ghcid

Start ghcid and monitor a Haskell file for changes

$ ghcid [path/to/Main.hs]

$ ghcid --command "[stack ghci Main.hs]"

$ ghcid --run=[action] [path/to/Main.hs]

$ ghcid --height=[height] --width=[width] [path/to/Main.hs]

$ ghcid --outputfile=[path/to/output_file.txt] [path/to/Main.hs]

$ ghcid --allow-eval [path/to/Main.hs]

ghcid [OPTIONS] [TARGET]

-c, --command COMMAND
    Command to run (default: ghci)

-a, --arguments ARGUMENTS
    Arguments to pass to the command

--ghci-arg ARG
    Argument to ghci (deprecated, use --arguments)

--ghci-args ARGUMENTS
    Arguments to ghci (deprecated, use --arguments)

-r, --reload RELOAD
    File(s)/dir to reload on (default: all project files)

-l, --lint
    Lint code with hlint

-W, --warnings
    Display compiler warnings

-E, --exit-on-error
    Exit on first error

--clear
    Clear screen between tests

--rerun-timings
    Print timing info for rerun tests

--top
    Run from project top-level

-o, --output OUTPUT
    Test output expression

-O, --outputfile OUTPUTFILE
    Output file (deprecated, use --output)

--ignore IGNORE
    Directories to ignore when watching

-T, --no-title
    Suppress banner/title

--future
    Assume future GHC version

-v, --verbose
    Verbose output

--list-files
    List watched files

-h, --help
    Show help

--version
    Print version

Ghcid is a lightweight, fast development tool for Haskell projects that launches GHCi and automatically reloads and retests code upon file changes. It monitors specified files or directories, recompiling and running tests in a continuous loop, providing immediate feedback during development.

Ideal for TDD workflows, ghcid supports custom commands (default: ghci), test expressions, linting with hlint, and warning display. It clears the screen between runs for clarity and handles errors by exiting or continuing based on flags. Target can be a .cabal, package.yaml, .hs file, or directory.

By avoiding full rebuilds, it achieves sub-second reload times, boosting productivity. Verbose mode aids debugging, and ignore patterns exclude irrelevant files like .git. Widely used in Haskell ecosystem for its simplicity and speed.

Requires GHC and Haskell project setup; high CPU on large projects with many files; no native Windows support (use WSL); reloads may fail on syntax errors until fixed.

Via cabal: cabal update && cabal install ghcid
Via stack: stack install ghcid
Or from Hackage/Nixpkgs.

Uses .ghcid file in project root for default flags, e.g., --command='stack ghci'.

Defaults to .cabal/package.yaml if present; otherwise scans for .hs files.

Created by Will Yager in 2014 to address slow GHCi reloads. Maintained by Nam P. Nguyen since 2020. Reached v9.1.4 in 2023, integrating with modern tools like HLS and Nix.

ghci(1), hlint(1), cabal(1), stack(1)