Cue Modules: Local Filesystem Paths For External Dependencies

Hey everyone! Let's dive into a cool feature request for the cue command-line tool. The main issue is how Cue handles external dependencies, especially when you're using build tools like Bazel. The goal? To make it easier to point Cue towards a local filesystem path for these external modules. Basically, we want Cue to play nice with tools that have already downloaded the dependencies, without forcing everyone to use OCI registries or the pkg subdirectory.

The Core Problem: Where Do External Modules Live?

So, the current Cue module system has a couple of assumptions about where external modules come from. It either expects them to be in an OCI registry (like a container registry, but for modules) or tucked away in a pkg subdirectory within your current module. The problem arises when you're working with tools like Bazel, which might download these dependencies for you and place them somewhere on your local filesystem. It is a bit of a pain, because Cue doesn't inherently know to look in these other places. This is where the feature request comes in. We need a way to tell Cue, "Hey, I've got the modules right here!" This would dramatically simplify integrating Cue into build processes that already manage their own dependency resolution.

Think of it like this: you have a bunch of LEGO bricks (your dependencies), and you've already got them sorted into different boxes (your local filesystem). You don't want to go to the LEGO store (the OCI registry) every time you need a brick. You just want to grab them from your pre-sorted boxes. Currently, Cue makes you go to the LEGO store or force all your bricks into a single box (pkg).

The Proposed Solution: A Local Filesystem Path

The ideal solution is a way to provide Cue with an extra location, similar to the pkg directory, where it can search for external modules. This could be a command-line flag, an environment variable, or even a configuration file. The specifics aren't as important as the functionality itself. The core idea is to allow build tools like Bazel to download dependencies, place them somewhere accessible, and then point Cue to that location.

This would provide a much cleaner and more flexible way to handle external dependencies. Instead of shoehorning everything into the pkg directory or relying solely on OCI registries, you'd have the freedom to manage your dependencies in a way that best suits your project's needs. It makes for a much more adaptable system.

This isn't just about convenience; it's about making Cue more compatible with the broader ecosystem of build tools and dependency management strategies. It allows you to tailor the workflow to meet your requirements.

Diving Deeper: How This Would Actually Work

So, how might this actually work in practice? Let's imagine a scenario using Bazel:

1. Bazel Downloads Dependencies: Bazel, using its dependency resolution magic, downloads all the necessary Cue modules. These are the external dependencies for your project.
2. Dependencies Stored Locally: Bazel stores those dependencies in a well-defined location on your filesystem (e.g., a dedicated cache directory). The precise location is something that Bazel manages.
3. Cue Knows Where to Look: You configure Cue (perhaps through a command-line flag like --module-path=/path/to/bazel/cache) to look in this Bazel cache directory when resolving module imports. You might add a flag in your Cue build file so it knows to search for the modules there.
4. Cue Resolves Imports: When Cue encounters an import statement for an external module, it checks the specified module path in addition to its default search locations. It finds the module in the Bazel cache and everything works smoothly.

This setup gives you the best of both worlds: Bazel manages the dependency fetching and caching, and Cue can still correctly resolve and use those dependencies during your build process. The key is the ability to tell Cue where to search for these external modules.

Alternatives Considered: What Doesn't Work

Let's talk about some of the alternatives that have been tried. The tutorial on working with the central registry and custom module registries in Cue docs have been explored, along with the cue help import command. These methods don't fully address the core problem.

• Working with the Central Registry: While the central registry is great for public modules, it might not be the best fit if you are dealing with internal modules, or if your build process already handles fetching the dependencies.
• Custom Module Registries: Custom registries offer more control, but they can introduce complexity and require you to set up and maintain your own infrastructure. It is overkill for situations where a build tool already manages dependency fetching and caching.
• cue help import: This command provides information about imports, but it doesn't directly provide a way to tell Cue to search a specific local filesystem path.

The main problem is that current solutions focus on external registries and registries rather than the local filesystem. The feature request gives a more streamlined and efficient experience by targeting the local filesystem directly.

Benefits of Implementing the Feature

• Improved Integration with Build Tools: The primary benefit is seamless integration with build tools like Bazel, which is a huge plus for teams that already rely on these tools for dependency management. The setup becomes more streamlined and less error-prone, as developers do not need to make dependency management more complex than necessary.
• Enhanced Flexibility: This offers more flexibility in managing external dependencies. You are not constrained to using OCI registries or the pkg directory. You can adapt the dependency management strategy to fit your project's needs.
• Simplified Development Workflow: When dependencies are managed locally, it can lead to faster build times, especially if you are working offline or if your network connection is unreliable. It simplifies the overall development workflow, as it reduces the friction in resolving dependencies.
• Better Offline Support: Imagine you're working on a project offline or in an environment with limited internet access. With this feature, you could pre-download your dependencies and point Cue to the local directory. This would allow you to continue developing without interruptions.

Potential Implementation Details

The implementation could take several forms, such as a new command-line flag (e.g., --module-path), an environment variable (e.g., CUE_MODULE_PATH), or configuration file entries. It is best to explore the options and find the right way to implement it.

• Command-Line Flag: A command-line flag provides direct control and is easy to use. It allows users to specify the path each time they run the cue command. The potential downside is that you may need to add the flag to every command.
• Environment Variable: An environment variable allows you to set the module path globally for your shell session or system. This is convenient if you are using a specific set of dependencies often. However, it might be less flexible if you need to switch between different sets of dependencies frequently.
• Configuration File: A configuration file can provide a more structured and persistent way to manage module paths. You could specify different paths for different projects. The downside is that it may involve more setup initially, depending on the nature of the configurations and builds.

Regardless of the specifics, the implementation should be easy to understand, and also easily integrated into existing workflows. The solution should feel natural to the end-user.

The Impact of Improved Dependency Management

The impact of the improved dependency management is a more efficient and streamlined development workflow. Developers can spend more time writing code and less time battling dependency issues. This also boosts productivity and promotes a collaborative environment.

Also, improving how Cue handles external modules can have a wider impact. More developers will be willing to adopt Cue, and the whole Cue ecosystem becomes richer. As more people use Cue, we can start to see a wider range of projects, and more libraries and tools that can further improve the way we work.

This feature request is important because it addresses a real-world need in projects that use build tools. By allowing Cue to use local filesystem paths, we can make Cue more flexible, and make it more integrated with existing tools.