Version: dev

Schema IDL

This document provides the syntax and semantic reference for Fory IDL.

For compiler usage and build integration, see Compiler Guide. For protobuf/FlatBuffers frontend mapping rules, see Protocol Buffers IDL Support and FlatBuffers IDL Support.

File Structure

An Fory IDL file typically consists of:

Optional package declaration
Optional file-level options
Optional import statements
Type definitions (enums, messages, and unions)

// Optional package declaration
package com.example.models;

// Optional file-level options
option java_package = "com.example.models";

// Import statements
import "common/types.fdl";

// Type definitions
enum Color [id=100] { ... }
message User [id=101] { ... }
message Order [id=102] { ... }
union Event [id=103] { ... }

Comments

Fory IDL supports both single-line and block comments:

// This is a single-line comment

/*
 * This is a block comment
 * that spans multiple lines
 */

message Example {
    string name = 1;  // Inline comment
}

Package Declaration

The package declaration defines the namespace for all types in the file.

package com.example.models;

You can optionally specify a package alias used for auto-generated type IDs:

package com.example.models alias models_v1;

Rules:

Optional but recommended
Must appear before any type definitions
Only one package declaration per file
Used for namespace-based type registration
Package alias is used for auto-ID hashing

Language Mapping:

Language	Package Usage
Java	Java package
Python	Module name (dots to underscores)
Go	Package name (last component)
Rust	Module name (dots to underscores)
C++	Namespace (dots to `::`)
C#	Namespace
JavaScript	Module name (last segment)
Dart	Library name (package segments)

File-Level Options

Options can be specified at file level to control language-specific code generation.

Syntax

option option_name = value;

Java Package Option

Override the Java package for generated code:

package payment;
option java_package = "com.mycorp.payment.v1";

message Payment {
    string id = 1;
}

Effect:

Generated Java files will be in com/mycorp/payment/v1/ directory
Java package declaration will be package com.mycorp.payment.v1;
Type registration still uses the Fory IDL package (payment) for cross-language compatibility

Go Package Option

Specify the Go import path and package name:

package payment;
option go_package = "github.com/mycorp/apis/gen/payment/v1;paymentv1";

message Payment {
    string id = 1;
}

Format: "import/path;package_name" or just "import/path" (last segment used as package name)

Effect:

Generated Go files will have package paymentv1
The import path can be used in other Go code
Type registration still uses the Fory IDL package (payment) for cross-language compatibility

C# Namespace Option

Override the C# namespace for generated code:

package payment;
option csharp_namespace = "MyCorp.Payment.V1";

message Payment {
    string id = 1;
}

Effect:

Generated C# files use namespace MyCorp.Payment.V1;
Output path follows namespace segments (MyCorp/Payment/V1/ under --csharp_out)
Type registration still uses the Fory IDL package (payment) for cross-language compatibility

Go Nested Type Style Option

Control Go naming for nested message/enum/union types:

package payment;
option go_nested_type_style = "camelcase";

message Envelope {
    message Payload {
        string id = 1;
    }
}

Values:

underscore (default): Envelope_Payload
camelcase: EnvelopePayload

The CLI flag --go_nested_type_style overrides this schema option when both are set.

Swift Namespace Style Option

Control how package namespace is reflected in Swift generated type names:

package payment.v1;
option swift_namespace_style = "flatten";

message Payment {
    string id = 1;
}

Values:

enum (default): namespace wrappers (for example Payment.V1.Payment)
flatten: package prefix on top-level types (for example Payment_V1_Payment)

Important: namespace wrapper/prefixing is only applied when package is non-empty. If package is empty, Swift emits top-level types directly for both styles.

The CLI flag --swift_namespace_style overrides this schema option when both are set.

Java Outer Classname Option

Generate all types as inner classes of a single outer wrapper class:

package payment;
option java_outer_classname = "DescriptorProtos";

enum Status {
    UNKNOWN = 0;
    ACTIVE = 1;
}

message Payment {
    string id = 1;
    Status status = 2;
}

Effect:

Generates a single file DescriptorProtos.java instead of separate files
All enums and messages become public static inner classes
The outer class is public final with a private constructor
Useful for grouping related types together

Generated structure:

public final class DescriptorProtos {
    private DescriptorProtos() {}

    public static enum Status {
        UNKNOWN,
        ACTIVE;
    }

    public static class Payment {
        private String id;
        private Status status;
        // ...
    }
}

Combined with java_package:

package payment;
option java_package = "com.example.proto";
option java_outer_classname = "PaymentProtos";

message Payment {
    string id = 1;
}

This generates com/example/proto/PaymentProtos.java with all types as inner classes.

Java Multiple Files Option

Control whether types are generated in separate files or as inner classes:

package payment;
option java_outer_classname = "PaymentProtos";
option java_multiple_files = true;

message Payment {
    string id = 1;
}

message Receipt {
    string id = 1;
}

Behavior:

`java_outer_classname`	`java_multiple_files`	Result
Not set	Any	Separate files (one per type)
Set	`false` (default)	Single file with all types as inner classes
Set	`true`	Separate files (overrides outer class)

Effect of java_multiple_files = true:

Each top-level enum and message gets its own .java file
Overrides java_outer_classname behavior
Useful when you want separate files but still specify an outer class name for other purposes

Example without java_multiple_files (default):

option java_outer_classname = "PaymentProtos";
// Generates: PaymentProtos.java containing Payment and Receipt as inner classes

Example with java_multiple_files = true:

option java_outer_classname = "PaymentProtos";
option java_multiple_files = true;
// Generates: Payment.java, Receipt.java (separate files)

Multiple Options

Multiple options can be specified:

package payment;
option java_package = "com.mycorp.payment.v1";
option go_package = "github.com/mycorp/apis/gen/payment/v1;paymentv1";
option deprecated = true;

message Payment {
    string id = 1;
}

Protobuf Extension Syntax

In .fdl files, use native Fory IDL syntax only (for example, [id=100], ref, optional, nullable=true).

