Rules
=====

**Status: Experimental**. Expect some breaking changes in the API.

Rule creation
-------------

In a Skylark extension, use the `rule` function to create a new rule and store
it in a global variable. [See example](cookbook.md#empty).

Attributes
----------

An attribute is a rule argument, such as `srcs` or `deps`. You must list
the attributes and their type when you define a rule.

```python
sum = rule(
  implementation=impl,
  attrs = {
      "number": attr.int(default = 1),
      "deps": attr.label_list(),
  },
)
```

If an attribute starts with `_`, it is private and users cannot set it. It
is useful in particular for label attributes (your rule will have an
implicit dependency on this label).

The following attributes are implicitly added to every rule: `name`,
`visibility`, `deprecation`, `tags`, `testonly`, `features`.

To access an attribute, use `ctx.attr.<attribute_name>`. The name and the
package of a rule are available with `ctx.label.name` and `ctx.label.package`.

[See example.](cookbook.md#attr)

The rule implementation function
--------------------------------

Every rule has to have an implementation function. This contains the actual
logic of the rule and is executed strictly in the Analysis Phase. The function
has exactly one input parameter, `ctx` and it may return
the [runfiles](#runfiles) and [providers](#providers)
of the rule. The input parameter `ctx` can be used to access attribute values,
outputs and dependent targets and files. It also has some helper functions.
See [the library](library.html#ctx) for more context. Example:

```python
def impl(ctx):
  ...
  return struct(
      runfiles = ...,
      my_provider = ...,
      ...
  )

my_rule = rule(
  implementation=impl,
  ...
)
```

Files
-----

There are two kinds of files: files stored in the file system and generated
files. For each generated file, there must be one and only one generating
action, and each action must generate one ore more output files. Otherwise
Bazel will throw an error.

Targets
-------

Every build rule corresponds to exactly one target. A target can create
[actions](#actions), can have dependencies (which can be files or
other build rules), [output files](#output-files) (generated by
its actions) and [providers](#providers).

A target `y` is depending on target `x` if `y` has a label or label list type
attribute where `x` is declared:

```python
my_rule(
    name = "x"
)

my_rule(
    name = "y",
    deps = [":x"]
)
```

In the above case it's possible to access targets declared in `my_rule.deps`:

```python
def impl(ctx):
  for dep in ctx.attr.deps:
    # Do something with dep
  ...

my_rule = rule(
  implementation=impl,
  attrs = {
      "deps": attr.label_list()
  }
  ...
)
```

Output files  {#output-files}
------------

A target can declare output files, which must be generated by the target's
actions. There are three ways to create output files in Skylark:

* If the rule is marked `executable`, it creates an output file of the same name
  as the rule's. [See example](cookbook.md#outputs-executable)

* The rule can declare default `outputs`, which are always generated.
  [See example](cookbook.md#outputs-default)

* The rule can have output or output list type attributes. In that case the
  output files come from the actual attribute values.
  [See example](cookbook.md#outputs-custom)

All output files must have exactly one generating action. See the
[library](library.html#modules.ctx.outputs) for more context.


Actions
-------

There are three ways to create actions:

* `ctx.action`
* `ctx.file_action`
* `ctx.template_action`

Actions take a set (can be empty) of input files and generate a (non-empty)
set of output files.
The set of input and output files must be known during the analysis phase. It
might depend on the value of attributes and information from dependencies, but
it cannot depend on the result of the execution. For example, if your action
runs the unzip command, you must specify which files you expect to be inflated
(before running unzip).

Actions are comparable to pure functions: They should depend only on the
provided inputs, and avoid accessing computer information, username, clock,
network or I/O devices (except for reading inputs and writing outputs).

**If a command generates a file that is not listed in the outputs**: It is fine.
The file will be ignored and cannot be used by other rules.

**If a command does not generate a file that is listed in the outputs**: It is an
execution error and the build will fail. This happens for instance when a
compilation fails.

**If a command generates an unknown number of outputs and you want to keep them
all**, you may group them in a zip file. This way, you will be able to declare
your output.

**If a command does not list a file it uses as an input**, the action execution
will most likely result in an error. The file is not guaranteed to be available
to the action, so if it **is** there, it's due to a coincidence or error.

**If a command lists a file as an input, but does not use it**: It is fine. However
it can affect the action execution order resulting in sub-optimal performance.

Dependencies are resolved by Bazel, which will decide which actions are
executed. It is an error if there is a cycle in the dependency graph. Creating
an action does not guarantee that it will be executed: It depends on whether
its outputs are needed for the build.

Providers
---------

Providers are used to access information from another rule. A rule depending on
another rule has access to the data the latter provides. These data can be e.g.
output files, the libraries the dependent rule is using to link or compile, or
anything the depending rule should know about. Using providers is the only way
to exchange data between rules.

A rule can only access data provided by its direct dependencies, not that of
transitive dependencies: if rule `top` depends on `middle` and `middle` depends
on `bottom`, then `middle` is a direct dependency of `top` and `bottom` is a
transitive dependency of `top`. In this scenario `top` can only access data
provided by `middle`. If `middle` also provides the data that `bottom` provided
to it then and only then can `top` access it.

Only the following data types are allowed to pass using providers:

* `bool`
* `integer`
* `string`
* `file`
* `label`
* `None`
* anything composed of these types and `lists`, `dicts`, `sets` or `structs`

Providers are created from the return value of the rule implementation function:

```python
def dependent_rule_implementation(ctx):
  ...
  return struct(
      transitive_data = set(["a", "b", "c"])
  )
```

A depending rule might access these data as struct fields of the depending
`target`:

```python
def depending_rule_implementation(ctx):
  ...
  s = set()
  for dep_target in ctx.attr.deps:
    s += dep_target.transitive_data
  ...
```

Providers are only available during the analysis phase. Examples of usage:

* [mandatory providers](cookbook.md#mandatory-providers)
* [optional providers](cookbook.md#optional-providers)

Runfiles
---------

Runfiles are a set of files used by the (often executable) output of a rule
during runtime (as opposed to build time, i.e. when the binary itself is
generated).
Bazel creates a directory tree containing symlinks pointing to the
runfiles during execution, to stage this environment for the binary which can
thus access them during runtime.

Runfiles can be added manually during rule creation and/or collected
transitively from dependent rules:

```python
def rule_implementation(ctx):
  ...
  transitive_runfiles = set()
  for dep in ctx.attr.special_dependencies:
     transitive_runfiles += dep.transitive_runtime_files

  runfiles = ctx.runfiles(
      # Add some files manually.
      files = [ctx.file.some_data_file]
      # Add transitive files from dependencies manually.
      transitive_files = transitive_runfiles
      # Collect runfiles from the common locations: transitively from srcs,
      # deps and data attributes.
      collect_default = True
  )
  # Add a field named "runfiles" to the return struct in order to actually
  # create the symlink tree.
  return struct(runfiles = runfiles)
```

Note that non-executable rule outputs can also have runfiles. For example, a
library might need some external files during runtime, and every depending
binary should know about it.

Also note that if an action uses an executable, the executable's runfiles can
be used when the action executes.