haskell-lsp

haskell-lsp

A Claude Code plugin providing comprehensive Haskell development support through:

• haskell-language-server (HLS) integration for IDE-like features
• Automated hooks for formatting, linting, and code quality
• Haskell toolchain integration (hlint, ormolu/fourmolu, cabal/stack)

Quick Setup

# Run the setup command (after installing the plugin)
/setup

Or manually:

# Install GHC and cabal via ghcup (recommended)
ghcup install ghc latest
ghcup install cabal latest
ghcup install hls latest

# Install development tools
cabal install hlint
cabal install ormolu
# Or alternatively: cabal install fourmolu

Features

LSP Integration

The plugin configures haskell-language-server for Claude Code via .lsp.json:

{
    "haskell": {
        "command": "haskell-language-server-wrapper",
        "args": ["--lsp"],
        "extensionToLanguage": {
            ".hs": "haskell",
            ".lhs": "haskell"
        },
        "transport": "stdio"
    }
}

Capabilities:

• Go to definition / references
• Hover documentation with type information
• Code actions and refactoring
• Workspace symbol search
• Real-time type checking and diagnostics
• Import management

Automated Hooks

All hooks run afterWrite and are configured in hooks/hooks.json.

Core Haskell Hooks

Hook	Trigger	Description
haskell-format-on-edit	**/*.hs	Auto-format with ormolu or fourmolu
haskell-lint-on-edit	**/*.hs	Lint with hlint
haskell-build-check	**/*.hs	Compile check with cabal/stack
haskell-todo-fixme	**/*.hs	Surface TODO/FIXME comments

Quality & Documentation

Hook	Trigger	Description
haskell-haddock-check	**/src/**/*.hs	Check Haddock documentation
haskell-warnings	**/*.hs	Show GHC warnings

Required Tools

Core

Tool	Installation	Purpose
ghcup	ghcup.haskell.org	Haskell toolchain manager
ghc	ghcup install ghc	Haskell compiler
cabal	ghcup install cabal	Build tool and package manager
haskell-language-server	ghcup install hls	LSP server

Recommended

Tool	Installation	Purpose
hlint	cabal install hlint	Linting and suggestions
ormolu	cabal install ormolu	Code formatter (opinionated)
fourmolu	cabal install fourmolu	Code formatter (configurable)
stack	ghcup install stack	Alternative build tool

Commands

/setup

Interactive setup wizard for configuring the complete Haskell development environment.

What it does:

1. Checks for ghcup - Ensures ghcup is installed
2. Installs GHC - Latest stable compiler
3. Installs cabal - Build tool and package manager
4. Installs HLS - Haskell Language Server
5. Installs dev tools - hlint, ormolu/fourmolu
6. Validates LSP config - Confirms .lsp.json is correct
7. Verifies hooks - Confirms hooks are properly loaded

Usage:

/setup

Quick install command (from the wizard):

ghcup install ghc latest && \
ghcup install cabal latest && \
ghcup install hls latest && \
cabal update && \
cabal install hlint ormolu

Command	Description
/setup	Full interactive setup for HLS and all tools

Configuration

Formatter Selection

By default, hooks use ormolu for formatting. To use fourmolu instead:

1. Install fourmolu: cabal install fourmolu
2. Edit hooks/hooks.json and change ormolu to fourmolu

Customizing Hooks

Edit hooks/hooks.json to:

• Disable hooks by setting "enabled": false
• Adjust output limits (head -N)
• Modify matchers for different file patterns
• Add project-specific hooks

Example - use fourmolu:

{
    "name": "haskell-format-on-edit",
    "event": "afterWrite",
    "hooks": [
        {
            "type": "command",
            "command": "command -v fourmolu >/dev/null && fourmolu -i $FILE || true"
        }
    ],
    "matcher": "**/*.hs"
}

Project Structure