Haskell language tools
Use types and diagnostics while editing, then follow definitions or rename a symbol across the project. Haskell Language Server works on the current buffers, including edits you have not saved yet.
Start HLS
Install Haskell Language Server for the GHC used by your package and put haskell-language-server-wrapper on PATH. Open the package or one of its .hs or .lhs files:
thc-edit .The editor starts haskell-language-server-wrapper --lsp in the detected project root. It looks for a Cabal package, cabal.project, hie.yaml, stack.yaml or Git root while walking upward from the file. HLS reads the project's normal configuration.
Set THC_EDIT_HLS to use another server executable. Tools > Restart language server reconnects after a configuration change. HLS runs independently of the interface; language requests do not stop you editing.
Types, definitions and completions
| Action | Keys or menu |
|---|---|
| Inspect the type at the cursor | Shift+F1 or Tools > Inspect type |
| Follow a definition | F12 or Search > Go to definition |
| Complete an identifier | Ctrl+Space or Edit > Complete identifier |
| Rename a symbol | Right-click source, then Rename |
| Quick fixes and refactorings | Tools > Code actions or right-click source, then Code actions |
Pausing the text cursor shows type information in the status bar. In graphical frontends, hovering over source does the same. The source context menu also offers type information, definitions and completion.
Rename applies HLS's edits to the affected buffers, with one undo step per file. Review those buffers and save them to write the result to disk. If a buffer or disk file changed while the request was pending, the editor rejects the stale edit. Text edits are supported; HLS file creation/deletion operations and executable completion commands are not applied. Completion snippets are disabled.
Code actions
Select source or place the cursor at a diagnostic, then choose Code actions. The chooser lists HLS quick fixes and refactorings for that range. Select an action and press Enter to apply its text edits to buffers; review and save them when ready. HLS may resolve an action lazily when you select it.
Actions can supply text edits, an advertised HLS command, or both. A supplied edit runs before its command. For example, HLS's Evaluate... action evaluates a doctest comment and inserts its result. Commands may have server-side effects; only text edits are confined to unsaved editor buffers.
Each edit batch rejects changed source, private or unrelated files, and file creation/deletion operations as a whole. A later command failure or cancellation keeps earlier accepted edits; review the status and changed buffers before saving. Only one command runs at a time per project. Every completed, failed, or canceled command retires that HLS process, so its late edits cannot affect another action. The next command needs a fresh action list and may incur HLS startup time; edit-only actions keep the existing server. Actions from an older list expire when you request another list or restart HLS.
Diagnostics
Source-line chevrons mark diagnostics. Tools > Messages opens their list at the bottom of the desktop. Click a message to select it; Enter or double-click opens its source. Alt+F8 visits the next message and Alt+F7 the previous one, opening the file when needed.
Copy a selected message with Ctrl+C, Ctrl+Insert, or Cmd+C on macOS. The copy includes the full message and source location. Right-click Messages for Go to source, Copy message, Copy all messages and Hide Messages.
Drag the Messages title bar vertically to make more room. Adjacent editors and the Files pane follow its edge. Hiding Messages leaves the source markers available. Editing clears diagnostics for older buffer versions while HLS checks the new text.
For commands that build from disk, save first. Running and debugging covers the THC target, terminals and debugger attachment.
Cabal project browser
Tools > Project browser lists the local components in Cabal's existing dist-newstyle/cache/plan.json. Use Prev and Next to move between pages, then Details to open a read-only window with the package, component, source root and library/build-tool dependencies. Unresolved dependency IDs remain visible when their units are absent from the bounded graph.
The chooser reports the plan's compiler, freshness and any omitted units. Changed manifests or unsaved manifest buffers mark it stale; unchanged timestamps do not prove it current. Refresh reads the plan again. If no plan exists, build with Cabal first. Browsing never starts a build, configures a project, or changes the run target.