Building an AST Parser Pt 1: Scanning

September 01, 2018

Before we get into the nitty gritty of writing the actual parser, let's get a lay of the land. Rob Nystrom does a great job with this and if you want the details you should check out A Map of the Territory chapter. I'll give you quick tour.

When we run or compile a text file containing source code there are a set of steps that take us to the machine code or bytecode that is eventually executed. The first step is scanning or lexing

A scanner (or “lexer”) takes in the linear stream of characters and chunks them together into a series of something more akin to “words”. In programming languages, each of these words is called a token. Some tokens are single characters, like ( and ,. Others may be several characters long, like numbers (123), string literals ("hi!"), and identifiers (min).

The next step is parsing. Parsing takes the tokens built in the scanning step and transforms it into a tree data structure that better represents the nested nature of the language's grammer.

After parsing there's static analyzing, generating intermediate languages and so on, none of which we'll get to in this series. I'll be doing that as I learn from the book but to solve the problem at hand all I need is that tree structure which is commonly called an abstract syntax tree.

In the scanning step, which is what we'll focus on here, we're going to scan the source code and generate a series of tokens. Swift's lexical grammar is pretty complex so we're going to start really simple.

We'll build this tool as a Swift library so we'll start with

$ mkdir SwiftInspector
$ cd SwiftInspector
$ swift package init

By default the package manage will create a library (as opposed to an executable). If you'd like to work in Xcode you can follow this up with

$ swift package generate-xcodeproj
$ open SwiftInspector.xcodeproj

We'll start in here by creating a new file named Token.swift. Given source code like this

var foo = "bar"

We can split this up into four distinct pieces - var, foo, = and "bar". We call each of these pieces a lexeme and in scanning or in lexical analysis our goal is to generate tokens for each lexeme. This seems like a simple bit of source code to parse but there's a fair bit going on here so we'll focus on defining tokens to represent single character lexemes. Inside Token.swift create the following type

public enum TokenType {
    // Single character
    case leftParen, rightParen
    case eof

For each token we're going to indicate what type of token it is and since there's a limited set defined in Swift's lexical grammar we can use an enum. Single character lexemes are the most straightforward to start with. As you will see in a bit, with lexemes that span multiple characters, generating a token means scanning forward until we can somehow determine the end of that particular lexeme. In a toy language this may be limited to whitespace, identifiers and so on but with a complex set of grammars like Swift we'll eventually need to expand this to account for multiline comments, multiline string literals and even arbitary whitespace when listing out array values.

Since we're taking baby steps here we're going to start by generating tokens for opening and closing parentheses. There's more than one way to approach token generation and I'm going to define tokens for individual characters like braces, brackets and parens. SwiftFormat, a command line Swift formatting tool, defines its own AST parser and the approach there is to define tokens for start and ends of scope with associated values carrying the type of scope. So instead of an indivual token type for an opening paren (, the lexer would emit a token called .startOfScope("("). I don't know which approach works better for my tool yet but we'll try the most straighforward approach first and refactor later if we need to.

We also need to generate a token when we reach the end of the source code and for that we'll define an end of file token type, eof.

Now we can define a type for Token.

public struct Token {
    let type: TokenType
    let line: Int
    public init(type: TokenType, line: Int) {
        self.type = type
        self.line = line

Token is fairly simple for now and defines the type of the token and the line number in the source file on which the lexeme was defined. Tokens typically carry information about column and line numbers so that when errors are reported we can direct users right to the spot they need to modify but line numbers should suffice for my needs. This way if I wanted to ask a student to do something on a specific line I can verify it in a generated token.


Once we have a token the next step is to define a lexer or a scanner that scans each character of the source code and generates a set of tokens.

public final class Lexer {
    private let source: String
    private var tokens: Array<Token>

    init(source: String) {
        self.source = source
        self.tokens = []

We'll initialize the lexer with the source code, which for now will be a String literal. Later on we'll define a separate initializer to accept a path to a file.

We'll also need a few variables to keep track of our position in the source code.

private var start = 0
public var current = 0
private var line = 1

start defines the start position, not of the file or source code, but the position of each lexeme. We'll need to keep track of the starting position of individual lexemes when we move to multi character lexemes. current defines the lexer's current scan position and line the line number we're currently on. As the lexer scans the source code we'll need a way to indicate that we've reached the end. Let's define a computed property for this

var isAtEnd: Bool {
    return current >= source.count

With these helper properties we can define a scan method.

public func scan() throws -> [Token] {
    while !isAtEnd {
        start = current
        let token = try tokenize()
    let token = Token(type: .eof, line: line)
    return tokens

scan() will be a throwing method in the event we reach an invalid token. While we're not at the end of the file, we'll set start to current. The logic here is that after every lexeme is tokenized, we'll set start to current to identify the starting position of the following lexeme.

When we're at the start, let's call tokenize() to get a token back. If this succeeds, we'll add the token to the tokens array. Once the loop condition terminates and we reach the end of the file, we'll add the eof token to the array and return it.

So what happens in tokenize()? A lot actually so let's break it down.

private func tokenize() throws -> Token {}

The first thing we need to do is check the current character. To do this let's define two helper methods. First is a method to get any character from the source string by providing an offset distance from the start of the source.

