and verifying its correctness.

Drawbacks, Alternatives, and Unknowns

Using colon to separate name from type

Since we're moving types to be second, we could also consider using the quite common : separator as is done in type theory, Rust, Kotlin, the ML languages (SML, Haskell, OCaml), Scala, Nim, Python, TypeScript, and many more:

    field: int32 rather than the proposed field int32

This proposal rejects this approach.

The : separator is primordially used to separate layouts from constraints. It is also used to indicate a "wrapped type" for enum and bits declarations. Finally it is used to denote envelopes in table and union declarations. Further overloading the : separator, especially in close grammatical proximity to its main use will lead to confusion (e.g. a table member 1: name: string:128;).

Omitting semicolons

It has been discussed to work to omit semicolons terminating declarations (be it member, const, or other).

This proposal chooses not to explore this simplification.

Removing semicolons makes little syntactic difference for FIDL authors. It's also not a key change to make, and should we want to explore this in the future it will be easy to modify (e.g. Go's approach to remove semicolons).

However, presence of semicolons to terminate members and declarations makes it much easier to guarantee unambiguous grammar rules especially as we explore constraints (use-site and declaration-site). For instance, with a declaration site layout constraint (C) such as struct Example { ... }:C; we delineate a constraint nicely between the : separator and the ; terminator.

Unifying enums and unions

From a type theoretic standpoint, an enumeration represents a sum of unit types, and a union represents a sum of any types. It is therefore tempting to seek to unify these two concepts into one. This is the approach taken by programming languages which support ADTs such as ML or Rust.

However, from a layout standpoint, a sum type of only unit types (an enumeration) can be represented much more efficiently than the extensible counterpart (a union). While both offer extensibility in light of adding new members, only unions offer extensibility to go from unit types (e.g. struct {}) to any types. This extensibility comes at a cost of an inline envelope.

We have chosen a pragmatic approach that balances the complexity of having two constructs, with the performance benefit of special casing enumerations.


On syntax

On extensible method arguments

On type aliasing and named types


While it may seem odd to prefer syntactic conciseness over explicitly choosing a wrapped type, having a sensible default provides greater consistency for enums across FIDL libraries. This in term provides a migration path to switch enums down the road, e.g. should a library define a general purpose ErrorStatus enum, it could be replaced later by another 'better' general purpose ErrorStatusV2.

  1. Or at least, not without a good understanding of the wire format and care, e.g. https://fxrev.dev/360015