Stephen Diehl’s article Vim and Haskell in 2016 does a great job of outlining the quickest way to get a quality Haskell development experience. I ran into a simple issue when I followed it due to a bit of an idiosyncrasy on OS X El Capitan. I also document a few of the extra steps I took which aren’t mentioned explicitly in his original article.

This post wouldn’t actually be necessary if it weren’t for one thing: System Integrity Protection in OS X El Capitan, also known as “rootless” mode. It affects Haskell’s build and install step in a subtle (but solved) way, and the rest of the article will cover what to watch out for when installing on OS X.

If you don’t follow these instructions and you’re on OS X El Capitan, you’ll likely run into an issue that looks like /usr/bin/ar: permission denied. The Stack installation instructions call this out specifically:

If you are on OS X 10.11 (“El Capitan”) or later, GHC 7.8.4 is incompatible with System Integrity Protection (a.k.a. “rootless”). GHC 7.10.2 includes a fix, so this only affects users of GHC 7.8.4.

At the risk of spoiling the surprise, we’re going to set up Stack while taking care to make sure that we get GHC >= 7.10. So let’s do just that!

Environment Prep (optional)

Note that this step is optional and has nothing to do with OS X; I just wanted to point it out quickly.

Under Dev Environment on Vim and Haskell in 2016, Stephen Diehl writes:

Times have changed quite a bit, and the new preferred way of installing GHC in 2016 is to forgo using the system package manager for installing ghc and use Stack to manage the path to the compiler executable and sandboxes.

Stack stores it’s metadata in the ~/.stack folder by default. If you’re not a huge fan of things cluttering up your home folder, set the STACK_ROOT variable in you bashrc or zshrc:

export XDG_DATA_HOME="$HOME/.local/share"

# Have Haskell Stack use XDG Base Directory spec
export STACK_ROOT="$XDG_DATA_HOME/stack"

Of course, the choice of location is up to you. In particular I’ve chosen to adhere to the XDG Base Directory specification here, which you may want to take a peek at if you’re unfamiliar. That’s why you see references to XDG_DATA_HOME.

Make sure you restart your terminal to pick up the new variables before continuing.

Install Stack from Homebrew

If you’re like me, you’ll want to take a second to purge anything Haskell-related from previous botched setup attempts:

# If you had GHC installed
$ brew uninstall ghc

# If you tried and failed at installing Stack already
$ brew uninstall haskell-stack
$ rm -r ~/.stack
# If you happened to also set STACK_ROOT
$ rm -r $STACK_ROOT

With that out of the way, we can actually get our hands on Stack. For OS X, we’ll install through Homebrew:

$ brew install haskell-stack

Configure Stack, avoiding “rootless” issues

Here’s the trick. We need to run stack setup for the first time to let Stack configure itself. But remember: we want to make sure that Stack doesn’t set itself up with version 7.8.4 of GHC. We can get around this by specifying an explicit resolver to the stack command (you can find more information on “resolvers” elsewhere):

# Change the resolver to the most up-to-date one. This is a hack to ensure that
# get GHC version > 7.8, because there's an issue with El Capitan's rootless
# mode.
stack --resolver=lts-6.10 setup

You’ll notice we use the --resolver=... flag to force Stack to use a specific resolver. This post will be out of date as soon as it’s written, so check Stackage to find the latest LTS snapshot.

Return to Vim and Haskell in 2016

That’s it for the environment setup! Now you’ll want to turn your attention to configuring Vim.

The steps to set up Vim are platform independent, so now that we’ve set up Stack correctly, you can head over to Vim and Haskell in 2016 to finish things out.