Buck2 build system for Haskell projects

Short summary: this is a cabal external command cabal buck2 that allows you to use Buck2 as the build system for your Cabal project.

Quick start

First download a buck2 binary, unpack it and put it on your PATH.

Then

cabal install cabal-buck2

Then in the root of your project or package:

git clone https://github.com/simonmar/haskell-buck2.git buck2
cabal buck2 --enable-tests

Then you can use buck2 as the build tool, e.g.

buck2 build //...

To build all the components, or

buck2 test //...

To run your tests.

Note that you need to re-run cabal buck2 --enable-tests if you modify the cabal.project or any of the .cabal files.

Not all Cabal features are supported - see limitations.

Why?

(skip this section if you know why you want buck2)

Why might you want to use buck2 as the build system compared with just using cabal? Well, first off let me be clear that you still need Cabal, because the Buck2 support doesn't know how to solve package dependencies or build them. So the workflow consists of first running cabal buck2 to solve and build the dependencies, but once you've done that you can switch to buck2 for building. The idea is that buck2 is a more pleasant experience because:

• It's faster than Cabal, particularly for rebuilds.

• It supports different build modes out of the box: the default is to build in dev mode (unoptimised with dynamic linking) but adding -m opt gives you optimisation and static linking. Note that Cabal doesn't have a purely dynamic build mode: it always uses -dynamic-too for libraries, which has a significant built-time performance cost.

• The Buck2 build is extensible. If you have anything that needs to be generated as part of your build, or any non-standard tooling, then hooking that up using Buck2 is far easier than Cabal. Furthermore Buck2 knows how to rebuild things correctly when either the build system or the code generator components change.

• It works a lot better than Cabal when you have non-Haskell code (e.g. C/C++ or Rust) in your project, because

  • Buck2 understands dependencies between C/C++ source files and header files (Cabal doesn't: issue #4306), so when you modify a C/C++ header the correct things are rebuilt.
  • Buck2 builds C/C++ files in parallel, while Cabal doesn't (issue #7127)

• You can use remote execution and caching (I haven't tried this with cabal buck2 yet).

Finally, if you have an existing codebase using Buck2 then this is the basis of something that could "buckify" Cabal packages to integrate into your build system. It needs a bit of work to be suitable for that use case, though: cabal buck2 builds all the external dependencies and installs them in the Cabal store, whereas to integrate with an existing build system you would want to satisfy those external dependencies from the build system itself.

How complete is it?

I've used it to build a few largish projects, in particular the Cabal project itself which consists of about 16 packages and a few hundred source files. It can also build Glean, which has some complex build requirements including custom codegen, FFI & hsc2hs.

There are a few limitations, however.

What cabal buck2 does

You can run cabal buck2 in a project or a single Cabal package. It does the following:

1. Solves the build-depends constraints of your package(s) and builds all the dependencies, much like cabal build all --only-dependencies would.

2. Generates some files, notably:

   • BUCK and BUCK.cabal.bzl in each package, these are the Buck2 build targets

   • cabal-buck2/autogen in each package, this is where we put the files that Cabal autogenerates, such as cabal_macros.h and Paths_<pkg>.hs.

   • third-party/haskell: tells Buck2 about all the prebuilt package dependencies, either in the Cabal store or in GHC's package DB. In here we also record the GHC version you're using, and the paths to any tool dependencies.

Buck2 quick start

To build your code:

buck2 build //...

The //... is Buck2's syntax for "all targets recursively below the current directory". You can also build specific target(s), for example buck2 build cabal-install:cabal would build the cabal target in the cabal-install package. For more details see Target Pattern in the Buck2 docs.

Next you can run your tests:

buck2 test //...

Customising the build

cabal buck2 will generate all the BUCK files if they don't exist, but you can also write your own if you want (cabal buck2 won't overwrite them).

The BUCK file usually goes in the same directory as your source files. The targets that cabal buck2 produces go in the BUCK.cabal.bzl file, and are generated by a call to generated_targets() from the BUCK file. This call takes some arguments that you can use to override or transform the generated targets - take a look at the comments in the generated code to see how.

You can also completely override the generated targets and write a BUCK file with your own rules, while still making use of the pre-built dependencies that cabal buck2 produces. For example, the BUCK file for a simple Haskell library might look something like

load("//buck2:haskell.bzl", "haskell_library")

haskell_library(
    name = "my-package",
    srcs = [
        "Some/Module.hs",
    ],
    packages = [
        "unordered-containers",
    ],
    visibility = ["PUBLIC"],
)

and the BUCK file for a test might look like

load("//buck2:haskell.bzl", "haskell_test")

haskell_test(
    name = "my-test",
    srcs = {
        "Main.hs" : "my-test.hs",
    },
    deps = [
        "//:my-package",
    ],
    packages = [
        "test-framework",
        "test-framework-hunit",
        "HUnit",
    ],
)

You can find docs on how to write BUCK files in the Buck2 docs, e.g. haskell_library.

Build modes

The Buck2 build system has two build modes:

• dev: the default, builds everything with -O0 and dynamic linking. This is intended to give you the quickest edit-compile-test turnaround.
• opt: enable -O and link statically. This takes longer but the code runs faster.

