The sickest experience possible for Haskell development in July 2022 is undoubtedly VS Code with the Haskell plugin powered by Haskell Language Server (HLS). When HLS works for your project, that is. This guide will demonstrate a simpler, lower-featured alternative that hopefully is reliable enough to work with your project in those cases where HLS does not.
--warningsto our Ghcid invocations. This causes tests to be run (updating ctags and hoogle) even in the presense of warnings.
On M1, the Stack distributed from haskellstack.org fails on some tasks. Use only the Stack distributed with GHCUp.
On M1, Hasktags might fail if you have LLVM 13 or 14. To work around this, install
brew install llvm@12and put its bindir near the front of your path.
Restart Ghcid (via
./start-ide.sh) whenever you modify stack.yaml, package.yaml, or your .cabal file.
HLS is superb and provides a rich Haskell experience that genuinely boosts productivity.
However, there are various reasons why HLS might not work with a particular Haskell project.
The project might be built using a version of GHC that HLS doesn’t support.
The project might be very
TemplateHaskell-heavy or otherwise just too damn big for HLS to function with acceptable latency.
That said, progress continues on HLS (and on GHC), making it faster and more stable, with the goal that we’ll be able to use it for all of our Haskell projects.
In the mean time, we still would like some of the luxuries our text editors are capable of providing when given a suitable source of information. Following this guide, you’ll get:
Basic code suggestions (i.e. autocomplete) based on textual analysis of your source file.
Goto definition for top-level symbols and modules within your project.
Project-specific Hoogle search and Haddock documentation for your project and its dependencies (and transitive dependencies).
Compiler errors and warnings will appear both in VS Code’s Problems pane, allowing you to easily jump to the source locatoin, and
Inline squiggly underlines in your source code.
Finally, we’ll discus tying these features together into a cohesive, convenient workflow.
Why VS Code?
This guide is for VS Code, because that’s the editor I know and use. I’m sure you can get similar features for Emacs and the various Vis using the approaches described here, though. If you do, drop a comment and link to your blog post :-D
I rely on Stack to reduce the number of dev tools I need to manage manually. Alternatively, the same setup should work if you install the necessary tools yourself (at the correct versions for your project). Or if you know of Cabal commands that more-or-less correspond to the Stack commands I use, then these workflows should work with those commands just fine.
You can use Stack for these editor integrations even if you don’t use Stack to build your project.
In your project root directory (rather, the directory that has the Cabal file defining your package), use
stack init to create a stack.yaml file.
Check stackage.org and find your version of GHC on the list Latest releases per GHC version; click into that.
Somewhere on the page, you’ll see something that looks more or less like
resolver property in your stack.yaml file with the one you found on Stackage.
You should be good to go, unless you’re on Apple Silicon.
Stack will work on Apple Silicon, but it takes a bit of finagling (please read on).
Stack on Apple silicon
Because of incompatibilities between GHC versions, Stack treats GHC as a project dependency: Stack will download and use the appropriate version of GHC for your project. However, the GHC binaries that Stack currently distributes are incompatible with Apple M1 (and presumably M2) chips. Moreover, I’ve gotten reports that the Stack binaries distributed by Homebrew and haskellstack.org exhibit unexpected errors. These are known issues, and they will hopefully be fixed soon.
Meanwhile, you’ll need to install both Stack and GHC from GHCUp. You’ll also need to configure Stack to use the GHC installed by GHCUp.
Remove any extand Stack distribution that didn’t come from GHCUp.
For example, if you installed Stack from Homebrew or from haskellstack.org, remove it.
Install GHCUp, and use it to install Stack and Cabal (latest versions).
Make sure GHCUp’s bin directory is included in your shell’s
For example, on my Mac OS 12.4 system, GHCUp’s bin directory is /Users/daniel/.ghcup/bin, and this directory appears near the front of my
Use GHCUp to install and set the version of GHC you want for your project.
Add a stack.yaml file to your project root if there isn’t one already.