Protobuf extension syntax with (fory). is for .proto files and the protobuf frontend only.

For protobuf extension options, see Protocol Buffers IDL Support.

Option Priority

For language-specific packages/namespaces:

Command-line package override (highest priority)
Language-specific option (java_package, go_package, csharp_namespace)
Fory IDL package declaration (fallback)

Example:

package myapp.models;
option java_package = "com.example.generated";

Scenario	Java Package Used
No override	`com.example.generated`
CLI: `--package=override`	`override`
No java_package option	`myapp.models` (fallback)

Cross-Language Type Registration

Language-specific options only affect where code is generated, not the type namespace used for serialization. This ensures cross-language compatibility:

package myapp.models;
option java_package = "com.mycorp.generated";
option go_package = "github.com/mycorp/gen;genmodels";

message User {
    string name = 1;
}

All languages will register User with namespace myapp.models, enabling:

Java serialized data → Go deserialization
Go serialized data → Java deserialization
Any language combination works seamlessly

Import Statement

Import statements allow you to use types defined in other Fory IDL files.

Basic Syntax

import "path/to/file.fdl";

Multiple Imports

import "common/types.fdl";
import "common/enums.fdl";
import "models/address.fdl";

Path Resolution

Import paths are resolved relative to the importing file:

project/
├── common/
│   └── types.fdl
├── models/
│   ├── user.fdl      # import "../common/types.fdl"
│   └── order.fdl     # import "../common/types.fdl"
└── main.fdl          # import "common/types.fdl"

Rules:

Import paths are quoted strings (double or single quotes)
Paths are resolved relative to the importing file's directory
Imported types become available as if defined in the current file
Circular imports are detected and reported as errors
Transitive imports work (if A imports B and B imports C, A has access to C's types)

Complete Example

common/types.fdl:

package common;

enum Status [id=100] {
    PENDING = 0;
    ACTIVE = 1;
    COMPLETED = 2;
}

message Address [id=101] {
    string street = 1;
    string city = 2;
    string country = 3;
}

models/user.fdl:

package models;
import "../common/types.fdl";

message User [id=200] {
    string id = 1;
    string name = 2;
    Address home_address = 3;  // Uses imported type
    Status status = 4;          // Uses imported enum
}

Unsupported Import Syntax

The following protobuf import modifiers are not supported:

// NOT SUPPORTED - will produce an error
import public "other.fdl";
import weak "other.fdl";

import public: Fory IDL uses a simpler import model. All imported types are available to the importing file only. Re-exporting is not supported. Import each file directly where needed.

import weak: Fory IDL requires all imports to be present at compile time. Optional dependencies are not supported.

Import Errors

The compiler reports errors for:

File not found: The imported file doesn't exist
Circular import: A imports B which imports A (directly or indirectly)
Parse errors: Syntax errors in imported files
Unsupported syntax: import public or import weak

Enum Definition

Enums define a set of named integer constants.

Basic Syntax

enum Status {
    PENDING = 0;
    ACTIVE = 1;
    COMPLETED = 2;
}

With Explicit Type ID

enum Status [id=100] {
    PENDING = 0;
    ACTIVE = 1;
    COMPLETED = 2;
}

Reserved Values

Reserve field numbers or names to prevent reuse:

enum Status {
    reserved 2, 15, 9 to 11, 40 to max;  // Reserved numbers
    reserved "OLD_STATUS", "DEPRECATED"; // Reserved names
    PENDING = 0;
    ACTIVE = 1;
    COMPLETED = 3;
}

Enum Type Options

Enum-level options are declared inline in [] after the enum name:

enum Status [deprecated=true] {
    PENDING = 0;
    ACTIVE = 1;
}

FDL does not support option ...; statements inside enum bodies.

Unsupported:

allow_alias is not supported. Each enum value must have a unique integer.

Language Mapping

Language	Implementation
Java	`enum Status { UNKNOWN, ACTIVE, ... }`
Python	`class Status(IntEnum): UNKNOWN = 0`
Go	`type Status int32` with constants
Rust	`#[repr(i32)] enum Status { Unknown }`
C++	`enum class Status : int32_t { ... }`
JavaScript	`export enum Status { UNKNOWN, ... }`
Dart	`enum Status { unknown, active, ... }`

Enum Prefix Stripping

When enum values use a protobuf-style prefix (enum name in UPPER_SNAKE_CASE), the compiler automatically strips the prefix for languages with scoped enums:

// Input with prefix
enum DeviceTier {
    DEVICE_TIER_UNKNOWN = 0;
    DEVICE_TIER_TIER1 = 1;
    DEVICE_TIER_TIER2 = 2;
}

Generated code:

Language	Output	Style
Java	`UNKNOWN, TIER1, TIER2`	Scoped enum
Rust	`Unknown, Tier1, Tier2`	Scoped enum
C++	`UNKNOWN, TIER1, TIER2`	Scoped enum
Python	`UNKNOWN, TIER1, TIER2`	Scoped IntEnum
Go	`DeviceTierUnknown, DeviceTierTier1, ...`	Unscoped const
JavaScript	`UNKNOWN, TIER1, TIER2`	Scoped enum
Dart	`unknown, tier1, tier2`	Scoped enum

Note: The prefix is only stripped if the remainder is a valid identifier. For example, DEVICE_TIER_1 is kept unchanged because 1 is not a valid identifier name.

