bazel-skylib/README.md

Skylib

Skylib is a standard library that provides functions useful for manipulating collections, file paths, and other features that are useful when writing custom build rules in Bazel.

This library is currently under early development. Be aware that the APIs in these modules may change during this time.

Each of the .bzl files in the lib directory defines a "module"—a struct that contains a set of related functions and/or other symbols that can be loaded as a single unit, for convenience.

Skylib also provides build rules under the rules directory.

Getting Started

WORKSPACE file

Add the following to your WORKSPACE file to import the Skylib repository into your workspace. Replace the version number in the tag attribute with the version you wish to depend on:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
load("@bazel_tools//tools/build_defs/repo:utils.bzl", "maybe")

maybe(
    http_archive,
    name = "bazel_skylib",
    url = "https://github.com/bazelbuild/bazel-skylib/releases/download/0.9.0/bazel_skylib-0.9.0.tar.gz",
    sha256 = "1dde365491125a3db70731e25658dfdd3bc5dbdfd11b840b3e987ecf043c7ca0",
)

If you want to use lib/unittest.bzl from Skylib versions released in or after December 2018, then you also should add to the WORKSPACE file:

load("@bazel_skylib//:workspace.bzl", "bazel_skylib_workspace")

bazel_skylib_workspace()

BUILD and *.bzl files

Then, in the BUILD and/or *.bzl files in your own workspace, you can load the modules (listed below) and access the symbols by dotting into those structs:

load("@bazel_skylib//lib:paths.bzl", "paths")
load("@bazel_skylib//lib:shell.bzl", "shell")

p = paths.basename("foo.bar")
s = shell.quote(p)

List of modules (in lib/)

• collections
• dicts
• partial
• paths
• selects
• sets - deprecated, use new_sets
• new_sets
• shell
• structs
• types
• unittest
• versions

List of rules (in rules/)

• analysis_test
• build_test
• cmd_maprule and bash_maprule

Writing a new module

Steps to add a module to Skylib:

1. Create a new .bzl file in the lib directory.

2. Write the functions or other symbols (such as constants) in that file, defining them privately (prefixed by an underscore).

3. Create the exported module struct, mapping the public names of the symbols to their implementations. For example, if your module was named things and had a function named manipulate, your things.bzl file would look like this:

   def _manipulate():
     ...
   
   things = struct(
       manipulate=_manipulate,
   )
   

4. Add unit tests for your module in the tests directory.

bzl_library

The bzl_library.bzl rule can be used to aggregate a set of Starlark files and its dependencies for use in test targets and documentation generation.

Troubleshooting

If you try to use unittest and you get the following error:

ERROR: While resolving toolchains for target //foo:bar: no matching toolchains found for types @bazel_skylib//toolchains:toolchain_type
ERROR: Analysis of target '//foo:bar' failed; build aborted: no matching toolchains found for types @bazel_skylib//toolchains:toolchain_type

then you probably forgot to load and call bazel_skylib_workspace() in your WORKSPACE file.