The Zokka compiler is a fork of the Elm compiler. The original README that comes with the Elm compiler can be found at the end of this document.

What is Zokka?

Zokka is an alternative compiler for Elm, forked off the same underlying codebase, alongside some additions to the Elm package ecosystem. Its main purpose is to provide a method for bug fixes and patches that have accumulated over the years for the compiler and foundational packages to be used by any Elm programmer.

To accomplish this, compared to the standard Elm compiler, Zokka adds:

1. The ability to use custom package repositories
2. The ability to override dependencies (both direct and indirect) with custom versions of your own

In addition to providing a method to incorporate bug fixes and patches, Zokka also provides them directly. That means it includes on an ongoing basis:

1. A collection of API-identical versions of foundational Elm packages that incorporate bug fixes that can be used to override their usual counterparts [this is not quite done yet]
2. A minimal alternative package repository that hosts these packages
3. Minor compiler bug fixes and quality of life improvements that do not change the Elm language (ongoing)

This aims to be the entirety of Zokka's mission until 2025. This means Zokka is a quite conservative extension of Elm. What this implies is:

• A commitment to bidirectional compatibility with Elm 0.19 through the end of 2024: any code that compiles with the default Elm compiler should compile with Zokka and vice versa. In particular, this means that Zokka will not just be backwards-compatible with Elm, all Zokka code that compiles with the Zokka compiler should compile with the Elm compiler, barring compiler bugs in the Elm compiler (or Zokka compiler!) or usage of packages not in the standard Elm package repository.
• What is considered a "bug fix" will be quite tightly scoped. E.g. something like elm/compiler#1773 that adds the ability to pattern match on negative number literals would be on the fence (and likely not considered to be part of Zokka's current mission). Common candidates for compiler bug fixes that would be considered acceptable are compiler crashes and improved error messages.
• Very little actual feature development. Some experimentation may happen on side branches that may show up in both private and public channels, but those will not be given an actual Github release until 2025 (if Zokka is still relevant then).

Again, Zokka's main function in the wider Elm community (at least until 2025) is to collect bug fixes submitted by the community and merge them, not to do major Elm development itself. As a (substantial) side effect of that Zokka provides private repositories as well, largely in part because this allows individuals to fix bugs privately and use those bug fixes without exposing them to the wider world if they would like.

How do I use Zokka?

Quick Start

You can always drop in Zokka as a replacement for Elm so for example the following will work.

1. Download the Zokka binary from Github releases https://github.com/changlinli/zokka-compiler/releases.
2. Run zokka make $YOUR_ELM_MAIN_FILE with whatever flags you usually pass to the Elm compiler.
3. Everything should work just the exact same as it did before!

But that's not terribly interesting so let's look at https://github.com/changlinli/zokka-basic-example. This repository showcases a bug in the elm/core library. Normally we would be stuck with this bug and have to work around it, because we cannot fork elm/core and use with the vanilla Elm compiler.

Zokka, however, allows this with a single override in your elm.json. Let's demonstrate that.

1. Run git clone https://github.com/changlinli/zokka-basic-example to get the repository.
2. cd zokka-basic-example.
3. Now let's first run vanilla Elm: elm make src/Main.elm.
4. Open the resulting index.html in your web browser of choice. Get ready to close the browser, because it should hang in an infinite loop!
5. Now let's try again with Zokka: zokka make src/Main.elm.
6. Again open the resulting index.html in your web browser of choice. You should now see a single string and not have a hanging browser!

Explanation of quick start

zokka aims to be command-line compatible with the usual elm binary (except for the publish command), so all elm subcommands are usable in zokka as well with their usual flags. So for example you can immediately run zokka make $YOUR_ELM_MAIN_FILE in the root of a pre-existing Elm project and it should run fine. Indeed if you wanted you could alias zokka as elm.

Now as for the zokka-basic-example repository, let's go ahead and take a look at the elm.json. You'll see that in addition to all the normal bits in a vanilla elm.json, we also have a new field called zokka-package-overrides. This field overrides elm/core with another package called zokka/elm-core-1-0-override. zokka/elm-core-1-0-override fixes the bug in elm/core, so by overriding elm/core with this package instead, we get the bug fix!