Grammar:

enum_def     := 'enum' IDENTIFIER [type_options] '{' enum_body '}'
type_options := '[' type_option (',' type_option)* ']'
type_option  := IDENTIFIER '=' option_value
enum_body    := (reserved_stmt | enum_value)*
reserved_stmt := 'reserved' reserved_items ';'
enum_value   := IDENTIFIER '=' INTEGER ';'

Rules:

Enum names must be unique within the file
Enum values must have explicit integer assignments
Value integers must be unique within the enum (no aliases)
Type ID ([id=100]) is optional for enums but recommended for cross-language use

Example with All Features:

// HTTP status code categories
enum HttpCategory [id=200] {
    reserved 10 to 20;           // Reserved for future use
    reserved "UNKNOWN";          // Reserved name
    INFORMATIONAL = 1;
    SUCCESS = 2;
    REDIRECTION = 3;
    CLIENT_ERROR = 4;
    SERVER_ERROR = 5;
}

Message Definition

Messages define structured data types with typed fields.

Basic Syntax

message Person {
    string name = 1;
    int32 age = 2;
}

With Explicit Type ID

message Person [id=101] {
    string name = 1;
    int32 age = 2;
}

Without Explicit Type ID

message Person {  // Auto-generated when enable_auto_type_id = true
    string name = 1;
    int32 age = 2;
}

Language Mapping

Language	Implementation
Java	POJO class with getters/setters
Python	`@dataclass` class
Go	Struct with exported fields
Rust	Struct with `#[derive(ForyObject)]`
C++	Struct with `FORY_STRUCT` macro
JavaScript	`export interface` declaration
Dart	`@ForyStruct` `final class`

Type IDs control cross-language registration for messages, unions, and enums. See Type IDs for auto-generation, aliases, and collision handling.

Reserved Fields

Reserve field numbers or names to prevent reuse after removing fields:

message User {
    reserved 2, 15, 9 to 11;       // Reserved field numbers
    reserved "old_field", "temp";  // Reserved field names
    string id = 1;
    string name = 3;
}

Message Type Options

Message-level options are declared inline in [] after the message name:

message User [deprecated=true] {
    string id = 1;
    string name = 2;
}

FDL does not support option ...; statements inside message or enum bodies.

Grammar:

message_def  := 'message' IDENTIFIER [type_options] '{' message_body '}'
type_options := '[' type_option (',' type_option)* ']'
type_option  := IDENTIFIER '=' option_value
message_body := (reserved_stmt | nested_type | field_def)*
nested_type  := enum_def | message_def | union_def

Rules:

Type IDs follow the rules in Type IDs.

Nested Types

Messages can contain nested message, enum, and union definitions. This is useful for defining types that are closely related to their parent message.

Nested Messages

message SearchResponse {
    message Result {
        string url = 1;
        string title = 2;
        list<string> snippets = 3;
    }
    list<Result> results = 1;
}

Nested Enums

message Container {
    enum Status {
        STATUS_UNKNOWN = 0;
        STATUS_ACTIVE = 1;
        STATUS_INACTIVE = 2;
    }
    Status status = 1;
}

Qualified Type Names

Nested types can be referenced from other messages using qualified names (Parent.Child):

message SearchResponse {
    message Result {
        string url = 1;
        string title = 2;
    }
}

message SearchResultCache {
    // Reference nested type with qualified name
    SearchResponse.Result cached_result = 1;
    list<SearchResponse.Result> all_results = 2;
}

Deeply Nested Types

Nesting can be multiple levels deep:

message Outer {
    message Middle {
        message Inner {
            string value = 1;
        }
        Inner inner = 1;
    }
    Middle middle = 1;
}

message OtherMessage {
    // Reference deeply nested type
    Outer.Middle.Inner deep_ref = 1;
}

Language-Specific Generation

Language	Nested Type Generation
Java	Static inner classes (`SearchResponse.Result`)
Python	Nested classes within dataclass
Go	Flat structs with underscore (`SearchResponse_Result`, configurable to camelcase)
Rust	Nested modules (`search_response::Result`)
C++	Nested classes (`SearchResponse::Result`)
JavaScript	Flat names (`Result`)
Dart	Flat classes with underscore (`SearchResponse_Result`)

Note: Go defaults to underscore-separated nested names; set option go_nested_type_style = "camelcase"; to use concatenated names. Rust emits nested modules for nested types.

Nested Type Rules

Nested type names must be unique within their parent message
Nested types can have their own type IDs
Numeric type IDs must be globally unique (including nested types); see Type IDs for auto-generation and collision handling
Within a message, you can reference nested types by simple name
From outside, use the qualified name (Parent.Child)

Union Definition

Unions define a value that can hold exactly one of several case types.

Basic Syntax

union Animal [id=106] {
    Dog dog = 1;
    Cat cat = 2;
}

Using a Union in a Message

message Person [id=100] {
    Animal pet = 1;
    optional Animal favorite_pet = 2;
}

Rules

Case IDs must be unique within the union
Cases cannot be optional or ref
Union cases do not support field options
Case types can be primitives, enums, messages, or other named types
Union type IDs follow the rules in Type IDs.

