Harlan Haskins

Software Engineer | About | Projects | Resume | Contact

Building a Compiler in Swift with LLVM, Part 1: Introduction and the Lexer

Compilers are some of the most fascinating programs because they touch every aspect of Computer Science, from CLI and API design to calling conventions and platform-specific optimizations. Yet, many developers have a fearful reverence for them that makes them seem unapproachable. I think this is unnecessary.

Difficulties in compiler development arise mainly because of human concerns: semantic correctness and following a strict standard. Those concerns are easier to satisfy when designing a new language, as the standard is usually malleable.

This is the first part in a 4-part series where we'll build a compiler for the basic form of the Kaleidoscope sample language.

Quick Links

Introduction to LLVM

Traditional compilers handle every part of the compilation process themselves. After lexing, parsing, and semantic analysis, they branch out into code generation paths for all the different target architectures they support. This means that the compiler has to duplicate optimizations and code generation logic, and that every compiler needs its own bespoke register allocator, optimizer, code generator, etc. for x86, ARM, PowerPC, and so on. Gross.

Enter LLVM

LLVM (the 'Low Level Virtual Machine') is a suite of tools that facilitate development of compilers. These tools are all packaged into individual libraries so it's easier to integrate them into other projects.

At the heart of the LLVM project is one specific tool: a programming language called LLVM Intermediate Representation — abbreviated LLVM IR. Instead of writing new code generation for every architecture, compiler engineers can lower their language to LLVM IR and get fantastic code generation and optimizations for free.

LLVM IR is a typed assembly language with support for high-level features like functions (with different calling conventions), named types, and abstractions for gritty details like pointer arithmetic. What’s more, LLVM IR is a very simple language. It’s easy to get started emitting basic IR, and later, when you need it, to opt in to more advanced features and optimizations.


Kaleidoscope is a simple imperative language that's used in the official LLVM tutorial. It has floating-point numbers and supports functions, recursion, and binary operator math.

We can define functions like this:

def foo(n) (n * 100.34);

And we can scale up to more complex expressions and definitions like so:

extern sqrt(n)

def foo(n) (n * sqrt(n * 200) + 57 * n % 2);

The Lexer

The first step in programming language development is reading in the source text and translating it to a stream of manipulatable tokens. These tokens are usually one of a set of values, some of which might have extra data attached. Naturally, a Swift enum makes the most sense for a token.

We'll want a specific case for each bit of syntax exposed in that grammar, so it should look something like this:

enum Token {
    case leftParen, rightParen, def, extern, comma, semicolon, `if`, then, `else`
    case identifier(String)
    case number(Double)
    case `operator`(BinaryOperator)

That BinaryOperator there corresponds to this simple enum:

enum BinaryOperator: Character {
    case plus = "+"
    case minus = "-"
    case times = "*"
    case divide = "/"
    case mod = "%"

Once we define this, we can write a small Lexer that will deconstruct the source text into a list of these tokens, while ignoring whitespace.

First, we want to define a class called Lexer that holds a reference to a String.

class Lexer {
    let input: String
    var index: String.Index

    init(input: String) {
        self.input = input
        self.index = input.startIndex

Once we do that, we'll want a method that reads in characters until it can spit out a token. We'll call it advanceToNextToken(). The full code is attached at the bottom of the post, but advanceToNextToken basically does this, in order:

  • Skip whitespace by advancing the index while the current character is a space.
  • Check if the non-whitespace character is one of the single-character tokens.
    • If it is, then return that token.
  • Otherwise, read an identifier.
    • If that identifier is an integer or decimal number, then return the number token.
    • Check if it's one of the keyword tokens, def, extern, if, then, and else.
      • If it is, return it.
    • Return a generic identifier token.

With this, we can take input from the programmer and read it out into a series of tokens that we can pass to the parser. Add one more method, lex() that reads all available tokens from the input and returns a list.

func lex() -> [Token] {
    var toks = [Token]()
    while let tok = advanceToNextToken() {
    return toks

Let's give it a shot!

let toks = Lexer(input: "def foo(n) (n * 100.35);").lex()
// [Token.def, Token.identifier("foo"), Token.leftParen, Token.identifier("n"), Token.rightParen, Token.leftParen, Token.identifier("n"), Token.operator(BinaryOperator.times), Token.number(100.34), Token.rightParen, Token.semicolon]

It works!

Hopefully you're now aware how simple it can be to write a lexer for a fairly feature-rich little toy language. In later parts we'll parse these tokens into an AST then generate the LLVM IR for them using a library I'm working on, LLVMSwift.

Happy compiling!

Code Listing

The full code is listed below and available as a gist: