Purple Yolk

Purple Yolk is an extension for Visual Studio Code that provides a simple IDE for Haskell projects.

Purple Yolk is designed to work with Haskell projects rather than individual Haskell files. That typically means something with a cabal.project or stack.yaml file. Although Purple Yolk can work with a single Haskell file, its utility is somewhat limited.

At a high level, Purple Yolk launches GHCi for you, reloads it when you make changes, and displays GHCi's output in VS Code. It is designed to work with any project and build tool that can provide a REPL. It includes built-in support for both Cabal and Stack. Other tools are supported via custom configuration.

Features

These are the features that Purple Yolk provides:

• Syntax highlighting for both Haskell and Cabal files.

• Integration with GHCi, displaying GHC's warnings and errors inline and in the problems tab. This supports both Cabal and Stack for projects as well as GHCi for individual files.

• Formatting of Haskell files. Out of the box this supports both Fourmolu and Ormolu. Additional formatters are supported via custom configuration as long as the support reading from STDIN and writing to STDOUT.

• Formatting of Cabal files. Out of the box this supports both cabal-fmt and Gild. Additional formatters are supported via custom configuration as long as they support reading from STDIN and writing to STDOUT.

• Linting of Haskell files. Out of the box this supports HLint. Additional linters are supported via custom configuration, but their output must be the same as HLint's.

Not Implemented

These features are not (yet?) implemented. Most of them require a GHCi session loaded for an individual module rather than the entire project.

• Automatically suggesting fixes. Consider using the Haskutil extension along with Purple Yolk.

• Contextual auto completion. This could be provided with GHCi's :complete command.

• Documentation on hover. This could be provided with GHCi's :doc and/or :info commands.

• Displaying the type at the cursor. This could be provided with GHCi's :type command.

• Sending commands directly to the REPL. This could be supported, but extra safeguards would have to be put in place to avoid breaking the integration. For example sending a bogus :load command could have unintended consequences.

• Jump to definition. This is not supported by GHCi.

• Find usages. This is not supported by GHCi.

Common Problems

If you delete a file/module, you'll have to restart GHCi for the build to succeed. Similarly if you add a new dependency, you'll have to restart GHCi for it to be picked up.

In general, if something goes wrong you should run the "Purple Yolk: Start Interpreter" command to (re)start GHCi. If things go really wrong you can try "Developer: Reload Window" to reload everything.

Purple Yolk produces some chatty output that can help debugging. Use the "Purple Yolk: Show Output" command to see it.

Development

To build Purple Yolk locally and install it, run the following commands:

$ npm install
$ rm -f purple-yolk-*.vsix
$ npx vsce package --pre-release
$ code --install-extension purple-yolk-*.vsix