Grammar:

union_def  := 'union' IDENTIFIER [type_options] '{' union_field* '}'
union_field := field_type IDENTIFIER '=' INTEGER ';'

Field Definition

Fields define the properties of a message.

Basic Syntax

field_type field_name = field_number;

With Modifiers

optional list<string> tags = 1;  // Nullable list
list<optional string> tags = 2;  // Elements may be null
ref list<Node> nodes = 3;        // Collection tracked as a reference
list<ref Node> nodes = 4;        // Elements tracked as references

Grammar:

field_def    := [modifiers] field_type IDENTIFIER '=' INTEGER ';'
modifiers    := { 'optional' | 'ref' }
field_type   := primitive_type | named_type | list_type | array_type | map_type
list_type    := 'list' '<' { 'optional' | 'ref' | scalar_encoding } field_type '>'
array_type   := 'array' '<' array_element_type '>'

Modifiers apply to the field/collection. Use list<...> to describe element modifiers. repeated is accepted as an alias for list.

Field Modifiers

`optional`

Marks the field as nullable:

message User {
    string name = 1;           // Required, non-null
    optional string email = 2; // Nullable
}

Generated Code:

Language	Non-optional	Optional
Java	`String name`	`String email` with `@ForyField(nullable=true)`
Python	`name: str`	`name: Optional[str]`
Go	`Name string`	`Name *string`
Rust	`name: String`	`name: Option<String>`
C++	`std::string name`	`std::optional<std::string> name`
JavaScript	`name: string`	`name?: string \| null`
Dart	`String name`	`String? email`

Default Values:

Type	Default Value
Non-optional types	Language default
Optional types	`null`/`None`/`nil`

`ref`

Enables reference tracking for shared/circular references:

message Node {
    string value = 1;
    ref Node parent = 2;     // Can point to shared object
    list<ref Node> children = 3;
}

Use Cases:

Shared objects (same object referenced multiple times)
Circular references (object graphs with cycles)
Tree structures with parent pointers

Generated Code:

Language	Without `ref`	With `ref`
Java	`Node parent`	`Node parent` with `@ForyField(ref=true)`
Python	`parent: Node`	`parent: Node = pyfory.field(ref=True)`
Go	`Parent Node`	`Parent *Node` with `fory:"ref"`
Rust	`parent: Node`	`parent: Arc<Node>`
C++	`Node parent`	`std::shared_ptr<Node> parent`
JavaScript	`parent: Node`	`parent: Node` (no ref distinction)
Dart	`Node parent`	`Node parent` with `@ForyField(ref: true)`

Rust uses Arc by default; use ref(thread_safe=false) or ref(weak=true) to customize pointer types. For protobuf option syntax, see Protocol Buffers IDL Support.

`list`

Marks the field as an ordered collection:

message Document {
    list<string> tags = 1;
    list<User> authors = 2;
}

Generated Code:

Language	Type
Java	`List<String>`
Python	`List[str]`
Go	`[]string`
Rust	`Vec<String>`
C++	`std::vector<std::string>`
JavaScript	`string[]`
Dart	`List<String>`

Combining Modifiers

Modifiers can be combined:

message Example {
    optional list<string> tags = 1;  // Nullable list
    list<optional string> aliases = 2; // Elements may be null
    ref list<Node> nodes = 3;          // Collection tracked as a reference
    list<ref Node> children = 4;       // Elements tracked as references
    optional ref User owner = 5;          // Nullable tracked reference
}

Modifiers before list apply to the field/collection. Modifiers after list apply to elements. repeated is accepted as an alias for list.

List modifier mapping:

Fory IDL	Java	Python	Go	Rust	C++	Dart
`optional list<string>`	`List<String>` + `@ForyField(nullable = true)`	`Optional[List[str]]`	`[]string` + `nullable`	`Option<Vec<String>>`	`std::optional<std::vector<std::string>>`	`List<String>?`
`list<optional string>`	`List<String>` (nullable elements)	`List[Optional[str]]`	`[]*string`	`Vec<Option<String>>`	`std::vector<std::optional<std::string>>`	`List<String?>`
`ref list<User>`	`List<User>` + `@ForyField(ref = true)`	`List[User]` + `pyfory.field(ref=True)`	`[]User` + `ref`	`Arc<Vec<User>>`	`std::shared_ptr<std::vector<User>>`	`List<User>` + `@ForyField(ref: true)`
`list<ref User>`	`List<User>`	`List[User]`	`[]*User` + `ref=false`	`Vec<Arc<User>>`	`std::vector<std::shared_ptr<User>>`	`List<User>` + `@ListField(element: DeclaredType(ref: true))`

Use ref(thread_safe=false) in Fory IDL (or [(fory).thread_safe_pointer = false] in protobuf) to generate Rc instead of Arc in Rust.

Field Numbers

Each field must have a unique positive integer identifier:

message Example {
    string first = 1;
    string second = 2;
    string third = 3;
}

Rules and best practices:

Numbers must be unique within a message.
Numbers must be positive integers.
Gaps are allowed and are useful when fields are removed.
Prefer sequential numbering from 1.
Never reuse a removed field number for a different field.

Type System

Fory IDL provides a cross-language type system for primitives, named types, and collections. Field modifiers (optional, ref) control nullability and reference tracking, while list<T> and array<T> choose collection schema kind (see Field Modifiers).

