docs/compiler/compiler-guide.md

title: Compiler Guide sidebar_position: 3 id: compiler_guide license: | Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

 http://www.apache.org/licenses/LICENSE-2.0

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This guide covers installation, usage, and integration of the Fory IDL compiler.

Installation

From Source

cd compiler
pip install -e .

Verify Installation

foryc --help

Command Line Interface

Basic Usage

foryc [OPTIONS] FILES...

foryc --scan-generated [OPTIONS]

Options

Compile options:

Option	Description	Default
`--lang`	Comma-separated target languages	`all`
`--output`, `-o`	Output directory	`./generated`
`--package`	Override package name from Fory IDL file	(from file)
`-I`, `--proto_path`, `--import_path`	Add directory to import search path (can be repeated)	(none)
`--java_out=DST_DIR`	Generate Java code in DST_DIR	(none)
`--python_out=DST_DIR`	Generate Python code in DST_DIR	(none)
`--cpp_out=DST_DIR`	Generate C++ code in DST_DIR	(none)
`--go_out=DST_DIR`	Generate Go code in DST_DIR	(none)
`--rust_out=DST_DIR`	Generate Rust code in DST_DIR	(none)
`--csharp_out=DST_DIR`	Generate C# code in DST_DIR	(none)
`--javascript_out=DST_DIR`	Generate JavaScript code in DST_DIR	(none)
`--swift_out=DST_DIR`	Generate Swift code in DST_DIR	(none)
`--go_nested_type_style`	Go nested type naming: `camelcase` or `underscore`	`underscore`
`--swift_namespace_style`	Swift namespace style: `enum` or `flatten`	`enum`
`--emit-fdl`	Emit translated FDL (for non-FDL inputs)	`false`
`--emit-fdl-path`	Write translated FDL to this path (file or directory)	(stdout)

For both go_nested_type_style and swift_namespace_style, schema-level file options are supported (option ... = ...;) and the CLI flag overrides the schema option when both are present.

Scan options (with --scan-generated):

Option	Description	Default
`--root`	Root directory to scan	`.`
`--relative`	Print paths relative to root	`false`
`--delete`	Delete matched generated files	`false`
`--dry-run`	Scan/print only, do not delete	`false`

Scan Generated Files

Use --scan-generated to find files produced by foryc. The scanner walks the tree recursively, skips build/, target/, and hidden directories, and prints each generated file as it is found.

# Scan current directory
foryc --scan-generated

# Scan a specific root
foryc --scan-generated --root ./src

# Print paths relative to the scan root
foryc --scan-generated --root ./src --relative

# Delete scanned generated files
foryc --scan-generated --root ./src --delete

# Dry-run (scan and print only)
foryc --scan-generated --root ./src --dry-run

Examples

Compile for all languages:

foryc schema.fdl

Compile for specific languages:

foryc schema.fdl --lang java,python,csharp,javascript,swift

Specify output directory:

foryc schema.fdl --output ./src/generated

Override package name:

foryc schema.fdl --package com.myapp.models

Compile multiple files:

foryc user.fdl order.fdl product.fdl --output ./generated

Compile a simple schema containing service definitions (Java + Python models):

foryc compiler/examples/service.fdl --java_out=./generated/java --python_out=./generated/python

Use import search paths:

# Add a single import path
foryc src/main.fdl -I libs/common

# Add multiple import paths (repeated option)
foryc src/main.fdl -I libs/common -I libs/types

# Add multiple import paths (comma-separated)
foryc src/main.fdl -I libs/common,libs/types,third_party/

# Using --proto_path (protoc-compatible alias)
foryc src/main.fdl --proto_path=libs/common

# Mix all styles
foryc src/main.fdl -I libs/common,libs/types --proto_path third_party/

Language-specific output directories (protoc-style):

# Generate only Java code to a specific directory
foryc schema.fdl --java_out=./src/main/java

# Generate multiple languages to different directories
foryc schema.fdl --java_out=./java/gen --python_out=./python/src --go_out=./go/gen --csharp_out=./csharp/gen --javascript_out=./javascript/src --swift_out=./swift/gen

# Combine with import paths
foryc schema.fdl --java_out=./gen/java -I proto/ -I common/

When using --{lang}_out options:

Only the specified languages are generated (not all languages)
The compiler writes under the specified directory (language-specific generators may still create package/module subdirectories)
This is compatible with protoc-style workflows

Inspect translated Fory IDL from proto/fbs input:

# Print translated Fory IDL to stdout
foryc schema.proto --emit-fdl

# Write translated Fory IDL to a directory
foryc schema.fbs --emit-fdl --emit-fdl-path ./translated

Import Path Resolution

When compiling Fory IDL files with imports, the compiler searches for imported files in this order:

Relative to the importing file (default) - The directory containing the file with the import statement is always searched first, automatically. No -I flag needed for same-directory imports.
Each -I path in order - Additional search paths specified on the command line

Same-directory imports work automatically:

// main.fdl
import "common.fdl";  // Found if common.fdl is in the same directory

