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I want to learn how interpreters and compilers work, so I'll build one :D
.package(url: "https://github.com/YamiDaisuke/hermes-swift.git", from: "v1.0.1")

Hermes Swift

macOS Test Linux Test codecov License: MIT

A Lexer, Parser, Interpreter, VM and compiler library built with swift, supporting the addition of new languages and currently implemented for the Monkey Language.


With this project I want to get a better understanding on how compilers and interpreters works, while also have fun in the process 😎🤓. For that purpose I'll be following this book series:

What I like about this two books is that requires no third party or existing libraries it takes you through the entire process of making the interpreter and the compiler. The result might not be a production ready software but will accomplish what I'm looking for which is to learn how they work internally without any black box magic.

The book uses a language called Monkey designed for the book. From the book page the language is described as follows:

Monkey has a C-like syntax, supports variable bindings, prefix and infix operators, has first-class and higher-order functions, can handle closures with ease and has integers, booleans, arrays and hashes built-in.

The book uses Go Lang for the implementation but I want to do it in swift just because I like swift ❤️. For starters I will be using an XCode to compile and test my implementation, later on I'll try to implement a standalone compilation. As in the book this implementation will not use third party libs only standard swift libraries. Maybe I'll include a lib to simplify the CLI implementation but nothing else.

Before you begin

After cloning this repo run ./scripts/install-hooks.sh to configure git hooks. This will ensure documentation is update on each commit and will prevent broken tests and lint errors to be pushed in the repo.


Documentation can be found here

Install Monkey CLI

Download the latest release binaries or compile using swift build -c release and include the monkey binary in your path. Then execute:

## To start the interactive REPL console
$ monkey

### To evaluate a file containing Monkey code
$ monkey filename.mky

Language Extra Features

Besides the language features include in the books, I have added the following by my own:

String scape sequences

Support of scape characters inside string literals, currently supported values are:

\n = line break
\t = tabulation
\" = Double quote character inside a string
\\ = Back slash character

Any other value will produce an error

String compare

Comparison of String values with any other value.

  • Two strings will be compared based on their characters
  • A string will be equal to boolean true if it is not empty
  • A string will be equal to boolean false if it is empty
  • A string will never be equal to any other value

Constants and variables

Support of declaration constants using let <identifier> = <expression>; and variables using var <identifier> = <expression>;.

Variables can later be assigned to new values but the value must be the same type for example the following lines will produce an error:

var a = 10;
a = true;


Support for single line and multiline comments using // and /**/ respectively

let a = 42; // Meaning of life
Well it actually is:
Answer to the Ultimate Question of Life, the Universe, and Everything


Outside the scope of the book my current stretch goals (so far) and aditional taks are:

  • Create each component as a swift package not tied to the Monkey language, the idea is that you can implement a series of protocols and classes and provide your very own language that should work with this library
  • Modify Lexer to include line and column information for each token
  • Modify Lexer to read from files and parse tokens line by line
  • Implement CI steps
  • Create an standalone build and test script to work without XCode
  • CI steps to build release artifacts
  • Support Linux

After this point TODO will be kept using github projects.


Initially, I named the project "Rosetta" thinking about the archeological artifact associated with language translation and interpretation, the name made sense however I completely forgot about Apple's Rosetta, only after a couple of days working on the repo the name clicked in my mind. To avoid the confusion the name can generate I have switched to the name "Hermes" after the greek god associated with language.

I'll remove all references to previous name, but I might miss a couple, please forgive me.


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Last commit: Yesterday

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Release Notes

Release v1.0.1
6 weeks ago


  • Monkey objects now support equality comparison
  • Monkey support single line (//) and multiline (/* */) comments


  • Code refactor and quality improvements


  • Fixed REPL copy and paste actions

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