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diff --git a/third_party/bazel/rules_haskell/docs/haskell.rst b/third_party/bazel/rules_haskell/docs/haskell.rst new file mode 100644 index 0000000000..353561111a --- /dev/null +++ b/third_party/bazel/rules_haskell/docs/haskell.rst @@ -0,0 +1,364 @@ +.. _guide: + +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 :ref:`Common Haskell build use cases <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 +---------------- + +To prepare for the tutorial, first `install Bazel`_ if you don't have +it installed already. Then, retrieve the ``rules_haskell`` GitHub +repository:: + + git clone https://github.com/tweag/rules_haskell/ + +The sample project for this tutorial is in the ``tutorial`` +directory and is structured as follows:: + + rules_haskell + └── tutorial + ├── WORKSPACE + ├── main + │ ├── BUILD.bazel + │ └── Main.hs + └── lib + ├── BUILD.bazel + └── Bool.hs + +The first thing to do is to:: + + $ cd tutorial + +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 an empty file +named ``WORKSPACE`` in that directory. + +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 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:: + + haskell_library( + 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: + bazel-bin/lib/libZSbooleans/libZSbooleans.conf + bazel-bin/lib/libZSbooleans/package.cache + 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:: + + haskell_library( + name = "booleans", + srcs = ["Bool.hs"], + ) + + haskell_toolchain_library(name = "base") + + haskell_binary( + name = "demorgan", + srcs = ["Main.hs"], + compiler_flags = ["-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: + bazel-bin/main/demorgan + 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:: + + haskell_library( + name = "booleans", + srcs = ["Bool.hs"], + visibility = ["//main:__pkg__"], + ) + +And at the ``main/BUILD.bazel`` file:: + + haskell_toolchain_library(name = "base") + + haskell_binary( + name = "demorgan", + srcs = ["Main.hs"], + compiler_flags = ["-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:: + + //path/to/package:target-name + +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 the most common :ref:`Common Haskell +build use cases <use-cases>`. Then, check out the following: + +* `External Dependencies`_ to learn more about working with local and + remote repositories. + +* The `Build Encyclopedia`_ to learn more about Bazel. + +* The `C++ build tutorial`_ to get started with building C++ + applications with Bazel. + +* The `Java build tutorial`_ to get started with building Java + applications with Bazel. + +* The `Android application tutorial`_ to get started with building + mobile applications for Android with Bazel. + +* The `iOS application tutorial`_ to get started with building mobile + applications for iOS with Bazel. + +Happy building! + +.. note:: This tutorial is adapted from the Bazel `C++ build tutorial`_. + +.. _install Bazel: https://docs.bazel.build/versions/master/install.html +.. _haskell_binary: http://api.haskell.build/haskell/haskell.html#haskell_binary +.. _haskell_toolchain_library: http://api.haskell.build/haskell/haskell.html#haskell_toolchain_library +.. _haskell_library: http://api.haskell.build/haskell/haskell.html#haskell_library +.. _graphviz: https://www.graphviz.org/ +.. _external dependencies: https://docs.bazel.build/versions/master/external.html +.. _build encyclopedia: https://docs.bazel.build/versions/master/be/overview.html +.. _C++ build tutorial: https://docs.bazel.build/versions/master/tutorial/cpp.html +.. _Java build tutorial: https://docs.bazel.build/versions/master/tutorial/java.html +.. _Android application tutorial: https://docs.bazel.build/versions/master/tutorial/android-app.html +.. _iOS application tutorial: https://docs.bazel.build/versions/master/tutorial/ios-app.html |