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Open source binary data format for Swift AST with tooling
.package(url: "https://github.com/App-Maker-Software/BinarySwiftAST.git", from: "0.50500.0")

Binary Swift AST

A small data medium for transferring Swift ASTs. Primarily used in conjunction with SwiftInterpreter.


While it is already possible to work with the Swift AST in Swift using SwiftSyntax, there are several problems solved with "Binary Swift AST".

  1. Transferability - There is no good way to transfer Swift AST data across a network (maybe it's possible with the .swiftmodule format, but that contains more information than just the AST). BinarySwiftAST solves this by converting the AST into a binary data format which can be saved as a file or sent over a network.
  2. AST Format Stability - Neither is there a stable public data format for the Swift AST. While the -dump-ast mode exists in the compiler, it isn't documented and is only meant for debugging purposes. BinarySwiftAST solves this with its binary definition of the AST which can remain stable.
  3. Implementation Scope - While you can access the Swift Compiler's AST Node through SwiftSyntax, there is a lot extra happening beyond what is needed to represent the AST. BinarySwiftAST solves this by implementing a compact AST in a flat byte array (without linked lists).
  4. AST Data Size - While -dump-ast and other ways of getting Swift AST data tend to rely on plaintext, BinarySwiftAST can represent the same data more compactly by "packing" the data to store only necessary information.
  5. Node Access Speed - Checking node types and walking the AST tree can be expensive and prone to stack overflows with SwiftSyntax, but BinarySwiftAST can represent the AST in its unpacked format which allows for fast reading and type casting.
  6. Footprint Size - All AST nodes are very similar, yet there are about 250 different node types in Swift. So by using SwiftSyntax's method of having a struct for each node type, we greatly increase the footprint size of an application. BinarySwiftAST solves this by providing two ways of consuming the AST--with and without the node type wrappers.


Declare SwiftPM dependency with release tag

Add this repository to the Package.swift manifest of your project:

// swift-tools-version:5.2
import PackageDescription

let package = Package(
  name: "MyTool",
  dependencies: [
    .package(name: "BinarySwiftAST", url: "https://github.com/App-Maker-Software/BinarySwiftAST.git", .exact("<#Specify Release tag#>")),
  targets: [
      name: "MyTool",
      dependencies: [
        .product(name: "<#Specify Product Name#>", package: "BinarySwiftAST")

Replace <#Specify Release tag#> with the version of BinarySwiftAST that you want to use (see the following table for mapping details).

Xcode Release Swift Release Tag BinarySwiftAST Release Tag
Xcode 13.0 swift-5.5-RELEASE 0.50500.0

Replace <#Specify Product Name#> with the product in BinarySwiftAST that you want to use (see the following table for product details).

Product Name Function
BinarySwiftAST Just the core ASTNode type and tooling
BinarySwiftASTWrappers Core plus AST node type wrappers
BinarySwiftASTConstructor Builds a BinarySwiftAST tree by using SwiftSyntax
ASTWrapperRemover A tool to remove node type wrapper usage from .swift source files


Usage examples are given for each product.

Using BinarySwiftAST

import BinarySwiftAST

// to start working with raw ast data
let rawASTData: Data = ...
let astNode: ASTNode = ArraySliceASTNode(data: rawASTData, offset: 0)

// print the node type id

// print the body type id, where the body type is defined in types.swift

// returns nil if syntaxType is not 3
let castedASTNode: ASTNode? = astNode.as(typeId: 3)

// returns a string, assumes type is TokenSyntax
let text: String? = astNode.text

// prints how many element sub-nodes there are, assuming we are a collection type

// gets all subnodes on node (properties/elements), shallow
let subNodes: [ASTNode] = astNode.allChildren

// gets a sub node property at index 1
let subNode1 = astNode.property(i: 1)

// gets a sub node list element at index 2
let subNode2 = astNode.element(i: 2)  

Using BinarySwiftASTWrappers

import BinarySwiftASTWrappers

// wrap an ast node
let someASTNode: ASTNode = ...
let wrappedASTNode: AnySyntax = AnySyntax.convert(from: someASTNode)

// get the underlying raw ast node
let astNode: ASTNode = wrappedASTNode._node

// print the node type id

// debug helpers
// print the syntax node type name

// prints the syntax tree
wrappedASTNode.printTree(depth: 2)

// cast and force unwrap
let tokenSyntax: TokenSyntax = wrappedASTNode._asTokenSyntax!

