Macros

Introduction

A macro is a function called from the BUILD file that can instantiate rules. Macros are mainly used for encapsulation and code reuse of existing rules and other macros. By the end of the loading phase, macros don’t exist anymore, and Bazel sees only the concrete set of instantiated rules.

Usage

The typical use-case for a macro is when you want to reuse a rule.

For example, we have a genrule in a BUILD file that generates a file using //:generator with a some_arg argument hardcoded in the command:

genrule(
    name = "file",
    outs = ["file.txt"],
    cmd = "$(location //:generator) some_arg > $@",
    tools = ["//:generator"],
)

Tip: $@ is a Make variable that refers to the execution-time locations of the files in the outs attribute list. It is equivalent to $(locations :file.txt).

If you want to generate more files with different arguments, you may want to extract this code to a macro function. Let’s call the macro file_generator, which has name and arg parameters. Replace the genrule with the following:

load("//path:generator.bzl", "file_generator")

file_generator(
    name = "file",
    arg = "some_arg",
)

file_generator(
    name = "file-two",
    arg = "some_arg_two",
)

file_generator(
    name = "file-three",
    arg = "some_arg_three",
)

Here, we are loading the file_generator symbol from a .bzl file located in the //path package. By putting macro function definitions in a separate .bzl file, we can keep our BUILD files clean and declarative, The .bzl file can be loaded from any package in the workspace.

Finally, in path/generator.bzl, let’s write the definition of the macro to encapsulate and parameterize our original genrule definition:

def file_generator(name, arg, visibility=None):
  native.genrule(
    name = name,
    outs = [name + ".txt"],
    cmd = "$(location //:generator) %s > $@" % arg,
    tools = ["//:generator"],
    visibility = visibility,
  )

You can also use macros to chain rules together. This example shows chained genrules, where a genrule uses the outputs of a previous genrule as inputs:

def chained_genrules(name, visibility=None):
  native.genrule(
    name = name + "-one",
    outs = [name + ".one"],
    cmd = "$(location :tool-one) $@",
    tools = [":tool-one"],
    visibility = ["//visibility:private"],
  )

  native.genrule(
    name = name + "-two",
    srcs = [name + ".one"],
    outs = [name + ".two"],
    cmd = "$(location :tool-two) $< $@",
    tools = [":tool-two"],
    visibility = visibility,
  )

Note that we only assigned the value of visibility to the second genrule. This enables macro authors hide outputs of intermediate rules from being depended upon by other targets in the workspace.

Tip: Similar to $@ for outputs, $< expands to the locations of files in the srcs attribute list.

Expanding macros

When you want to investigate what a macro does, use the query command with --output=build to see the expanded form:

$ bazel query --output=build :file
# /absolute/path/test/ext.bzl:42:3
genrule(
  name = "file",
  tools = ["//:generator"],
  outs = ["//test:file.txt"],
  cmd = "$(location //:generator) some_arg > $@",
)

Instantiating native rules

Native rules (i.e. rules that don’t need a load() statement) can be instantiated from the native module, e.g.

def my_macro(name, visibility=None):
  native.cc_library(
    name = name,
    srcs = ["main.cc"],
    visibility = visibility,
  )

If you need to know the package name (i.e. which BUILD file is calling the macro), use the function native.package_name().

Debugging

bazel query --output=build //my/path:all will show you how the BUILD file looks after evaluation. All macros, globs, loops are expanded. Known limitation: select expressions are currently not shown in the output.
You may filter the output based on generator_function (which function generated the rules) or generator_name (the name attribute of the macro), e.g.
```
$ bazel query --output=build 'attr(generator_function, my_macro, //my/path:all)'
```
To find out where exactly the rule foo is generated in a BUILD file, you can try the following trick. Insert this line near the top of the BUILD file: cc_library(name = "foo"). Run Bazel. You will get an exception when the rule foo is created (due to a name conflict), which will show you the full stack trace.
You can also use print for debugging. It displays the message as a DEBUG log line during the loading phase. Except in rare cases, either remove print calls, or make them conditional under a debugging parameter that defaults to False before submitting the code to the depot.

Errors

If you want to throw an error, use the fail function. Explain clearly to the user what went wrong and how to fix their BUILD file. It is not possible to catch an error.

def my_macro(name, deps, visibility=None):
  if len(deps) < 2:
    fail("Expected at least two values in deps")
  # ...

Conventions

All public functions (functions that don’t start with underscore) that instantiate rules must have a name argument. This argument should not be optional (don’t give a default value).
Public functions should use a docstring following Python conventions.
In BUILD files, the name argument of the macros must be a keyword argument (not a positional argument).
The name attribute of rules generated by a macro should include the name argument as a prefix. For example, macro(name = "foo") can generate a cc_library foo and a genrule foo_gen.
In most cases, optional parameters should have a default value of None. None can be passed directly to native rules, which treat it the same as if you had not passed in any argument. Thus, there is no need to replace it with 0, False, or [] for this purpose. Instead, the macro should defer to the rules it creates, as their defaults may be complex or may change over time. Additionally, a parameter that is explicitly set to its default value looks different than one that is never set (or set to None) when accessed through the query language or build-system internals.
Macros should have an optional visibility argument.