# No -I needed for same-directory imports
foryc main.fdl

Example project structure:

project/
├── src/
│   └── main.fdl          # import "common.fdl";
└── libs/
    └── common.fdl

Without -I (fails):

$ foryc src/main.fdl
Import error: Import not found: common.fdl
  Searched in: /project/src

With -I (succeeds):

$ foryc src/main.fdl -I libs/
Compiling src/main.fdl...
  Resolved 1 import(s)

Supported Languages

Language	Flag	Output Extension	Description
Java	`java`	`.java`	POJOs with Fory annotations
Python	`python`	`.py`	Dataclasses with type hints
Go	`go`	`.go`	Structs with struct tags
Rust	`rust`	`.rs`	Structs with derive macros
C++	`cpp`	`.h`	Structs with FORY macros
C#	`csharp`	`.cs`	Classes with Fory attributes
JavaScript	`javascript`	`.ts`	Interfaces with registration function
Swift	`swift`	`.swift`	`@ForyObject` Swift models

Output Structure

Java

generated/
└── java/
    └── com/
        └── example/
            ├── User.java
            ├── Order.java
            ├── Status.java
            └── ExampleForyRegistration.java

One file per type (enum or message)
Package structure matches Fory IDL package
Registration helper class generated

Python

generated/
└── python/
    └── example.py

Single module with all types
Module name derived from package
Registration function included

Go

generated/
└── go/
    └── example/
        └── example.go

Single file with all types
Directory and package name are derived from go_package or the Fory IDL package
Registration function included

Rust

generated/
└── rust/
    └── example.rs

Single module with all types
Module name derived from package
Registration function included

C++

generated/
└── cpp/
    └── example.h

Single header file
Namespace matches package (dots to ::)
Header guards and forward declarations

JavaScript

generated/
└── javascript/
  └── example.ts

Single .ts file per schema
export interface declarations for messages
export enum declarations for enums
Discriminated unions with case enums
Registration helper function included

C#

generated/
└── csharp/
    └── example/
        └── example.cs

Single .cs file per schema
Namespace uses csharp_namespace (if set) or Fory IDL package
Includes registration helper and ToBytes/FromBytes methods
Imported schemas are registered transitively (for example root.idl importing addressbook.fdl and tree.fdl)

Swift

generated/
└── swift/
    └── addressbook/
        └── addressbook.swift

Single .swift file per schema
Package segments are mapped to nested Swift enums (for example addressbook.* -> Addressbook.*)
Generated messages/unions/enums use @ForyObject and @ForyField(id: ...)
Union types are generated as tagged enums with associated payload values
Each schema includes ForyRegistration and toBytes/fromBytes helpers
Imported schemas are registered transitively by generated registration helpers

C# IDL Matrix Verification

Run the end-to-end C# IDL matrix (FDL/IDL/Proto/FBS generation plus roundtrip tests):

cd integration_tests/idl_tests
./run_csharp_tests.sh

This runner executes schema-consistent and compatible roundtrips across:

addressbook, auto_id, complex_pb primitives
collection and union/list variants
optional_types
any_example (.fdl) and any_example (.proto)
tree and graph reference-tracking cases
monster.fbs and complex_fbs.fbs
root.idl cross-package import coverage
evolving schema compatibility cases

Swift IDL Matrix Verification

Run the end-to-end Swift IDL matrix (FDL/IDL/Proto/FBS generation plus roundtrip tests):

cd integration_tests/idl_tests
./run_swift_tests.sh

This runs:

local Swift IDL roundtrip tests in both compatible and schema-consistent modes
Java-driven peer roundtrip validation with IDL_PEER_LANG=swift

The script also sets DATA_FILE* variables so file-based roundtrip paths are exercised.

Build Integration

Maven (Java)

Add to your pom.xml:

<build>
  <plugins>
    <plugin>
      <groupId>org.codehaus.mojo</groupId>
      <artifactId>exec-maven-plugin</artifactId>
      <version>3.1.0</version>
      <executions>
        <execution>
          <id>generate-fory-types</id>
          <phase>generate-sources</phase>
          <goals>
            <goal>exec</goal>
          </goals>
          <configuration>
            <executable>foryc</executable>
            <arguments>
              <argument>${project.basedir}/src/main/fdl/schema.fdl</argument>
              <argument>--java_out</argument>
              <argument>${project.build.directory}/generated-sources/fory</argument>
            </arguments>
          </configuration>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>

Add generated sources:

<build>
  <plugins>
    <plugin>
      <groupId>org.codehaus.mojo</groupId>
      <artifactId>build-helper-maven-plugin</artifactId>
      <version>3.4.0</version>
      <executions>
        <execution>
          <phase>generate-sources</phase>
          <goals>
            <goal>add-source</goal>
          </goals>
          <configuration>
            <sources>
              <source>${project.build.directory}/generated-sources/fory</source>
            </sources>
          </configuration>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>

Gradle (Java/Kotlin)