In stack.yaml, add the following top-level properties:
Stack will now use the GHC binary provided by GHCUp.
One more thing. I’ve heard reports of Haskell projects unexpectedly failing to build on M1 due to LLVM incompatibilities. We need to install LLVM version 12 and we need to put its bindir on our path so that it’s the first LLVM found.
Basic code suggestions
VS Code ships with a rudimentary form of code suggestions (a.k.a. autocomplete) built-in. It’s not very smart, being driven by a simple textual analysis of your source file, but it’s better than nothing.
Here are VS Code settings that work for me. You might want to tweak some of them to fit your preferred workflow.
With these settings, you’ll see the top suggestion as ghost text. You can apply the suggestion by hitting the Tab key. You can see the full list of suggestions by hitting Control+Space.
I have these settings in my user-level VS Code settings because I want them to apply to every workspace. (Workspace is just the word VS Code uses for what I’ve been calling your project.) Alternatively, you might want these settings to apply only in specific workspaces. If so, create a .vscode/settings.json in your project and put these properties there.
This feature depends on the ctagsx VS Code plugin.
We will use a Haskell tool called Hasktags to generate a file named tags. The tags file contains symbol information from your project source code. ctagsx relies on this file to provide Goto definition functionality in VS Code.
It’s not as great as what you get with HLS. HLS is able to index all of the defined entities in your source code, including local variables. Hasktags is only capable of indexing module-level definitions, but that’s better than nothing. It indexes types, classes, and variables—whether or not they’re exported—and it indexes modules. Right click on the entity you want to chase, and pick Goto Definition.
Caveat: to jump to a module you’ll need to select the whole module name before you right click.
For example, to be taken to the source code for a module named
Foo.Bar you need to select that entire module name—from
r—before right clicking.
All you need to do is install the ctagsx plugin and generate a tags file for your source code.
Generate tags file:
If you want an up-to-date index, you’ll need to regenerate your tags file every time you modify your code. Obviously, doing this manually would be very tedious. We’ll automate it later.
Hoogle and Haddock
This feature depends on the vscode-goto-documentation plugin.
By now we should have Goto Definition working for the types, functions, classes, and modules in our project. Unfortunately, Goto Definition won’t work for entities defined in our dependencies. The next best thing, though, is having Hoogle search and Haddock documentation for them (and Haddock includes source code, so we kinda do get Goto Definition in a roundabout way).
Build initial Hoogle database (also builds docs):
Start Hoogle server (server will listen at localhost:8080):
You’ll now have Hoogle and Haddock for your project and its dependencies when you open your browser to localhost:8080.
As with our tags file, we’ll have to update Hoogle’s database every time our source code changes. Fortunately, we can do this while the Hoogle server is running. This needs to be done every time we change a source file (so we’ll automate it later).
Hot-rebuild Hoogle database:
When we’re done working for the day, we’ll want to tear down our server to free up Port 8080.
Kill Hoogle server:
Now, opening our browser, navigating to localhost:8080, and keying in the name of a function or type is laborious. Fortunately, we can get all this with just a right click once we configure our plugin appropriately. Create a .vscode/settings.json in your project if there isn’t one already, and add the following property:
You can now right-click on a function, class, module, or type in your source code and pick Goto Document to get taken to the Hoogle results. This works both for symbols we get from our dependencies and symbols we’ve defined ourselves. The best part is we don’t even need a live internet connection.
This feature depends on the haskell-ghcid VS Code plugin.
Here’s the real meat of our faux-IDE setup.
Ghcid is a simple, reliable file watcher that spawns a Ghci session and presses
:r for you (Ghci’s reload command) whenever a Haskell source file changes.
It’s reliable precisely because it has no notion of projects, build tools, or text editors.
It relies simply on Ghci.
It’ll even let you choose how to start Ghci, so that all of your dependencies will be registered and all of your modules will be in scope.
(I don’t know why I felt the need to explain all that. It’s hard to imagine anyone is writing Haskell and doesn’t know about Ghcid.)
