Haskell "God Mode" Sandbox

May 15, 2015

Sandboxes have abolished cabal hell but in many cases have introduced long redundant compilation (looking at you, haskell-src-exts and lens). Recently I’ve given Stackage a try to share a common sandbox between projects while pinning compatible package versions.

Stackage has worked great, but I thought of a way to get even crazier with it: preemptively build all of hackage into a huge shared sandbox pinned to a given Stackage LTS version. This would take forever, right? Yep, it did. Over a week on my Mac running around the clock. But now that it’s done virtually any haskell project builds on my machine immediately without having to build its dependencies.

If you’re also using OS X 10.10, GHC 7.8 and Stackage LTS 2.5 then you can get the speed too without having to turn your computer into a space heater:

mkdir -p ~/.stackage
curl | tar xJ -C ~/.stackage
# it's about 800MB

Next install stackage-cli. Then, inside any project you’d like to build run

stackage sandbox init lts-2.5

Building the project will use the sandbox and be fast. If cabal install hangs on resolving resources, try the flag --solver=topdown which uses more memory but appears to get past that problem.

Creating a new megasandbox

To build god mode for another architecture you need to get the list of all packages on hackage and xargs it into cabal. Here’s my hacky little solution:

npm install -g json
mkdir -d /tmp/build && cd $_
cabal sandbox init
curl -H "Accept: application/json" | \
  json | grep packageName | cut -f 4 -d '"' | \
  xargs -L 1 cabal install -j --disable-documentation --max-backjumps=10

# Clean the executables themselves (they're big!)
rm ~/.stackage/ghc-7.8.4/lts-2.5/bin/*

Note that the first packages in the list begin with capital letters and these packages are mostly old and bogus. You could remove them from the list with grep ^[a-z] without doing too much harm because the useful capital letter packages will end up being included by other packages.

Some packages failed to build on my machine because certain specialized C libraries were missing. That’s OK by me since it built all the general purpose packages. Also the max-backjumps was important because otherwise cabal would freeze on certain packages as the sandbox got big. The magic number is open to experimentation. I think it did cause some packages to fail when they shouldn’t have.


CircleCI now supports OS X builds in addition to Linux. We could use it to do the builds for new Stackage LTS versions as they are released. Have Circle run the build, compress the folder, and deploy to S3.

Having full prebuilt Stackage sandboxes available across platforms could help make Haskell development faster and more fun for everybody.