Primitive Types

Type	Description	Size
`bool`	Boolean value	1 byte
`int8`	Signed 8-bit integer	1 byte
`int16`	Signed 16-bit integer	2 bytes
`int32`	Signed 32-bit integer, varint by default	4 bytes
`int64`	Signed 64-bit integer, PVL varint by default	8 bytes
`uint8`	Unsigned 8-bit integer	1 byte
`uint16`	Unsigned 16-bit integer	2 bytes
`uint32`	Unsigned 32-bit integer, varint by default	4 bytes
`uint64`	Unsigned 64-bit integer, PVL varint by default	8 bytes
`float16`	IEEE 754 binary16 floating point	2 bytes
`bfloat16`	Brain floating point	2 bytes
`float32`	32-bit floating point	4 bytes
`float64`	64-bit floating point	8 bytes
`string`	UTF-8 string	Variable
`bytes`	Binary data	Variable
`date`	Calendar date	Variable
`timestamp`	Date and time with timezone	Variable
`duration`	Duration	Variable
`decimal`	Decimal value	Variable
`any`	Dynamic value (runtime type)	Variable

Boolean

Language	Type	Notes
Java	`boolean` / `Boolean`	Primitive or boxed
Python	`bool`
Go	`bool`
Rust	`bool`
C++	`bool`
JavaScript	`boolean`
Dart	`bool`

Integer Types

Fory IDL provides fixed-width signed integers (varint encoding for 32/64-bit by default):

Fory IDL Type	Size	Range
`int8`	8-bit	-128 to 127
`int16`	16-bit	-32,768 to 32,767
`int32`	32-bit	-2^31 to 2^31 - 1
`int64`	64-bit	-2^63 to 2^63 - 1

Language Mapping (Signed):

Fory IDL	Java	Python	Go	Rust	C++	JavaScript	Dart
`int8`	`byte`	`pyfory.Int8`	`int8`	`i8`	`int8_t`	`number`	`int`
`int16`	`short`	`pyfory.Int16`	`int16`	`i16`	`int16_t`	`number`	`int`
`int32`	`int`	`pyfory.Int32`	`int32`	`i32`	`int32_t`	`number`	`int`
`int64`	`long`	`pyfory.Int64`	`int64`	`i64`	`int64_t`	`bigint \| number`	`Int64`

Fory IDL provides fixed-width unsigned integers (varint encoding for 32/64-bit by default):

Fory IDL	Size	Range
`uint8`	8-bit	0 to 255
`uint16`	16-bit	0 to 65,535
`uint32`	32-bit	0 to 2^32 - 1
`uint64`	64-bit	0 to 2^64 - 1

Language Mapping (Unsigned):

Fory IDL	Java	Python	Go	Rust	C++	JavaScript	Dart
`uint8`	`short`	`pyfory.UInt8`	`uint8`	`u8`	`uint8_t`	`number`	`int`
`uint16`	`int`	`pyfory.UInt16`	`uint16`	`u16`	`uint16_t`	`number`	`int`
`uint32`	`long`	`pyfory.UInt32`	`uint32`	`u32`	`uint32_t`	`number`	`int`
`uint64`	`long`	`pyfory.UInt64`	`uint64`	`u64`	`uint64_t`	`bigint \| number`	`Uint64`

Integer Encoding Modifiers

For 32/64-bit integers, Fory IDL uses variable-length encoding by default. Add a scalar encoding modifier when you need a different wire encoding:

Modifier	Valid types	Notes
`varint`	`int32`, `int64`, `uint32`, `uint64`	Explicit spelling of default
`fixed`	`int32`, `int64`, `uint32`, `uint64`	Fixed-width little-endian
`tagged`	`int64`, `uint64`	Tagged 64-bit encoding

Modifiers are part of the scalar type expression, so they can be used in nested list and map positions:

fixed int32 id = 1;
list<fixed int32> offsets = 2;
map<string, tagged uint64> counters = 3;

Underscore spellings for integer encoding are not FDL type names.

Floating-Point Types

Fory IDL Type	Size	Precision
`float32`	32-bit	~7 digits
`float64`	64-bit	~15-16 digits

Language Mapping:

Fory IDL	Java	Python annotation/value	Go	Rust	C++	JavaScript	Dart
`float16`	`Float16`	`pyfory.Float16` / `float`	`float16.Float16`	`Float16`	`fory::float16_t`	`number`	`double`
`bfloat16`	`BFloat16`	`pyfory.BFloat16` / `float`	`bfloat16.BFloat16`	`BFloat16`	`fory::bfloat16_t`	`number`	`double`
`float32`	`float`	`pyfory.Float32`	`float32`	`f32`	`float`	`number`	`Float32`
`float64`	`double`	`pyfory.Float64`	`float64`	`f64`	`double`	`number`	`double`

String Type

Language	Type	Notes
Java	`String`	Immutable
Python	`str`
Go	`string`	Immutable
Rust	`String`	Owned, heap-allocated
C++	`std::string`
JavaScript	`string`
Dart	`String`

Bytes Type

Language	Type	Notes
Java	`byte[]`
Python	`bytes`	Immutable
Go	`[]byte`
Rust	`Vec<u8>`
C++	`std::vector<uint8_t>`
JavaScript	`Uint8Array`
Dart	`Uint8List`

Temporal Types

Date

Language	Type	Notes
Java	`java.time.LocalDate`
Python	`datetime.date`
Go	`time.Time`	Time portion ignored
Rust	`chrono::NaiveDate`	Requires `chrono` crate
C++	`fory::serialization::Date`
JavaScript	`Date`
Dart	`LocalDate`	Fory package type

