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Update README to describe `cabal buck2`
simonmar Sep 26, 2026
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Add some perf measurements
simonmar Sep 29, 2026
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simonmar Sep 30, 2026
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# Buck2 build system for Haskell projects

This is a version of the [Buck2 prelude](https://github.com/facebook/buck2/tree/main/prelude) with a few tweaks (that will
hopefully be upstreamed at some point) and some supporting tooling to
enable Haskell projects to be built with [Buck2](https://buck2.build/).

Why might you want to do that compared with, say, just using Cabal to
build your code? Well, first off let me be clear that you *still need
Cabal*, because this build system doesn't know how to solve package
dependencies or build them. But once you've built your package
dependencies using Cabal, you can use Buck2 for your edit-compile-test
loop, and in many ways it's a more pleasant experience than using
Cabal because:

* It's much faster than Cabal
* It's much more extensible than Cabal. 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 when you have C/C++ components in your project, because
* Buck2 understands dependencies between C/C++ source files and header files (Cabal doesn't), so when you modify a C/C++ header the correct things are rebuilt.
* Buck2 builds C/C++ files in parallel (Cabal doesn't)
* You can integrate remote caching and execution (although I haven't tried that).

The main disadvantage of course is that you have to maintain
additional build instructions in the form of `BUCK` files for your
project, which duplicate some of what is in your `.cabal` file. In the
future I hope to be able to generate the `BUCK` files from the
`.cabal` file, but how to do that in the presence of custom build
stuff is an open question.
Short summary: you can use [Buck2](https://buck2.build) as the build system for your Cabal
project. There is support for auto-generating the `BUCK` files from
the `.cabal` files for a complete Cabal project, and once generated
you can use `buck2` to build, rebuild, and run the tests for the whole
project or individual components.

# How to use it
# Why?

First, clone this repo as `buck2/` in your Cabal project
(skip this section if you know why you want buck2)

Next, create a `.buckconfig` containing
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:

```
[cells]
root = .
prelude = buck2/prelude
toolchains = buck2/toolchains
third-party-haskell = third-party/haskell
none = none
* It's [faster than Cabal, particularly for rebuilds](#performance).

* 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](https://github.com/haskell/cabal/issues/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](https://github.com/haskell/cabal/issues/7127))

* You can use [remote execution and caching](https://buck2.build/docs/users/remote_execution/) (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.

# What is this repo?

This is a version of the [Buck2
prelude](https://github.com/facebook/buck2/tree/main/prelude) with a
few tweaks (that will hopefully be upstreamed at some point).

[cell_aliases]
config = prelude
ovr_config = prelude
fbcode = none
fbsource = none
buck = none
You also need a modified version of `cabal-install` (see below) that
supports the `cabal buck2` command.

[parser]
target_platform_detector_spec = target:root//...->prelude//platforms:default \
target:prelude//...->prelude//platforms:default \
target:toolchains//...->prelude//platforms:default \
target:third-party-haskell//...->prelude//platforms:default
# 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 thousand
source files. It can also build [Glean](https://glean.software), which
has some complex build requirements including custom codegen, FFI &
hsc2hs.

There are a few [limitations](#limitations), however.

# How to use it

Build a modified version of `cabal-install` that has the `cabal buck2`
command:

[build]
execution_platforms = prelude//platforms:default
```
git clone https://github.com/simonmar/cabal.git -b buck2
cd cabal
cabal install cabal-install
```

Next, create `PACKAGE` containing
Next, clone this repo as `buck2` in the root of your Cabal project.

```
load("//buck2:cfg_constructor_if_standalone.bzl", "cfg_constructor_if_standalone", "dev_modifiers_if_standalone")
load("@prelude//cfg/modifier/set_cfg_modifiers.bzl", "set_cfg_modifiers")
cd <my-project>
git clone https://github.com/simonmar/haskell-buck2.git buck2
```

cfg_constructor_if_standalone()
Next, build dependencies and set up the buck2 build system:

set_cfg_modifiers(
cfg_modifiers = dev_modifiers_if_standalone(),
)
```
cabal buck2 --enable-tests
```

Next ask Cabal to build your dependencies:
This will generate 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.

Then build your code:

```
cabal build all --only-dependencies --enable-tests
buck2 build //...
```

And set up the Buck2 build. This script is going to analyse Cabal's
build plan and create Buck2 `haskell_prebuilt_library()` declarations
for all the external Haskell libraries that your project depends
on. It will also fish out the GHC version from Cabal and tell Buck2
about your Haskell toolchain.
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](https://buck2.build/docs/concepts/target_pattern/) in the
Buck2 docs.

Next you can run your tests:

```
python3 buck2/gen-haskell-prebuilt.py
buck2 test //...
```

Next, create `BUCK` file(s) containing your build targets. These
should go in the same directory as your source files. For example, the
`BUCK` file for a simple Haskell library might look something like
# Custom `BUCK` files

`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. For example, the `BUCK` file for a simple Haskell library might
look something like

```
load("//buck2:haskell.bzl", "haskell_library")
Expand Down Expand Up @@ -132,29 +162,17 @@ haskell_test(

You can find docs on how to write `BUCK` files in the Buck2 docs, e.g. [haskell_library](https://buck2.build/docs/prelude/rules/haskell/haskell_library/).

Then build your code:

```
buck2 build //...
```

and run your tests:

```
buck2 test //...
```

# 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 `-O2` and link statically. This takes longer but the code runs faster.
* `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 //some/target -m opt
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.
Expand Down Expand Up @@ -186,18 +204,87 @@ buck2 test -m prof //...
build --only-dependencies` this all depends on) on every push and pull
request, in `dev`, `opt` and `prof` mode.

# 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.

![Buck2 vs Cabal build times](perf-chart.svg)

**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](github.com/yesodweb/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

**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.
## 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](https://github.com/tweag/buck2-haskell).

## 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 iself a
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
Expand All @@ -212,20 +299,59 @@ the reason for this limitation.
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.
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](https://glean.software). This project just fixes a few things
and adds some wrappers that make it more suitable for external use.
and adds some functionality needed to support building Cabal projects.

# Related projects

[Tweag](https://tweag.io) also worked on a [Haskell integration for
Buck2](https://www.youtube.com/watch?v=bbFnrTAIK9Q). This project has
nothing in common with theirs, except for the shared upstream prelude
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.
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