Using Custom Repositories

To take advantage of Zokka-specific support for custom repositories, you can look at the custom-package-repository-config.json file, which is located in the $ELM_HOME/0.19.1/zokka/ directory (this is usually ~/.elm/0.19.1/zokka/). Much like how $ELM_HOME is generated on the first invocation of an elm subcommand, this file is generated the first time you successfully run a zokka subcommand (e.g. zokka make). By default the file looks like the below:

{
    "repositories": [
        {
            "repository-type": "package-server-with-standard-elm-package-server-api",
            "repository-url": "https://package.elm-lang.org"
        },
        {
            "repository-type": "package-server-with-standard-elm-package-server-api",
            "repository-url": "https://package-server.zokka-lang.com"
        }
    ],
    "single-package-locations": []
}

The custom-package-repository-config.json file controls all configuration of package repositories used by zokka.

Custom Multi-File Repositories

As can be seen, by default this configuration file includes both the standard Elm package repository as well as a separate Zokka package repository. Either repository can be deleted. For example if you would like to make use of the Zokka compiler with its bug fixes, but would otherwise like to ensure that you have a vanilla Elm project, you can delete the zokka-lang.com repository. Likewise, if you set up an internal package repository for your company and would like to guarantee that all packages come from that repository and not the standard Elm repository, you can also delete the elm-lang.org repository and substitute your own.

The package-server-with-standard-elm-package-server-api means that those URLs adhere to the API provided by the usual Elm package server. You can add any new URL that also adheres to the API of the usual Elm package server as a new entry in repositories. We are exploring adding other repository-types.

The set of packages available to the zokka compiler to download and compile is the union of all packages among your repositories.

So e.g. if you wish to set up a private repository for e.g. CI/CD purposes, you can delete both default repositories and include only the URL to your private repository.

$ELM_HOME is usually viewed as disposable, while often custom-package-repository-config.json has some data that we'd rather like to keep. One way to get around this is to store custom-package-repository-config.json somewhere else and symlink it into the proper location in $ELM_HOME. That way if you ever delete $ELM_HOME, you can always just re-symlink your configuration.

As for why we choose to put the custom-package-repository-config.json file in $ELM_HOME, check out the FAQs.

Custom Single-File Repositories

Sometimes though, you don't want to spin up an entire website just to be able to serve one or two custom packages. You might have just one package that you don't want to publish to the public Elm repository, but still want to use in your own projects.

In those cases, you can directly just tell Zokka the exact location of one or two packages by adding them as entries to single-package-locations.

{
    "repositories": [
        {
            "repository-type": "package-server-with-standard-elm-package-server-api",
            "repository-url": "https://package.elm-lang.org"
        },
        {
            "repository-type": "package-server-with-standard-elm-package-server-api",
            "repository-url": "https://package-server.zokka-lang.com"
        }
    ],
    "single-package-locations": [
        {
            "file-type": "zipfile",
            "package-name": "someauthor/somecoolpackage",
            "version": "1.0.0",
            "url": "https://example.com/my-custom-library-1-0-0.zip"
            "hash-type": "sha-1",
            "hash": "somerealhashherelike293b123abb920323d127b"
        }
    ]
}

Note that your package-name and version must match exactly what is contained in any elm.json that wishes to use that package in order for Zokka to resolve that package correctly.

Currently zipfile is the only file-type supported, but we may add more later on.

Dependency Overrides

Sometimes there are bug fixes to packages to which you cannot contribute the bug fix. For most direct dependencies this is not a big deal: you can fork the package and depend on your fork. However, for packages that are indirect dependencies or packages that are in the elm or elm-explorations GitHub organizations that is not feasible.

Zokka lets you instead override dependencies. So for example if you depend on elm/core version 1.0.5, but would like to incorporate the bug fix in elm/core#1092, you can add zokka-package-overrides to your elm.json and run zokka make.

{
    "type": "application",
    "source-directories": ...,
    "elm-version": "0.19.1",
    "dependencies": {
        "direct": {
            "elm/core": "1.0.5",
            ...
        },
        "indirect": { ... }
    },
    "test-dependencies": {... },
    "zokka-package-overrides": [
        {
            "original-package-name": "elm/core",
            "original-package-version": "1.0.5",
            "override-package-name": "zokka/elm-core-1-0-override",
            "override-package-version": "1.0.0"
        }
    ]
}

The way to read this declaration is that we are overriding version 1.0.5 of the package named elm/core using version 1.0.0 of the package named zokka/elm-core-1-0-override.

Note that zokka/elm-core-1-0-override exists only on https://package-server.zokka-lang.com/ not the standard Elm package repository https://package.elm-lang.org/, so you'll need to make sure that the former is present in $ELM_HOME/0.19.1/zokka/custom-repository-config.json.

If you wish to create your own package for a package override, please see ./CREATING_A_NEW_PACKAGE_FOR_PACKAGE_OVERRIDES.md.