// get token's text body, in case of corrupted data will be nil
let text: String = tokenSyntax.text!

// type erase
let anySyntax: AnySyntax = tokenSyntax.toAny()

// another cast
let codeBlockItemListSyntax: CodeBlockItemListSyntax = wrappedASTNode._asCodeBlockItemListSyntax!

// prints how many elements there are

// gets first element
let first: CodeBlockItemSyntax? = codeBlockItemListSyntax._first

// gets last element
let last: CodeBlockItemSyntax? = codeBlockItemListSyntax._last

// gets all elements
let els: [CodeBlockItemSyntax] = codeBlockItemListSyntax._els

// another cast
let ifStmtSyntax: IfStmtSyntax = wrappedASTNode._asIfStmtSyntax!

// sub property which should be present unless we have bad data
let ifKeyword: TokenSyntax = ifStmtSyntax._ifKeyword_pos_2!

// sub property which might or might not be present
let elseKeyword: TokenSyntax? = ifStmtSyntax._elseKeyword_pos_105

Using BinarySwiftASTConstructor

import BinarySwiftASTConstructor

let sourceCode: String = """
  var message = "hello"
  message = message + " binary swift ast!"
  return message

let rawASTData: Data = BinarySwiftASTConstructor.constructAST(from: sourceCode)

// later if you wish to use the ast data
import BinarySwiftAST
let astNode: ASTNode = ArraySliceASTNode(data: rawASTData, offset: 0)

// and if you wish to use wrappers
import BinarySwiftASTWrappers
let wrappedASTNode: SourceFileSyntax = SourceFileSyntax.convert(from: astNode)

Using ASTWrapperRemover

The purpose of this module is to convert a .swift file which uses BinarySwiftASTWrappers to an identical one that only relies on BinarySwiftAST (without wrappers). The reason you would want to remove the wrappers is that they leave a large footprint, which is undesirable for some wanting to have the AST usable somewhere like a mobile device.

Example of a conversion:

let ifKeyword: TokenSyntax = ifStmtSyntax._ifKeyword_pos_2!
let ifKeyword: ASTNode = ifStmtSyntax.property(i: 2)!

It was able to convert it to a property access for element 2 because of the pos_2 in the property name.

This this product is used by the Swift Interpreter's Developer CLI.

Building .gyb Files

GYB is used to generate many source files, but most of this is already set up for you. Just have Python installed and run ./build-script to generate Swift files. If you are trying to add support for a new version of Swift, you will need to update the gyb_syntax_support folder to reflect what's in the Swift compiler at your desired version. You can probably find the correct files already here.

Building Products

Building instructions are given for each product.

Building BinarySwiftAST, BinarySwiftASTWrappers, and ASTWrapperRemover

Nothing complicated. Both should build easily as they're pure Swift.

Building BinarySwiftASTConstructor

As this product depends on SwiftSyntax, it must be linked with the Swift compiler's libSyntax library. The problem is that libSyntax is not included in this repo, so you must link it yourself. For most, running swift package generate-xcodeproj and then building on a Mac with Xcode will work because for some reason Xcode needs a project file to have a proper search paths setup. But even then many times (usually between Swift version updates) the expected libSyntax version will differ from what's installed in the Xcode toolchain. Follow instructions on the SwiftSyntax readme for information for how to get the specific lib_InternalSwiftSyntaxParser.dylib you need for linking.

If you are just wishing to generate some BinarySwiftAST data, you can use the swiftast CLI tool maintained by App Maker. That way you don't have to build BinarySwiftASTConstructor from source.

brew install App-Maker-Software/tools/swift-ast


There are currently 6 suits of tests:

  1. BinarySwiftASTTests
  2. BinarySwiftASTWrappersTests
  3. WrapperBuildersTests
  4. BinarySwiftASTConstructorTests
  5. ASTWrapperRemoverTests
  6. PackingAndUnpackingTests

All tests are currently passing.

Xcode may not play nice with the SwiftSyntax dependency, so the easiest way to test is to run swift test.



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