/// Returns character at specified distance from the start of the source string
/// - Parameters:
///   - source: The string to scan through
///   - offset: The distance to the offset index
/// - Returns: UnicodeScalar at specified index position
public func character(in source: String, atIndexOffsetBy offset: Int) -> UnicodeScalar {
    let index = source.unicodeScalars.index(source.unicodeScalars.startIndex, offsetBy: offset)
    return source.unicodeScalars[index]

Since this is a library let's add some unit tests to make sure our function performs exactly as we expect. Add a new file to the SwiftInspectorTests group named LexerTests.swift.

import XCTest
@testable import SwiftInspector

class LexerTests: XCTestCase {

    let source = "var abc = 1"
    lazy var lexer: Lexer = {
        return Lexer(source: source)
    func testCharacterAtIndexOffsetBy() {
        let first: UnicodeScalar = "v"
        let middle: UnicodeScalar = "b"
        let last: UnicodeScalar = "1"
        XCTAssertEqual(lexer.character(in: source, atIndexOffsetBy: 0), first)
        XCTAssertEqual(lexer.character(in: source, atIndexOffsetBy: 5), middle)
        XCTAssertEqual(lexer.character(in: source, atIndexOffsetBy: source.count - 1), last)

Here we've defined a simple unit test to make sure that the characters specified at the offset indexes are the ones we get back. Hit Cmd+U to run the tests; everything should pass.

Back to Lexer.swift, let's define an advance() function.

/// Consume character at `current` position and return it
/// Moves current forward after consuming character at `current`
/// - Returns: UnicodeScalar value at source at position offset by `current`
public func advance() -> UnicodeScalar {
    // Consume character at current before moving current forward
    let c = character(in: source, atIndexOffsetBy: current)
    current += 1
    return c

As the documentation states advance() consumes the character at current, returns it and increments current. Let's add a test for this in LexerTests.swift as well.

func testAdvanceConsumesCharacterAndMovesForward() {
    XCTAssertTrue(lexer.current == 0)
    let c = lexer.advance()
    XCTAssertTrue(c == UnicodeScalar("v"))
    XCTAssertTrue(lexer.current == 1)

These methods are definitely implementation details of Lexer that we don't need to expose but remember that you can only test the public interface of an object. In addition, to properly test all of our code we need to avoid, as much as possible, manipulating hidden state inside of a function, passing all values as arguments instead. For example, inside advance(), we're manipulating the value of current and while we are testing this behavior, its much too tightly coupled to the internals of the lexer. Perhaps I'll fix this in a refactor.

Let's use these methods to provide an implementation for tokenize(). The first thing we need to do is to get the current character. We'll use the advance() method for this so that after getting the character back we'll move current forward.

let character = advance()

Next, we'll use a switch statement to switch on the value we just obtained.

switch character {
    case "(": return Token(type: .leftParen, line: line)
    default: throw LexerError.invalidToken

If the character matches a opening or right parenthesis, we'll return a token instance representing it. In the default case, we'll assume the character we've consumed is invalid and we'll throw an error. We need to define a type for this, which we'll add to the top of the file.

public enum LexerError: Error {
    case invalidToken

This should be enough to scan a really simple string. Before we can do that though we need to add Equatable conformance to Token and TokenType so that we can compare them.

extension TokenType: Equatable {
    public static func ==(lhs: TokenType, rhs: TokenType) -> Bool {
        switch (lhs, rhs) {
        case (.rightParen, .rightParen): return true
        case (.leftParen, .leftParen): return true
        case (.eof, .eof): return true
        default: return false


extension Token: Equatable {
    public static func ==(lhs: Token, rhs: Token) -> Bool {
        return lhs.type == rhs.type && lhs.line == rhs.line

extension Token: CustomStringConvertible {
    public var description: String {
        return "\(type) - line: \(line)"

Let's add a new file called TokenTests.swift to validate the tokens we get back from the scanning operation.

import XCTest
@testable import SwiftInspector

class TokenTests: XCTestCase {}

We'll start by testing the base case of an empty string as source. Running scan() on this should return a single EOF token.

func testEmptyString() {
    let input = ""
    let output: [Token] = [
        Token(type: .eof, line: 1)
    let lexer = Lexer(source: input)
    XCTAssertEqual(try! lexer.scan(), output)

For the next test case we'll look for a leftParen token.

func testSingleParen() {
    let input = "("
    let output: [Token] = [
        Token(type: .leftParen, line: 1),
        Token(type: .eof, line: 1)
    let lexer = Lexer(source: input)
    XCTAssertEqual(try! lexer.scan(), output)

Run your tests and everything should pass. Generating tokens for the remaining single character lexemes is no different. These are defined under the Punctuation section of the Lexical Structure document. Let's add them to our token type enum.

public enum TokenType {
    // Punctuation
    case leftParen, rightParen
    case leftBrace, rightBrace
    case leftBracket, rightBracket
    case dot, comma, colon, semicolon
    case eof

I'll leave it up to you to implement the logic for these tokens. As an aside you might be wondering why I've excluded characters like = or + above. While those are technically single character lexemes, the existence of lexemes like == and += means we need to add some additional logic to the lexer to be able to handle it. All in due time.

In addition, you'll notice that to be able to run tests on the new token types we'll need to extend Equatable conformance to cover all the new cases. This is going to get really tedious as the type grows. I highly recommend using Sourcery to automate this and if you're interested you can read my follow up post.

In the next post in this series, let's tackle lexemes that longer than a single character.