Haskell Language Server (HLS) Guide

Setting Up and Using Haskell Language Server in VS Code

Introduction

The Haskell Language Server (HLS) is a powerful tool that provides IDE-like features for Haskell development. It integrates directly with VS Code through the Haskell extension, enabling features such as code completion, error diagnostics, refactoring, and more. This guide covers setting up HLS in VS Code specifically for Courseography and highlights the most useful features to make the developer experience more enjoyable. :)

Installation

Make sure to have all the dependencies stated in the project's README. The specific steps for installing HLS in VS Code is under the Developers Section.

Features of Haskell Language Server for Courseography

Category	Feature	Description
Diagnostics	GHC Compiler Errors	Displays errors and warnings from GHC.
	HLint Hints	Provides style and refactoring suggestions from HLint.
	Cabal Parse Errors	Reports errors from Cabal configuration files.
Code Assistance	Auto-completion	Provides intelligent code suggestions as you type.
	Hover Documentation	Displays type signatures and documentation on hover.
	Go to Definition	Jump to the definition of a function or type.
	Find References	Locate all occurrences of a function, type, or variable.
	Jump to Type Definition	Navigate to the definition of a type used in the code.
Refactoring	Rename Symbol	Allows renaming functions and variables project-wide.
	Extract Function	Extracts selected code into a separate function.
	Jump to Implementation	Navigate to function implementations.
Code Navigation	Workspace Symbols	Search for symbols across the project.
	Document Symbols	Lists symbols defined in a module.
	Call Hierarchy	Displays call relationships between functions.
	Highlight References	Highlights occurrences of variables across the code.
Formatting & Style	Automatic Code Formatting	Uses ormolu or fourmolu for consistent code formatting.
	Organize Imports	Automatically sorts and removes unused imports.
	Fix Module Names	Corrects mismatched module names to match file paths.
	Selection Range	Supports incremental code selection.
	Folding Range	Enables code folding for large sections.

Detailed Feature Walkthrough

GHC Compiler Errors

HLS continuously checks your code for errors and warnings. Issues appear in the Problems tab (Cmd+Shift+M / Ctrl+Shift+M). Hovering over red underlined code also provides error descriptions. To quickly jump between errors, use F8.

HLint Hints

HLint suggests improvements to your code style. These appear as yellow warnings, and you can apply fixes by clicking the lightbulb icon or using Quick Fix (Cmd+. / Ctrl+.).

Rename Symbol

To rename a function or variable across the project:

1. Right-click the symbol.
2. Select Rename Symbol (F2).
3. Enter the new name and press Enter.

Limitations:

• Cross-module renaming requires indexing all project components.

Extract Function

To extract a block of code into a separate function:

1. Highlight the code.
2. Right-click → Refactor… → Extract Function.
3. Name the function and apply changes.

Jump to Definition & Type Definition

• Go to Definition: Right-click on a function/type and select Go to Definition to navigate to its source.
• Jump to Type Definition: Works similarly but navigates to the type declaration instead.

Limitations:

• Only works for local definitions.

Call Hierarchy

Displays incoming and outgoing function calls:

1. Right-click on a function.
2. Select Show Call Hierarchy.
3. Expand nodes to see relationships between functions.

Fix Module Names

Ensures that module names match file paths:

1. Right-click the module declaration.
2. Select Fix Module Name.

Highlight References

To highlight all occurrences of a variable:

1. Click on a variable or function name.
2. All references in the file will be highlighted automatically.

Automatic Code Formatting

To format Haskell code:

1. Right-click → Format Document (Shift+Alt+F).
2. Ensure ormolu or fourmolu is installed (stack install ormolu).

Selection and Folding Range

• Selection Range: Supports incremental selection of expressions or blocks.
• Folding Range: Allows collapsing and expanding large sections of code.

Additional Notes

This does not exhaust all features of HLS, just the most relevant/useful ones for Courseography. If you want to look at other features please refer to the documentation and update this guide if anything not listed here becomes very useful.