ghcid

ghcid [options] [module ...]

ghcid is "GHCi as a daemon": it starts a GHCi session, watches your source files, and reloads whenever something changes, formatting the errors and warnings to fit the console. The author describes it as GHC plus a bit of an IDE.Its value is the speed of the feedback loop. Because GHCi stays resident, a reload only recompiles what changed, so errors typically appear within a fraction of a second of saving. The output is deliberately truncated to the terminal height, so a clean build shows All good and a broken one shows the first error at the top, where you are already looking.With no arguments ghcid picks a sensible repl command for the project, using cabal repl or stack ghci when it detects one and falling back to plain ghci. --test turns it into a continuous test runner by evaluating an expression after each successful load, which combined with --warnings gives a tight red/green cycle without leaving the editor.The tool is intentionally narrow: it does not integrate with editors, expose GHCi to you directly, depend on the GHC API, or run a web server. --outputfile exists so editors can consume its results instead.

-c, --command=COMMAND

Command to run. Defaults to ghci, or cabal repl / stack ghci when a project is detected.

Component to build, e.g. lib:foo for Cabal or foo:lib for Stack. May be repeated.

GHCi expression to run after a successful load.

Like --test, but defaults to main.

Message shown before running the test. Defaults to Running test....

Allow tests to run even when the code produced warnings.

Run a linter when there are no errors. Defaults to hlint.

Commands passed to ghci on stdin at startup, usually :set something. May be repeated.

Also reload when this file or directory changes. Defaults to none.

Restart the whole command when this path changes. Defaults to .ghci and any .cabal file, unless a custom command or stack is in use.

Set the current directory. Defaults to ..

Write the full output to a file, in a format editors can parse.

Project name shown in the title. Defaults to the current directory.

Clear the screen when reloading.

Suppress status messages.

Reverse the output order. Works best with --no-height-limit.

Disable the height limit and print all messages.

Number of lines to use. Defaults to the console height.

Number of columns to use. Defaults to the console width.

Maximum number of messages to print.

Colour the output. Defaults to auto.

Do not update the shell title or icon.

Keep the window on top. Windows only.

Poll for changes every N seconds instead of using filesystem notifications. Defaults to 0.1.

Execute REPL commands written in source comments.

Keep going when no files are loaded. Requires --reload.

Disable the default +RTS -N -RTS passed to an auto-selected cabal repl.

Enable the local ghcid socket server.

Display help information.

ghcid is only as reliable as the underlying repl. If cabal repl or ghci cannot load your project by hand, ghcid will not fix it; test the command directly first and pass a working one with --command.Reloading only picks up changes GHCi itself can see. Changes to .cabal files, CPP includes, or generated code often need a full restart, which is what --restart is for. Non-Haskell assets need an explicit --restart or --reload.Filesystem notifications can be unreliable over network mounts, in containers and in some virtual machines; use --poll when saves seem to go unnoticed.--test runs inside the same GHCi session, so an expression that loops forever or calls exitWith will hang or kill the daemon.--allow-eval executes code found in comments, so avoid it on code you have not read.

ghcid was written by Neil Mitchell, with significant contributions from JP Moresmau, and released around 2014. It became a standard part of many Haskell development setups by offering fast, editor-independent feedback, and is distributed through Hackage and Stackage.

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

Source code · Documentation