Hermetic Bazel + Haskell + Nix setup

Context

Minimal (?) repo that contains Haskell build set up with rules_bazel, with a nixpkgs-sourced GHC, and is able to build the results using Bazel from within nix-build itself with the same toolchain.

Latter is contrieved, since nix-build can't be run inside nix-build, so rules_haskell can't source its dependent toolchain from nix when bazel is being run inside a nix-build. See WORKSPACE.nix.part file and release.nix for details.

Running inside nix-build is needed if you want to use Hydra CI, for example, or depend on Bazel-built binaries from other nix files (maybe for a nixops deployment).

Code tour

The contents of the nix dir implement niv-style nix version pinning, so the deps are hermetic and independent of the nix-channel version of the user.

The release.nix file is the main nix-build target, resulting in binaries, tests and test results.

The treetide dir is the monorepo top.

• treetide/thirdparty/haskell/BUILD along with ghc.nix define the libraries we import from Nix.

• treetide/haskell/build_defs contains some Bazel shorthands for declaring common deps.

• treetide/haskell/extended contains some extended upstream libs, using the extended pattern (thus the name extended... ok you likely get it).

• Other dirs in the monorepo are laid out according to project structure. Now we just have the foo_* family of a toy library, test and binary, along with their BUILD rules.

• .bazelignore can be used to exclude eventual node_modules inside the monorepo, so Bazel doesn't get confused by BUILD files in there.

• .bazelrc sets up some Bazel compatibility flags, and instructs rules_haskell to pull toolchains from Nix. Note: latter instruction is dropped when building using nix-build, see release.nix.

Building

Having nix installed is a prerequisite.

Building from command line interactively

To build locally while developing:

1. Get a nix shell using nix-shell.

2. Then bazel build -c opt //treetide/....

3. Test using LC_ALL=C.UTF-8 ./bazel-bin/treetide/foo_test.

Note: can also execute single-target REPL, or set up REPL rules for multi-target repl.

Note: can test directly using bazel test -c opt //treetide/..., but that needs a custom main wrapper that forces GHC into UTF-8 mode. But that's a different story... Ping if interested.

Building using nix (suitable for Hydra etc)

Execute nix-build release.nix. This will build all targets and run the tests. The symlinked results output has a directory for the binaries (executables) and tests, respectively. The results-2 dir contains the test logs (if successful).

Tooling

Note: there are some tools available in the nix-shell, see README.haskell.md for details.

How can you help?

Cleanup PRs, additional tooling welcome!