To build with opt, use -m opt, e.g.

buck2 build my-package:my-program -m opt

There are other build options that can be selected in a similar way, such as -m prof to enable profiling. See constraints/BUCK for details.

Performance

I ran some experiments building the Cabal project itself - 16 packages and 641 source files (one package, hackage-security, is not part of the project but has to be built locally nonetheless because it depends on Cabal-syntax which is part of the project).

Buck2 shines when it comes to rebuilds: the dependency graph is cached in memory, and it knows when build steps can be omitted because the inputs haven't changed.

Caveats

• Results tend to be +/- a few seconds from run to run
• I didn't dig into the results in any detail
• It's just one set of data points. Different projects and different choices of edits could give different results. However, I did perform a similar experiment with the persistent project, and got similar results.

Raw results and details

Clean build

• Optimised:

  • Default Cabal build: 280s
    • cabal build all --enable-tests --enable-benchmarks -j
  • Buck2 build (opt mode, including cabal buck2): 259s
    • cabal buck2 --enable-tests --enable-benchmarks && buck2 build //... -m opt
    • Not much difference here, as we expect.

• Unoptimised / dynamic:

  • Cabal build with -O0 -dynamic: 136s
    • cabal build all --enable-tests --enable-benchmarks -j --disable-optimisation --enable-executable-dynamic
  • Buck2 build (dev mode, including cabal buck2): 78s
    • cabal buck2 --enable-tests --enable-benchmarks && buck2 build //... -m dev
    • Cabal is using -dynamic-too for libraries, while Buck2 is building everything purely dynamic.

Edit + rebuild

Next I made a single edit (added an extension to Language.Haskell.Extension) and rebuilt everything:

• Optimised:

  • Cabal: 197s
  • Buck2: 179s

• Unoptimised / dynamic:

  • Cabal: 85s
  • Buck2: 55s

Limitations

It's an external command, not builtin to cabal-install

This has some implications:

• cabal passes only the arguments after buck2, so global flags given before it (cabal --store-dir=... buck2) don't reach the tool. Use the environment (CABAL_DIR) instead.

• Nothing checks that your cabal binary matches the version of the cabal-install library that cabal-buck2 was built against. Try to make sure they match, or confusion will undoubtedly ensue.

Builds currently use --make

The current Buck2 prelude uses ghc --make to build each component (library, executable). Ideally we should expose the full per-module dependencies to Buck2 so that it can exploit parallelism across packages for faster builds/rebuilds. It's entirely possible to do this, indeed the functionality already exists in Tweag's Haskell/Buck2 integration.

Custom build type

The cabal buck2 command doesn't run the actual Setup.hs code for a package with the (legacy) Custom build type. If you rely on this, use Hooks instead.

Template Haskell and prof

A module that defines a splice must live in a different haskell_library() from any module that uses it, when profiling (-m prof). If not, the build will likely complain about a link error or a missing object file at compile-time.

The situation with Template Haskell and profiling is complex, as is the reason for this limitation.

• Without -fexternal-interpreter: GHC loads object code at compile-time into its own process. Since GHC is itself a dynamically-linked non-profiled executable, the objects it loads must be shared, non-profiled, objects. So we have to build all the dependencies of the current packages as shared libraries. This is fine, except for the current package: GHC expects to find the .dyn_o objects for the current package in the current -odir. But Buck2 doesn't work this way: it builds the two instances of the package separately. It's not clear if this is easily fixable.

• With -fexternal-interpreter, we could load the profiled non-shared objects. However, this method uses the RTS runtime linker, which is known to have some limitations and can't load some objects, particularly on certain architectures. This is the main reason that GHC switched to dynamic linking. So we don't go this route.

No support for Cabal's foreign-library

Nothing fundamental blocking this, it's just a TODO.

Preprocessors like hspec-discover

The hspec-discover preprocessor is designed to be invoked by GHC via the -pgmF flag to specify a custom preprocessor. The problem is that hspec-discover searches the filesystem to find other source files; these other source files amount to implicit inputs to the compilation, but when using Buck2 all inputs must be explicit (this is so that compilation steps can be executed remotely).

To build an hspec-discover test with Buck2, you have to run the preprocessor using a genrule() that takes all the source files as an input. For example, if your test is in test/Spec.hs:

filegroup(
    name = "srcs",
    srcs = glob(["**/*.hs"])
)

genrule(
    name = 'spec-gen',
    cmd = "$(location third-party-haskell//:hspec-discover-exe) $(location :srcs)/test/Spec.hs test/Spec.hs ${OUT}",
    out = "test/Spec.hs"
)

haskell_test(
    name = 'spec',
    srcs = {
        'Main.hs': ':spec-gen',
        ...
    },
    ...
)

Acknowledgments

Most of the code and modifications to the standard Buck2 prelude were developed with the help of Claude Code using Claude Sonnet 5/5.5.

The Haskell support already in the Buck2 prelude was developed by Meta and is in production use internally for building Glean. This project just fixes a few things and adds some functionality needed to support building Cabal projects.

Related projects

Tweag also worked on a Haskell integration for Buck2. This project has no code in common with theirs, except for the shared upstream prelude code. Tweag's integration is more sophisticated and was aimed at using Buck2's improved scalability to build large Haskell projects.