In the old days, we were forced to use Ghcid in a crude way. We’d have it running in a terminal, side-by-side with our editor. Today, we have editor plugins for Emacs, Neovim, and VS Code. Go install the plugin, you’ll be happy you did!
The plugin is very simple. It reads a file, named ghcid.txt. It assumes the file contains info on compiler errors and warnings, and it parses that info and feeds it into VS Code’s problems reporting API. Your errors and warnings show up in VS Codes Problems pane (which provides Jump-to-source-location functionality) and inlined into your source code as squiggly underlines.
Your responsibility here is to bootstrap this whole process by starting Ghcid in a terminal and then forgetting about it.
You’ll need to use Ghcid’s
--outputfile option to tell Ghcid that it should be writing info to ghcid.txt.
Start Ghcid with file output:
stack repl ... with whatever command you use to load your project into Ghci.
For example, if you use Cabal to build your project, it might be something like this
The most important thing here is
This Ghcid option it to write errors and warnings to ghcid.txt, where our VS Code plugin is expecting to find them.
The second most important thing here is
-fno-code -ignore-dot-ghci -ferror-spans.
These options tell Ghci that it should only be type checking (for faster reloads) and that it should be reporting full source spans for errors (so your editor can make squiggles).
-ignore-dot-ghci is more superstitious than anything, but it’s in there because sometimes your
.ghci file can mess up your project REPL.
(If you rely on a .ghci file to load your project REPL, then you probably already know enough about Ghci to know that you should overrule my suggestion and omit that option.)
Now, just minimize your terminal and pull your editor up. Chasing compiler errors has never been easier!
Putting it all together
Our problems reporting (and squiggles!) are powered by Ghcid running in a minimized terminal somewhere.
Well, Ghcid has a little-known feature that allows you to run an arbitrary Ghci command every time it reloads (so, every time a source file in your project changes).
We’ll use this feature to automate the re-indexing of our tags file and our Hoogle database.
This Ghcid feature is controlled by the
But we don’t want to run a Ghci command, we want to run a couple shell commands.
Well, Ghci has a little-known feature that allows you to run arbitrary shell commands.
At the Ghci prompt, type
:! and then a space and then any shell command.
Try it with something innocuous, like
That’s rather delightful!
So, when we invoke Ghcid, we’ll use the
--test option to get Ghci to run the shell commands that re-index our tags file and Hoogle database.
Here’s everything conveniently in a shell script:
(You can find the name of your package in your project’s Cabal file.)
So, when I go sit down to work, I open a terminal to my project directory and I run
Then I minimize that terminal and forget about it.
I open my project in VS Code, and I have
- basic code suggestions;
- goto definition for my types, classes, functions, and modules;
- goto documentation for all types, classes, functions, and modules;
- errors and warnings in VS Code’s Problems pane, with goto source location; and
- inline squigglies in my source code for errors and warnings.
And I have it all reliably, whether or not HLS works for my project.
Here are the steps written as tersely as possible, free of commentary, while still being totally complete. Some users will be able to skip some of these steps.
If on an Apple M1 or M2 system (optional otherwise), install GHCUp and:
Use it to install a version of GHC appropriate for your project.
Configure your shell so that GHCUp’s bindir is on your
Install ctagsx VS Code plugin.
Install vscode-goto-documentation plugin.
Install haskell-ghcid VS Code plugin.
Create .vscode/settings.json in your project directory (contents shown below):
If your project doesn’t have a stack.yaml file, create one with
stack init, and
Check stackage.org to find the latest resolver that matches your version of GHC.
resolverproperty in stack.yaml to the resolver you just found.
If on an Apple M1 or M2 system, modify your stack.yaml (contents show below):
Create an executable script named start-ide.sh in your project dir with contents:
Every time you work on your project:
./start-ide.shin a terminal and minimize it.
Each time you modify your .cabal file, stack.yaml, or package.yaml, you need to kill Ghcid and restart