Timestamp

Language	Type	Notes
Java	`java.time.Instant`	UTC-based
Python	`datetime.datetime`
Go	`time.Time`
Rust	`chrono::NaiveDateTime`	Requires `chrono` crate
C++	`fory::serialization::Timestamp`
JavaScript	`Date`
Dart	`Timestamp`	Fory package type

Any

Language	Type	Notes
Java	`Object`	Runtime type written
Python	`Any`	Runtime type written
Go	`any`	Runtime type written
Rust	`Box<dyn Any>`	Runtime type written
C++	`std::any`	Runtime type written
JavaScript	`any`	Runtime type written
Dart	`Object?`	Runtime type written

Example:

enum EventType [id=120] {
    CREATED = 0;
    DELETED = 1;
}

message UserCreated [id=121] {
    string user_id = 1;
}

message Envelope [id=122] {
    EventType type = 1;
    any payload = 2;
}

Generated Code (Envelope.payload):

Language	Generated Field Type
Java	`Object payload`
Python	`payload: Any`
Go	`Payload any`
Rust	`payload: Box<dyn Any>`
C++	`std::any payload`
JavaScript	`payload: any`
Dart	`Object? payload`

Notes:

any always writes a null flag (same as nullable) because values may be empty.
Allowed runtime values are limited to bool, string, enum, message, and union. Other primitives (numeric, bytes, date/time) and list/map are not supported; wrap them in a message or use explicit fields instead.
ref is not allowed on any fields (including list/map values). Wrap any in a message if you need reference tracking.
The runtime type must be registered in the target language schema/IDL registration; unknown types fail to deserialize.

Named Types

Reference other messages, enums, or unions by name:

enum Status { ... }
message User { ... }

message Order {
    User customer = 1;    // Reference to User message
    Status status = 2;    // Reference to Status enum
}

Collection Types

List (`list`)

Use the list<...> type for list fields. repeated is accepted as an alias. See Field Modifiers for modifier combinations and language mapping.

Nested collection support is target-capability based. The C++ generator accepts nested collection specs such as list<list<...>>, list<map<...>>, and map<..., list<...>>; targets that have not implemented nested field specs continue to reject them. Use a message wrapper when you need portable schemas across all targets.

Array (`array`)

Use array<T> for dynamic-length dense numeric data. array<T> is a distinct schema kind from list<T> and uses the packed primitive-array wire payload.

message Embedding {
    array<int32> indices = 1;
    array<float32> values = 2;
    array<uint8> pixels = 3;
}

array<T> accepts bool, integer, and floating-point element domains only. It does not accept optional, ref, named/object types, string, bytes, maps, or scalar integer encoding modifiers such as array<fixed int32>; array elements are always fixed-width by the array contract.

Generated carriers are language-specific, but the schema kind is not:

IDL schema	Java default	Python default	Dart default	JavaScript/TypeScript
`list<bool>`	`BoolList` / `List<Boolean>`	`List[bool]`	`List<bool>`	`Type.list(Type.bool())`
`array<bool>`	`boolean[]`	`pyfory.BoolArray`	`BoolList`	`Type.boolArray()`
`array<int8>`	`@Int8Type byte[]`	`pyfory.Int8Array`	`Int8List`	`Type.int8Array()`
`array<int16>`	`short[]`	`pyfory.Int16Array`	`Int16List`	`Type.int16Array()`
`array<int32>`	`int[]`	`pyfory.Int32Array`	`Int32List`	`Type.int32Array()`
`array<int64>`	`long[]`	`pyfory.Int64Array`	`Int64List`	`Type.int64Array()`
`array<uint8>`	`@UInt8Type byte[]`	`pyfory.UInt8Array`	`Uint8List`	`Type.uint8Array()`
`array<uint16>`	`@UInt16Type short[]`	`pyfory.UInt16Array`	`Uint16List`	`Type.uint16Array()`
`array<uint32>`	`@UInt32Type int[]`	`pyfory.UInt32Array`	`Uint32List`	`Type.uint32Array()`
`array<uint64>`	`@UInt64Type long[]`	`pyfory.UInt64Array`	`Uint64List`	`Type.uint64Array()`
`array<float16>`	`Float16Array`	`pyfory.Float16Array`	`Float16List`	`Type.float16Array()`
`array<bfloat16>`	`BFloat16Array`	`pyfory.BFloat16Array`	`Bfloat16List`	`Type.bfloat16Array()`
`array<float32>`	`float[]`	`pyfory.Float32Array`	`Float32List`	`Type.float32Array()`
`array<float64>`	`double[]`	`pyfory.Float64Array`	`Float64List`	`Type.float64Array()`

For handwritten Dart models, array<bool> requires BoolList plus @ArrayField(element: BoolType()) or @ForyField(type: ArrayType(element: BoolType())); List<bool> remains list<bool>. For handwritten Java models, unsigned primitive arrays use type-use annotations on the element type, for example private @UInt32Type int[] ids;.