Add to build.gradle:

task generateForyTypes(type: Exec) {
    commandLine 'foryc',
        "${projectDir}/src/main/fdl/schema.fdl",
        '--java_out', "${buildDir}/generated/sources/fory"
}

compileJava.dependsOn generateForyTypes

sourceSets {
    main {
        java {
            srcDir "${buildDir}/generated/sources/fory"
        }
    }
}

Python (setuptools)

Add to setup.py or pyproject.toml:

# setup.py
from setuptools import setup
from setuptools.command.build_py import build_py
import subprocess

class BuildWithForyIdl(build_py):
    def run(self):
        subprocess.run([
            'foryc',
            'schema.fdl',
            '--python_out', 'src/generated'
        ], check=True)
        super().run()

setup(
    cmdclass={'build_py': BuildWithForyIdl},
    # ...
)

Go (go generate)

Add to your Go file:

//go:generate foryc ../schema.fdl --lang go --output .
package models

Run:

go generate ./...

Rust (build.rs)

Add to build.rs:

use std::process::Command;

fn main() {
    println!("cargo:rerun-if-changed=schema.fdl");

    let status = Command::new("foryc")
        .args(&["schema.fdl", "--rust_out", "src/generated"])
        .status()
        .expect("Failed to run foryc");

    if !status.success() {
        panic!("Fory IDL compilation failed");
    }
}

CMake (C++)

Add to CMakeLists.txt:

find_program(FORY_COMPILER foryc)

add_custom_command(
    OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/generated/example.h
    COMMAND ${FORY_COMPILER}
        ${CMAKE_CURRENT_SOURCE_DIR}/schema.fdl
        --cpp_out ${CMAKE_CURRENT_SOURCE_DIR}/generated
    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/schema.fdl
    COMMENT "Generating Fory IDL types"
)

add_custom_target(generate_fory_idl DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/generated/example.h)

add_library(mylib ...)
add_dependencies(mylib generate_fory_idl)
target_include_directories(mylib PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/generated)

Bazel

Create a rule in BUILD:

genrule(
    name = "generate_fdl",
    srcs = ["schema.fdl"],
    outs = ["generated/example.h"],
    cmd = "$(location //:fory_compiler) $(SRCS) --cpp_out $(RULEDIR)/generated",
    tools = ["//:fory_compiler"],
)

cc_library(
    name = "models",
    hdrs = [":generate_fdl"],
    # ...
)

Error Handling

Syntax Errors

Error: Line 5, Column 12: Expected ';' after field declaration

Fix: Check the indicated line for missing semicolons or syntax issues.

Duplicate Type Names

Error: Duplicate type name: User

Fix: Ensure each enum and message has a unique name within the file.

Duplicate Type IDs

Error: Duplicate type ID 100: User and Order

Fix: Assign unique type IDs to each type.

Unknown Type References

Error: Unknown type 'Address' in Customer.address

Fix: Define the referenced type before using it, or check for typos.

Service RPC request and response types are validated in the same way: an RPC such as rpc SayHello (HelloRequest) returns (HelloReply); must reference defined message types, otherwise the validator reports an Unknown type '...' error on the RPC line.

Duplicate Field Numbers

Error: Duplicate field number 1 in User: name and id

Fix: Assign unique field numbers within each message.

Best Practices

Project Structure

project/
├── fdl/
│   ├── common.fdl       # Shared types
│   ├── user.fdl         # User domain
│   └── order.fdl        # Order domain
├── src/
│   └── generated/       # Generated code (git-ignored)
└── build.gradle

Version Control

Track: Fory IDL schema files
Ignore: Generated code (can be regenerated)

Add to .gitignore:

# Generated Fory IDL code
src/generated/
generated/

CI/CD Integration

Always regenerate during builds:

# GitHub Actions example
steps:
  - name: Install Fory IDL Compiler
    run: pip install ./compiler

  - name: Generate Types
    run: foryc fdl/*.fdl --output src/generated

  - name: Build
    run: ./gradlew build

Schema Evolution

When modifying schemas:

Never reuse field numbers - Mark as reserved instead
Never change type IDs - They're part of the binary format
Add new fields - Use new field numbers
Use optional - For backward compatibility

message User [id=100] {
    string id = 1;
    string name = 2;
    // Field 3 was removed, don't reuse
    optional string email = 4;  // New field
}

Troubleshooting

Command Not Found

foryc: command not found

Solution: Ensure the compiler is installed and in your PATH:

pip install -e ./compiler
# Or add to PATH
export PATH=$PATH:~/.local/bin

Permission Denied

Permission denied: ./generated

Solution: Ensure write permissions on the output directory:

chmod -R u+w ./generated

Import Errors in Generated Code

Java: Ensure Fory dependency is in your project:

<dependency>
  <groupId>org.apache.fory</groupId>
  <artifactId>fory-core</artifactId>
  <version>${fory.version}</version>
</dependency>

Python: Ensure pyfory is installed:

pip install pyfory

Go: Ensure fory module is available:

go get github.com/apache/fory/go/fory

Rust: Ensure fory crate is in Cargo.toml:

[dependencies]
fory = "x.y.z"

C++: Ensure Fory headers are in include path.