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Code Structure

I tried to make the code of the compiler and vlib as simple and readable as possible. One of V's goals is to be open to developers with different levels of experience in compiler development. Compilers don't need to be black boxes full of magic that only few people understand.

The V compiler is modular, and can be used by other applications. It is located in cmd/v/ and vlib/v/.

The most important and useful command to remember when working on the V compiler is v self. It rebuilds the V compiler.

Be careful, if you introduce a breaking change and rebuild V, you will no longer be able to use V to build itself. So it's a good idea to make a backup copy of a working compiler executable.

But don't worry, you can always simply run make (or make.bat), it will download the C version of the compiler and rebuild it from scratch.

The architecture of the compiler is very simple and has three distinct steps:

Parse/generate AST (v.parser) => Check types (v.checker) => Generate C/JavaScript/machine code (v.gen)

The main files are:

  1. cmd/v/v.v The entry point.
  1. vlib/v/scanner The scanner's job is to parse a list of characters and convert them to tokens.

  2. vlib/v/token This is simply a list of all tokens, their string values, and a couple of helper functions.

  3. vlib/v/parser The parser. It converts a list of tokens into an AST. In V, objects can be used before declaration, so unknown types are marked as unresolved. They are resolved later in the type checker.

  4. vlib/v/table V creates one table object that is shared by all parsers. It contains all types, consts, and functions, as well as several helpers to search for objects by name, register new objects, modify types' fields, etc.

  5. vlib/v/checker Type checker and resolver. It processes the AST and makes sure the types are correct. Unresolved types are resolved, type information is added to the AST.

  6. vlib/v/gen/c C backend. It simply walks the AST and generates C code that can be compiled with Clang, GCC, Visual Studio, and TCC.

  7. vlib/v/gen/js JavaScript backend. It simply walks the AST and generates JS code that can be executed on the browser or in NodeJS/Deno.

  8. vlib/v/gen/c/json.v defines the json code generation. This file will be removed once V supports comptime code generation, and it will be possible to do this using the language's tools.

  9. vlib/v/gen/native is the directory with all the machine code generation logic. It defines a set of functions that translate assembly instructions to machine code and build the binary from scratch byte by byte. It manually builds all headers, segments, sections, symtable, relocations, etc. Right now it only has basic support of the native platform (ELF, MACHO format).

The rest of the directories are vlib modules: builtin/ (strings, arrays, maps), time/, os/, etc. Their documentation is pretty clear.

Example Workflow for Contributing

(provided by @spytheman)

(If you don't already have a GitHub account, please create one. Your GitHub username will be referred to later as 'YOUR_GITHUB_USERNAME'. Change it accordingly in the steps below.)

  1. Fork https://github.com/vlang/v using GitHub's interface to your own account. Let's say that the forked repository is at https://github.com/YOUR_GITHUB_USERNAME/v .

  2. Clone the main v repository https://github.com/vlang/v to a local folder on your computer, say named nv/ (git clone https://github.com/vlang/v nv)

  3. cd nv

  4. git remote add pullrequest https://github.com/YOUR_GITHUB_USERNAME/v NB: the remote named pullrequest should point to YOUR own forked repo, not the main v repository! After this, your local cloned repository is prepared for making pullrequests, and you can just do normal git operations such as: git pull git status and so on.

  5. When finished with a feature/bugfix/change, you can: git checkout -b fix_alabala

    • Don't forget to keep formatting standards, run v fmt -w YOUR_MODIFIED_FILES before committing
  6. git push pullrequest # (NOTE: the pullrequest remote was setup on step 4)

  7. On GitHub's web interface, go to: https://github.com/vlang/v/pulls

    Here the UI shows a dialog with a button to make a new pull request based on the new pushed branch. (Example dialog: https://url4e.com/gyazo/images/364edc04.png)

  8. After making your pullrequest (aka, PR), you can continue to work on the branch fix_alabala ... just do again git push pullrequest when you have more commits.

  9. If there are merge conflicts, or a branch lags too much behind V's master, you can do the following:

    1. git pull --rebase origin master # solve conflicts and do git rebase --continue
    2. git push pullrequest -f # this will overwrite your current remote branch with the updated version of your changes.

The point of doing the above steps, is to never directly push to the main V repository, only to your own fork. Since your local master branch tracks the main V repository's master, then git checkout master, as well as git pull --rebase origin master will continue to work as expected (these are actually used by v up) and git can always do it cleanly.

Git is very flexible, so there are other ways to accomplish the same thing. See the GitHub flow, for more information.

Using Github's hub CLI tool

You can download the hub tool from https://hub.github.com/ . Using hub, you will not need to go through the (sometimes) slow website to make PRs. Most remote operations can be done through the hub CLI command.

NB: You still need to have a GitHub account.


(steps 1..3 need to be done just once):

  1. hub clone vlang/v my_v

  2. cd my_v

  3. hub fork --remote-name pullrequest

  4. git checkout -b my_cool_feature # Step 4 is better done once per each new feature/bugfix that you make.

Improve V by making commits:

  1. git commit -am "math: add a new function copysign"

Testing your commits locally:

You can test locally whether your changes have not broken something by running: v test-all. See TESTS.md for more details.

Publishing your commits to GitHub:

  1. git push pullrequest

Making a PR with hub:

(so that your changes can be merged to the main V repository)

  1. hub pull-request

Optionally, you can track the status of your PR CI tests with:

  1. hub ci-status --verbose

Fixing failing tests:

If everything is OK, after 5-10 minutes, the CI tests should pass for all platforms. If not, visit the URLs for the failing CI jobs, see which tests have failed and then fix them by making more changes. Just use git push pullrequest to publish your changes. The CI tests will run with your updated code. Use hub ci-status --verbose to monitor their status.


V allows you to pass custom flags using -d my_flag that can then be checked at compile time (see the documentation about compile-time if). There are numerous flags that can be passed when building the compiler with v self or when creating a copy of the compiler, that will help you when debugging.

Beware that the flags must be passed when building the compiler, not the program, so do for example: v -d time_parsing cmd/v or v -d trace_checker self. Some flags can make the compiler very verbose, so it is recommended to create a copy of the compiler rather than replacing it with v self.

Flag Usage
debugscanner Prints debug information during the scanning phase
debug_codegen Prints automatically generated V code during the scanning phase
debug_interface_table Prints generated interfaces during C generation
debug_interface_type_implements Prints debug information when checking that a type implements in interface
print_vweb_template_expansions Prints vweb compiled HTML files
time_checking Prints the time spent checking files and other related information
time_parsing Prints the time spent parsing files and other related information
trace_autofree Prints details about how/when -autofree puts free() calls
trace_autostr Prints details about .str() method auto-generated by the compiler during C generation
trace_ccoptions Prints options passed down to the C compiler
trace_checker Prints details about the statements being checked
trace_gen Prints strings written to the generated C file. Beware, this flag is very verbose
trace_parser Prints details about parsed statements and expressions
trace_thirdparty_obj_files Prints details about built thirdparty obj files
trace_usecache Prints details when -usecache is used
trace_embed_file Prints details when $embed_file is used
embed_only_metadata Embed only the metadata for the file(s) with $embed_file('somefile'); faster; for development, not distribution