Either "GHCi as a daemon" or "GHC + a bit of an IDE". To a first approximation, it opens ghci
and runs :reload
whenever your source code changes, formatting the output to fit a fixed height console. Unlike other Haskell development tools, ghcid
is intended to be incredibly simple. In particular, it doesn't integrate with any editors, doesn't depend on GHC the library and doesn't start web servers.
Acknowledgements: This project incorporates significant work from JPMoresmau, who is listed as a co-author.
Run stack install ghcid
or cabal update && cabal install ghcid
to install it as normal. Then run ghcid "--command=ghci Main.hs"
. The command
is how you start your project in ghci
. If you omit --command
then it will default to stack ghci
if you have the stack.yaml
file and .stack-work
directory, default to ghci
if you have a .ghci
file in the current directory, and otherwise default to cabal repl
.
Personally, I always create a .ghci
file at the root of all my projects, which usually reads something like:
:set -fwarn-unused-binds -fwarn-unused-imports
:set -isrc
:load Main
After that, resize your console and make it so you can see it while working in your editor. On Windows you may wish to pass --topmost
so the console will sit on top of all other windows. On Linux, you probably want to use your window manager to make it topmost or use a tiling window manager.
On every save you'll see a list of the errors and warnings in your project. It uses ghci
under the hood, so even relatively large projects should update their status pretty quickly. As an example:
Main.hs:23:10:
Not in scope: `verbosit'
Perhaps you meant `verbosity' (imported from System.Console.CmdArgs)
Util.hs:18:1: Warning: Defined but not used: `foo'
Or, if everything is good, you see:
All good
Please report any bugs you find.
There are a few plugins that integrate Ghcid into editors, notably:
- If you have a
.ghcid
file in the current folder, or a parent folder, the contents of that file will be used as command line arguments. For example, if you always want to pass--command=custom-ghci
then putting that in a.ghcid
file will free you from writing it each time. - There is an article on auto-reloading threepenny-gui apps during development.
- There are a list of general tips for using Ghcid.
- This isn't as good as full IDE. I've gone for simplicity over features. It's a point in the design space, but not necessarily the best point in the design space for you. For "real" IDEs see the Haskell wiki.
- If I delete a file and put it back it gets stuck. Yes, that's a bug in GHCi. If you see GHCi getting confused just kill
ghcid
and start it again. - I want to run my tests when files change. You can pass any
ghci
expression with the--test
flag, e.g.--test=:main
, which will be run whenever the code is warning free (or pass--warnings
for when the code is merely error free). - I want to run arbitrary commands when arbitrary files change. This project reloads
ghci
when files loaded byghci
change. If you want a more general mechanism something like Steel Overseer or Watchman will probably work better. - I want syntax highlighting in the error messages. One option is to use Neovim or Emacs and run the terminal in a buffer whose file type is set to Haskell. Another option is to pipe
ghcid
through source-highlight (ghcid | source-highlight -s haskell -f esc
). - I'm not seeing pattern matching warnings. Ghcid automatically appends
-fno-code
to the command line, which makes the reload cycle about twice as fast. Unfortunately GHC 8.0 and 8.2 suffer from bug 10600 which means-fno-code
also disables pattern matching warnings. On these versions, either accept no pattern match warnings or use-c
to specify a command line to startghci
that doesn't include-fno-code
. From GHC 8.4 this problem no longer exists. - I get "During interactive linking, GHCi couldn't find the following symbol". This problem is a manifestation of GHC bug 8025, which is fixed in GHC 8.4 and above. Ghcid automatically appends
-fno-code
to the command line, but for older GHC's you can supress that with--test "return ()"
(to add a fake test) or-c "ghci ..."
to manually specify the command to run. - I only see source-spans or error messages on errors/warnings after the first load. Due to limitations in
ghci
, these flags are only set after the first load. If you want them to apply from the start, pass them on the command line toghci
with something like `-c "ghci -ferror-spans -fdiagnostics-color=always". - I want to match on the file/line/column to get jump-to-error functionality in my editor. You will variously see
file:line:col:message
,file:line:col1-col2:msg
andfile:(line1,col1)-(line2,col2):message
, as these are the formats GHC uses. To match all of them you can use a regular expression such as^(\\S*?):(?|(\\d+):(\\d+)(?:-\\d+)?|\\((\\d+),(\\d+)\\)-\\(\\d+,\\d+\\)):([^\n]*)
.