Introduction to Bazel: Building a Haskell project

In this tutorial, you’ll learn the basics of building Haskell applications with Bazel. You will set up your workspace and build a simple Haskell project that illustrates key Bazel concepts, such as targets and BUILD.bazel files. After completing this tutorial, take a look at Common Haskell build use cases for information on more advanced concepts such as writing and running Haskell tests.

What you’ll learn

In this tutorial you’ll learn how to:

  • build a target,

  • visualize the project’s dependencies,

  • split the project into multiple targets and packages,

  • control target visibility across packages,

  • reference targets through labels.

Before you begin

On a Unix system you will need the following tools installed.

  • gcc

  • libffi

  • libgmp

  • libtinfo5

  • make

  • python3 (python also needs to be available in $PATH. Depending on your distribution, this might require installing the python meta-package, which might use Python 2 or 3, rules_haskell works with both.)

  • JDK (e.g., OpenJDK 8 or 11)

On Ubuntu you can obtain them by installing the following packages.

build-essential libffi-dev libgmp-dev libtinfo5 libtinfo-dev python python3 openjdk-11-jdk

On Windows you will need.

  • msys2

  • python3

Next, install Bazel if you don’t have it installed already. Then, retrieve the rules_haskell GitHub repository:

git clone

The sample project for this tutorial is in the tutorial directory and is structured as follows:

└── tutorial
   ├── main
   │  ├── BUILD.bazel
   │  └── Main.hs
   └── lib
      ├── BUILD.bazel
      └── Bool.hs

The first thing to do is to:

$ cd tutorial

If you use the NixOS distribution, also run the following command:

$ echo 'build --host_platform=@rules_nixpkgs_core//platforms:host' >> .bazelrc.local

Build with Bazel

Set up the workspace

Before you can build a project, you need to set up its workspace. A workspace is a directory that holds your project’s source files and Bazel’s build outputs. It also contains files that Bazel recognizes as special:

  • the WORKSPACE file, which identifies the directory and its contents as a Bazel workspace and lives at the root of the project’s directory structure,

  • one or more BUILD.bazel files, which tell Bazel how to build different parts of the project. (A directory within the workspace that contains a BUILD.bazel file is a package. You will learn about packages later in this tutorial.)

To designate a directory as a Bazel workspace, create a file named WORKSPACE in that directory. This file defines external dependencies.

When Bazel builds the project, all inputs and dependencies must be in the same workspace. Files residing in different workspaces are independent of one another unless linked, which is beyond the scope of this tutorial.

Understand the WORKSPACE file

The file tutorial/WORKSPACE defines how to obtain rules_haskell. This file only works within the rules_haskell repository: for your own project, run the start script to create a WORKSPACE file that makes rules_haskell available by downloading it.

The WORKSPACE file also defines how to obtain the compiler toolchain, as detailed in section picking a compiler.

Understand the BUILD file

It is recommended to use a .bazel extension for each BUILD file to avoid clashing with files or folders already using that name.

A BUILD.bazel file contains several different types of instructions for Bazel. The most important type is the build rule, which tells Bazel how to build the desired outputs, such as executable binaries or libraries. Each instance of a build rule in the BUILD.bazel file is called a target and points to a specific set of source files and dependencies. A target can also point to other targets.

Take a look at the BUILD.bazel file in the tutorial/lib directory:

    name = "booleans",
    srcs = ["Bool.hs"],

In our example, the booleans target instantiates the haskell_library rule. The rule tells Bazel to build a reusable (statically or dynamically linked) library from the Bool.hs source file with no dependencies.

The attributes in the target explicitly state its dependencies and options. While the name attribute is mandatory, many are optional. For example, in the booleans target, name is self-explanatory, and srcs specifies the source file(s) from which Bazel builds the target.

Build the project

Let’s build your sample project. Run the following command:

$ bazel build //lib:booleans

Notice the target label - the //lib: part is the location of our BUILD.bazel file relative to the root of the workspace, and booleans is what we named that target in the BUILD.bazel file. (You will learn about target labels in more detail at the end of this tutorial.)

Bazel produces output similar to the following:

INFO: Found 1 target...
Target //lib:booleans up-to-date:
INFO: Elapsed time: 2.288s, Critical Path: 0.68s

Congratulations, you just built your first Bazel target! Bazel places build outputs in the bazel-bin directory at the root of the workspace. Browse through its contents to get an idea for Bazel’s output structure.

Review the dependency graph

A successful build has all of its dependencies explicitly stated in the BUILD.bazel file. Bazel uses those statements to create the project’s dependency graph, which enables accurate incremental builds.

Let’s visualize our sample project’s dependencies. First, generate a text representation of the dependency graph (run the command at the workspace root):

bazel query --nohost_deps --noimplicit_deps \
  'deps(//lib:booleans)' --output graph

The above command tells Bazel to look for all dependencies for the target //lib:booleans (excluding host and implicit dependencies) and format the output as a graph.