Map

Maps with typed keys and values:

message Config {
    map<string, string> properties = 1;
    map<string, int32> counts = 2;
    map<int32, User> users = 3;
}

Language Mapping:

Fory IDL	Java	Python	Go	Rust	C++	JavaScript	Dart
`map<string, int32>`	`Map<String, Integer>`	`Dict[str, int]`	`map[string]int32`	`HashMap<String, i32>`	`std::map<std::string, int32_t>`	`Map<string, number>`	`Map<String, int>`
`map<string, User>`	`Map<String, User>`	`Dict[str, User]`	`map[string]User`	`HashMap<String, User>`	`std::map<std::string, User>`	`Map<string, User>`	`Map<String, User>`

Key Type Restrictions:

string (most common)
bool
Integer types (int8, int16, int32, int64, uint8, uint16, uint32, uint64)
Temporal scalar types (date, timestamp, duration)
Enums

Map keys do not support binary bytes, floating-point types, decimal, list<T>, array<T>, or nested map<K, V> types. Put those types in map values or wrap them in a message with a portable scalar or enum key.

Type Compatibility Matrix

This matrix shows which type conversions are safe across languages:

From -> To	bool	int8	int16	int32	int64	float32	float64	string
bool	Y	Y	Y	Y	Y	-	-	-
int8	-	Y	Y	Y	Y	Y	Y	-
int16	-	-	Y	Y	Y	Y	Y	-
int32	-	-	-	Y	Y	-	Y	-
int64	-	-	-	-	Y	-	-	-
float32	-	-	-	-	-	Y	Y	-
float64	-	-	-	-	-	-	Y	-
string	-	-	-	-	-	-	-	Y

Y = Safe conversion, - = Not recommended

Best Practices

Use int32 as the default for most integers; use int64 for large values.
Use string for text data (UTF-8) and bytes for binary data.
Use optional only when the field may legitimately be absent.
Use ref only when needed for shared or circular references.
Prefer list for ordered sequences and map for key-value lookups.

Type IDs

Type IDs enable efficient cross-language serialization and are used for messages, unions, and enums. When enable_auto_type_id = true (default) and id is omitted, the compiler auto-generates one using MurmurHash3(utf8(package.type_name)) (32-bit) and annotates it in generated code. When enable_auto_type_id = false, types without explicit IDs are registered by namespace and name instead. Collisions are detected at compile-time across the current file and all imports; when a collision occurs, the compiler raises an error and asks for an explicit id or an alias.

enum Color [id=100] { ... }
message User [id=101] { ... }
union Event [id=102] { ... }

Enum type IDs remain optional; if omitted they are auto-generated using the same hash when enable_auto_type_id = true.

With Explicit Type ID

message User [id=101] { ... }
message User [id=101, deprecated=true] { ... }  // Multiple options

Without Explicit Type ID

message Config { ... }  // Auto-generated when enable_auto_type_id = true

You can set [alias="..."] to change the hash source without renaming the type.

Practical Notes

If a type omits id and enable_auto_type_id = true, Fory generates an ID with MurmurHash3(utf8(package.type_name)) (32-bit).
Package alias and type alias change the hash input and can be used to resolve hash collisions without renaming public types.
Manual IDs in the small varint range (0-127) are compact on the wire; auto IDs are typically larger and usually consume 4-5 bytes.

ID Assignment Strategy

// Enums: 100-199
enum Status [id=100] { ... }
enum Priority [id=101] { ... }

// User domain: 200-299
message User [id=200] { ... }
message UserProfile [id=201] { ... }

// Order domain: 300-399
message Order [id=300] { ... }
message OrderItem [id=301] { ... }

Complete Example

// E-commerce domain model
package com.shop.models;

// Enums with type IDs
enum OrderStatus [id=100] {
    PENDING = 0;
    CONFIRMED = 1;
    SHIPPED = 2;
    DELIVERED = 3;
    CANCELLED = 4;
}

enum PaymentMethod [id=101] {
    CREDIT_CARD = 0;
    DEBIT_CARD = 1;
    PAYPAL = 2;
    BANK_TRANSFER = 3;
}

// Messages with type IDs
message Address [id=200] {
    string street = 1;
    string city = 2;
    string state = 3;
    string country = 4;
    string postal_code = 5;
}

message Customer [id=201] {
    string id = 1;
    string name = 2;
    optional string email = 3;
    optional string phone = 4;
    optional Address billing_address = 5;
    optional Address shipping_address = 6;
}

message Product [id=202] {
    string sku = 1;
    string name = 2;
    string description = 3;
    float64 price = 4;
    int32 stock = 5;
    list<string> categories = 6;
    map<string, string> attributes = 7;
}

message OrderItem [id=203] {
    ref Product product = 1;  // Track reference to avoid duplication
    int32 quantity = 2;
    float64 unit_price = 3;
}

message Order [id=204] {
    string id = 1;
    ref Customer customer = 2;
    list<OrderItem> items = 3;
    OrderStatus status = 4;
    PaymentMethod payment_method = 5;
    float64 total = 6;
    optional string notes = 7;
    timestamp created_at = 8;
    optional timestamp shipped_at = 9;
}

// Config without explicit type ID (auto-generated when enable_auto_type_id = true)
message ShopConfig {
    string store_name = 1;
    string currency = 2;
    float64 tax_rate = 3;
    list<string> supported_countries = 4;
}

For protobuf-specific extension options and (fory). syntax, see Protocol Buffers IDL Support.