Combining Package Overrides and Custom Package Repositories

The Elm community has had some other efforts to include bug fixes to Elm's core libraries. For example, there is the Elm Janitor project, which form the foundation of Zokka's forked versions of Elm's core libraries (thank you!).

By combining package overrides and custom package repositories, Zokka lets you seamlessly use those custom packages if you would like. For example, if you wish to depend solely on Elm Janitor's version of elm/core and not Zokka's forked version, then you can use the following custom-package-repository-config.json file, which eliminates the Zokka repository and adds the Elm Janitor version of elm/core as a custom package location.

{
    "repositories": [
        {
            "repository-type": "package-server-with-standard-elm-package-server-api",
            "repository-url": "https://package.elm-lang.org"
        }
    ],
    "single-package-locations": [
        {
            "file-type": "zipfile",
            "package-name": "elm-janitor/core",
            "version": "1.0.0",
            "url": "https://github.com/elm-janitor/core/archive/refs/heads/stack-1.0.5.zip",
            "hash-type": "sha-1",
            "hash": "f8b911c7d976533bce684b3c8da8f93151092bb2a"
        }
    ]
}

Then in your elm.json, you can add the following override:

{
    "type": "application",
    "source-directories": ...,
    "elm-version": "0.19.1",
    "dependencies": {
        "direct": {
            "elm/core": "1.0.5",
            ...
        },
        "indirect": { ... }
    },
    "test-dependencies": {... },
    "zokka-package-overrides": [
        {
            "original-package-name": "elm/core",
            "original-package-version": "1.0.5",
            "override-package-name": "elm-janitor/core",
            "override-package-version": "1.0.0"
        }
    ]
}

Note that in this case, as long as your elm.json and custom-package-repository-config.json agree on the overriding package name and version, you can call the package name and version whatever you want (so e.g. instead of elm-janitor/core, you could've called it some-author/some-package with version 5.3.1).

However, you should always give the overriding package a name that is different from the original package, either by giving it a different author or a different project name. If you don't, this can cause problems for both Elm and Zokka because of how Elm uses a global package cache. If for example you decide to call your overriding package elm/core with version 1.0.10, if elm/core ever publishes version 1.0.10 you can end up with weird transitive dependency errors when those two conflict.

This isn't a huge deal; you can always rename your package again if that happens. It can just be difficult to diagnose so can cause pain and annoyance that would be easily avoided by just making sure you give your overriding packages names that are unique.

Publishing Packages

The Zokka package repository currently disables publishing to it. If you set up your own instance of the Elm package server, you should be able to publish to it just fine with Zokka.

The zokka publish command is a bit different from elm publish. It takes a mandatory argument, which is the URL of the package repository (running an instance of the Elm package repository server) you wish to publish to.

Zokka is hard-coded to disallow publishing to the https://package.elm-lang.org/ package repository to preserve the integrity of the standard Elm repository. While Zokka packages could depend on packages that do not exist in the standard Elm repository, this is not the most important reason we disallow publishing to the standard Elm repository, since there are ways we could detect this and just selectively disallow those cases. The biggest reason is that the Zokka compiler may include bug fixes for Elm compiler crashes, which means that if we want to make sure that a Zokka-published package on the standard Elm package repository is usable in a vanilla Elm project, we must replicate all of the Elm compiler's bugs, which goes against one of the goals of Zokka. Otherwise we could end up with a Zokka-published package that causes a compiler crash when used in a vanilla Elm project.

If you wish to publish a package developed with Zokka to the standard Elm package repository, take advantage of bidirectional compatibility and use the standard Elm binary instead to publish the package. Assuming you are not using packages from non-stanard repositories, this should be a drop-in substitution (file a bug in Zokka if this is not the case!). You can still use Zokka while developing your package so that you can enjoy Zokka's improved error messages and other small quality of life improvements, but you are also thereby guaranteed that whatever you publish on https://package.elm-lang.org/ is usable by other Elm developers.

If you would like you can also manually publish a package. The easiest way to do this is just to bundle it up as a zipfile, throw it up somewhere, and then expose that location as a new entry under single-package-locations in your custom-package-repository-config.json file in $ELM_HOME/0.19.1/zokka.

Governance Structure

See ./GOVERNANCE.md

Design Choices Reasoning

See ./DESIGN_CHOICES.md.

FAQs

See ./FAQs.md.

The following is the README for the original Elm compiler:

Elm

A delightful language for reliable webapps.

Check out the Home Page, Try Online, or The Official Guide

Install

✨ Install ✨

For multiple versions, previous versions, and uninstallation, see the instructions here.

Help

If you are stuck, ask around on the Elm slack channel. Folks are friendly and happy to help with questions!