Fourmolu Warning

Fourmolu Warning reports Haskell files that do not pass a Fourmolu formatting check, without modifying them automatically.

Features

• checks saved .hs files;
• publishes a VS Code warning and a preferred formatting quick fix;
• supports standalone files, simple workspaces and multi-root workspaces;
• provides configurable include and exclude globs;
• supports additional Fourmolu arguments and a custom executable path;
• distinguishes formatting differences from Fourmolu parser or execution errors.

Examples

Import ordering

Documentation formatting

Generic formatting warning

Requirements

Install fourmolu in the environment where the extension runs. For remote SSH, WSL and devcontainer workspaces, this means the remote environment rather than the local machine.

By default, the extension runs fourmolu from PATH. Set fourmoluWarning.executablePath when the executable is elsewhere.

Usage

Open a Haskell workspace and save a .hs file. An unformatted file receives a warning at the start of the document and in the Problems panel. Use the lightbulb quick fix or one of these commands:

• Fourmolu: Check Current File
• Fourmolu: Format Current File

Settings

Example workspace configuration:

{
  "fourmoluWarning.include": [
    "services/**/*.hs",
    "libs/**/*.hs"
  ],
  "fourmoluWarning.extraArguments": [
    "-o", "-XImportQualifiedPost",
    "-o", "-XOverloadedRecordDot",
    "-o", "-XOverloadedLabels"
  ]
}

Relative executable paths and ${workspaceFolder} are resolved separately for each workspace folder:

{
  "fourmoluWarning.executablePath": "tools/fourmolu"
}

Development

The tests use Node.js built-ins and a fake Fourmolu process; they do not modify Haskell files.

npm test

To launch an Extension Development Host from this directory:

code --new-window --extensionDevelopmentPath="$PWD" /path/to/haskell-workspace

Packaging and publishing

Install the official VS Code extension CLI, then use the package scripts:

npm install --global @vscode/vsce
npm run package
npm run publish

The publisher field in package.json must match a publisher created in the Visual Studio Marketplace before publishing.