Grammar Summary

file         := [package_decl] file_option* import_decl* type_def*

package_decl := 'package' package_name ['alias' package_name] ';'
package_name := IDENTIFIER ('.' IDENTIFIER)*

file_option  := 'option' option_name '=' option_value ';'
option_name  := IDENTIFIER

import_decl  := 'import' STRING ';'

type_def     := enum_def | message_def | union_def

enum_def     := 'enum' IDENTIFIER [type_options] '{' enum_body '}'
enum_body    := (reserved_stmt | enum_value)*
enum_value   := IDENTIFIER '=' INTEGER ';'

message_def  := 'message' IDENTIFIER [type_options] '{' message_body '}'
message_body := (reserved_stmt | nested_type | field_def)*
nested_type  := enum_def | message_def | union_def
field_def    := [modifiers] field_type IDENTIFIER '=' INTEGER [field_options] ';'

union_def    := 'union' IDENTIFIER [type_options] '{' union_field* '}'
union_field  := ['repeated'] field_type IDENTIFIER '=' INTEGER [field_options] ';'
option_value := 'true' | 'false' | IDENTIFIER | INTEGER | STRING

reserved_stmt := 'reserved' reserved_items ';'
reserved_items := reserved_item (',' reserved_item)*
reserved_item := INTEGER | INTEGER 'to' INTEGER | INTEGER 'to' 'max' | STRING

modifiers    := { 'optional' | 'ref' | 'repeated' }

field_type   := [scalar_encoding] (primitive_type | named_type | list_type | array_type | map_type)
primitive_type := 'bool'
               | 'int8' | 'int16' | 'int32' | 'int64'
               | 'uint8' | 'uint16' | 'uint32' | 'uint64'
               | 'float16' | 'bfloat16' | 'float32' | 'float64'
               | 'string' | 'bytes'
               | 'date' | 'timestamp' | 'duration' | 'decimal'
               | 'any'
scalar_encoding := 'varint' | 'fixed' | 'tagged'
named_type   := qualified_name
qualified_name := IDENTIFIER ('.' IDENTIFIER)*   // e.g., Parent.Child
list_type    := 'list' '<' { 'optional' | 'ref' | scalar_encoding } field_type '>'
array_type   := 'array' '<' array_element_type '>'
map_type     := 'map' '<' field_type ',' field_type '>'

type_options := '[' type_option (',' type_option)* ']'
type_option  := IDENTIFIER '=' option_value         // e.g., id=100, deprecated=true
field_options := '[' field_option (',' field_option)* ']'
field_option := IDENTIFIER '=' option_value         // e.g., deprecated=true, ref=true

STRING       := '"' [^"\n]* '"' | "'" [^'\n]* "'"
IDENTIFIER   := [a-zA-Z_][a-zA-Z0-9_]*
INTEGER      := '-'? [0-9]+

File Structure​

Comments​

Package Declaration​

File-Level Options​

Syntax​

Java Package Option​

Go Package Option​

C# Namespace Option​

Go Nested Type Style Option​

Swift Namespace Style Option​

Java Outer Classname Option​

Java Multiple Files Option​

Multiple Options​

Protobuf Extension Syntax​

Option Priority​

Cross-Language Type Registration​

Import Statement​

Basic Syntax​

Multiple Imports​

Path Resolution​

Complete Example​

Unsupported Import Syntax​

Import Errors​

Enum Definition​

Basic Syntax​

With Explicit Type ID​

Reserved Values​

Enum Type Options​

Language Mapping​

Enum Prefix Stripping​

Message Definition​

Basic Syntax​

With Explicit Type ID​

Without Explicit Type ID​

Language Mapping​

Reserved Fields​

Message Type Options​

Nested Types​

Nested Messages​

Nested Enums​

Qualified Type Names​

Deeply Nested Types​

Language-Specific Generation​

Nested Type Rules​

Union Definition​

Basic Syntax​

Using a Union in a Message​

Rules​

Field Definition​

Basic Syntax​

With Modifiers​

Field Modifiers​

optional​

ref​

list​

Combining Modifiers​

Field Numbers​

Type System​

Primitive Types​

Boolean​

Integer Types​

Integer Encoding Modifiers​

Floating-Point Types​

String Type​

Bytes Type​

Temporal Types​

Date​

Timestamp​

Any​

Named Types​

Collection Types​

List (list)​

Array (array)​

Map​

Type Compatibility Matrix​

Best Practices​

Type IDs​

With Explicit Type ID​

Without Explicit Type ID​

Practical Notes​

File Structure

Comments

Package Declaration

File-Level Options

Syntax

Java Package Option

Go Package Option

C# Namespace Option

Go Nested Type Style Option

Swift Namespace Style Option

Java Outer Classname Option

Java Multiple Files Option

Multiple Options

Protobuf Extension Syntax

Option Priority

Cross-Language Type Registration

Import Statement

Basic Syntax

Multiple Imports

Path Resolution

Complete Example

Unsupported Import Syntax

Import Errors

Enum Definition

Basic Syntax

With Explicit Type ID

Reserved Values

Enum Type Options

Language Mapping

Enum Prefix Stripping

Message Definition

Basic Syntax

With Explicit Type ID

Without Explicit Type ID

Language Mapping

Reserved Fields

Message Type Options

Nested Types

Nested Messages

Nested Enums

Qualified Type Names

Deeply Nested Types

Language-Specific Generation

Nested Type Rules

Union Definition

Basic Syntax

Using a Union in a Message

Rules

Field Definition

Basic Syntax

With Modifiers

Field Modifiers

`optional`

`ref`

`list`

Combining Modifiers

Field Numbers

Type System

Primitive Types

Boolean

Integer Types

Integer Encoding Modifiers

Floating-Point Types

String Type

Bytes Type

Temporal Types

Date

Timestamp

Any

Named Types

Collection Types

List (`list`)

Array (`array`)

Map

Type Compatibility Matrix

Best Practices

Type IDs

With Explicit Type ID

Without Explicit Type ID

Practical Notes