Then, paste the text into GraphViz.

On Ubuntu, you can view the graph locally by installing GraphViz and the xdot Dot Viewer:

sudo apt update && sudo apt install graphviz xdot

Then you can generate and view the graph by piping the text output above straight to xdot:

xdot <(bazel query --nohost_deps --noimplicit_deps \
         'deps(//lib:booleans)' --output graph)

As you can see, the first stage of the sample project has a single target that builds a single source file with no additional dependencies:

digraph booleans { node [shape=box]; "//lib:booleans" "//lib:booleans" -> "//lib:Bool.hs" "//lib:Bool.hs" }

Now that you have set up your workspace, built your project, and examined its dependencies, let’s add some complexity.

Refine your Bazel build

While a single target is sufficient for small projects, you may want to split larger projects into multiple targets and packages to allow for fast incremental builds (that is, only rebuild what’s changed) and to speed up your builds by building multiple parts of a project at once.

Specify multiple build targets

Let’s split our sample project build into two targets. Take a look at the BUILD.bazel files in the tutorial/lib and tutorial/main directories. The contents of both files could have been kept in a single BUILD.bazel as follows:

    name = "booleans",
    srcs = ["Bool.hs"],

haskell_toolchain_library(name = "base")

    name = "demorgan",
    srcs = ["Main.hs"],
    ghcopts = ["-threaded"],
    deps = [":base", ":booleans"],

With this single BUILD.bazel file, Bazel first builds the booleans library (using the haskell_library rule), then the demorgan binary (which as an example uses the booleans library to check one of the De Morgan laws). The deps attribute in the demorgan target tells Bazel that the :booleans library is required to build the demorgan binary. The binary also requires the base built-in library that ships with GHC, to perform I/O among other things. Libraries like base, bytestring and others that ship with GHC are special in that they are prebuilt outside of Bazel. To import them as regular targets, we use the haskell_toolchain_library rule.

Let’s build this new version of our project:

$ bazel build //main:demorgan

Bazel produces output similar to the following:

INFO: Found 1 target...
Target //main:demorgan up-to-date:
INFO: Elapsed time: 2.728s, Critical Path: 1.23s

Now test your freshly built binary:

$ bazel-bin/main/demorgan

Or alternatively:

$ bazel run //main:demorgan

If you now modify Bool.hs and rebuild the project, Bazel will usually only recompile that file.

Looking at the dependency graph:

digraph demorgan { node [shape=box]; "//main:demorgan" "//main:demorgan" -> "//main:base\n//main:Main.hs" "//main:demorgan" -> "//lib:booleans" "//lib:booleans" "//lib:booleans" -> "//lib:Bool.hs" "//lib:Bool.hs" "//main:base\n//main:Main.hs" }

You have now built the project with two targets. The demorgan target builds one source file and depends on one other target (//lib:booleans), which builds one additional source file.

Use multiple packages

Let’s now split the project into multiple packages.

Notice that we actually have two sub-directories, and each contains a BUILD.bazel file. Therefore, to Bazel, the workspace contains two packages, lib and main.

Take a look at the lib/BUILD.bazel file:

    name = "booleans",
    srcs = ["Bool.hs"],
    visibility = ["//main:__pkg__"],

And at the main/BUILD.bazel file:

haskell_toolchain_library(name = "base")

    name = "demorgan",
    srcs = ["Main.hs"],
    ghcopts = ["-threaded"],
    deps = [":base", "//lib:booleans"],

As you can see, the demorgan target in the main package depends on the booleans target in the lib package (hence the target label //lib:booleans) - Bazel knows this through the deps attribute.

Notice that for the build to succeed, we make the //lib:booleans target in lib/BUILD.bazel explicitly visible to targets in main/BUILD.bazel using the visibility attribute. This is because by default targets are only visible to other targets in the same BUILD.bazel file. (Bazel uses target visibility to prevent issues such as libraries containing implementation details leaking into public APIs.)

You have built the project as two packages with three targets and understand the dependencies between them.

Use labels to reference targets

In BUILD.bazel files and at the command line, Bazel uses labels to reference targets - for example, //main:demorgan or //lib:booleans. Their syntax is:


If the target is a rule target, then path/to/package is the path to the directory containing the BUILD.bazel file, and target-name is what you named the target in the BUILD.bazel file (the name attribute). If the target is a file target, then path/to/package is the path to the root of the package, and target-name is the name of the target file, including its full path.

When referencing targets within the same package, you can skip the package path and just use //:target-name. When referencing targets within the same BUILD.bazel file, you can even skip the // workspace root identifier and just use :target-name.

Further reading

Congratulations! You now know the basics of building a Haskell project with Bazel. Next, read up on Common Haskell build use cases and have a look cat_hs for an example build description of a full application. Then, check out the following:

Happy building!


This tutorial is adapted from the Bazel C++ build tutorial.