ghciwatch

[!NOTE] Mercury is migrating our backend build to Buck2 after outgrowing the Cabal build system. As a result, ghciwatch is currently in maintenance mode: we are not developing new features and code review bandwidth is limited.

For more information on Mercury's Buck2 migration, see some of the conference talks by our engineers and contractors: - "Integrating Nix and Buck2 for fun and profit" by Claudio Bley (Tweag) at NixCon 2024 (31 minutes). - "Remote Execution with Buck2 and Nix" by Claudio Bley (Tweag) at FOSDEM 2025 (20 minutes). - "Haskell Builds at Scale: Comparing Bazel and Buck2" by Andreas Herrmann (Tweag) at BazelCon 2025 (32 minutes).

Ghciwatch loads a GHCi session for a Haskell project and reloads it when source files change.

Features

• GHCi output is displayed to the user as soon as it's printed.
• Ghciwatch can handle new modules, removed modules, or moved modules without a hitch
• A variety of lifecycle hooks let you run Haskell code or shell commands on a variety of events.
• Run a test suite with --test-ghci TestMain.testMain.
• Refresh your .cabal files with hpack before GHCi starts using --before-startup-shell hpack.
• Format your code asynchronously using --before-reload-shell async:fourmolu.
• Custom globs can be supplied to reload or restart the GHCi session when non-Haskell files (like templates or database schema definitions) change.
• Ghciwatch can clear the screen between reloads.
• Compilation errors can be written to a file with --error-file, for compatibility with ghcid's --outputfile option.
• Comments starting with -- $> can be evaluated in GHCi.
• Eval comments have access to the top-level bindings of the module they're defined in, including unexported bindings.
• Multi-line eval comments are supported with {- $> ... <$ -}.

Demo

Check out a quick demo to see how ghciwatch feels in practice:

Learn More

Read the manual here.

Developing ghciwatch

See CONTRIBUTING.md for information on hacking ghciwatch.