Please basics

If you're familiar with Blaze / Bazel, Buck or Pants you will probably find Please very familiar. Otherwise, don't worry, it's not very complicated...

BUILD files

Please targets are defined in files named BUILD. These are analogous to Makefiles in that they define buildable targets for that directory. Fortunately, they do so in a rather nicer syntax.
We refer to a directory containing a BUILD file as a package.

For more information on the format of BUILD files, read the language specification here

Build targets

Each BUILD file can contain a number of build targets. These are units of buildable code which can be reused by other build targets. An example is probably worth 1000 words at this point:

Python
Java
Go
C
C++

        python_library(
            name = 'my_library',
            srcs = ['file1.py', 'file2.py'],
        )

        python_binary(
            name = 'my_binary',
            main = 'my_main.py',
            deps = [':my_library'],
        )

        python_test(
            name = 'my_library_test',
            srcs = ['my_library_test.py'],
            deps = [':my_library'],
        )
                

        java_library(
            name = 'my_library',
            srcs = ['File1.java', 'File2.java'],
        )

        java_binary(
            name = 'my_binary',
            main = 'Main.java',
            deps = [':my_library'],
        )

        java_test(
            name = 'my_library_test',
            srcs = ['MyLibraryTest.java'],
            deps = [':my_library'],
        )
                

        go_library(
            name = 'my_library',
            srcs = ['file1.go', 'file2.go'],
        )

        go_binary(
            name = 'my_binary',
            main = 'main.go',
            deps = [':my_library'],
        )

        go_test(
            name = 'my_library_test',
            srcs = ['my_library_test.go'],
            deps = [':my_library'],
        )
                

        cc_library(
            name = 'my_library',
            srcs = ['file1.cpp', 'file2.cpp'],
            hdrs = ['file1.hpp'],
        )

        cc_binary(
            name = 'my_binary',
            main = 'main.cpp',
            deps = [':my_library'],
        )

        cc_test(
            name = 'my_library_test',
            srcs = ['my_library_test.cpp'],
            deps = [':my_library'],
        )
                

        c_library(
            name = 'my_library',
            srcs = ['file1.c', 'file2.c'],
            hdrs = ['file1.h'],
        )

        c_binary(
            name = 'my_binary',
            main = 'main.c',
            deps = [':my_library'],
        )

        c_test(
            name = 'my_library_test',
            srcs = ['my_library_test.c'],
            deps = [':my_library'],
        )
                
This snippet defines three build targets; a build target is simply a thing that can be built by Please and typically appears as a single object in the build file (as the python_library, python_binary and python_test are above). This illustrates the core points of Please; every rule clearly defines its inputs - its own sources and its dependencies - and since these are known Please can be aggressive about parallelising, caching and reusing build artifacts.

Okay, great, so how do I actually use them?

Let's assume the build rules given above are defined in a file in your repo named package/BUILD. You can do the following:

Build labels

That brings us to the topic of build labels, which are identifiers of build targets. As you've just seen, these are of the form //package:target, where package is the path from the repo root to that package, and target is the name of the target within that package.
This is the most common form to identify targets absolutely, but there is also a convenient shorthand where we omit the part before the colon (as you saw earlier as well) to refer to a target within the current package.

The convention is to use lower_case_with_underscores for both package names and target names.
This is not just for looks; in many languages (eg. Python) the file path appears within the source files, and so it's important to choose something that's a valid lexical identifier.

There are also some special pseudo-labels:

Build targets can't use these as dependencies; these are primarily for using on the command line or in the visibility specification for a target.

The BUILD file format

You may have noticed that the invocations in BUILD files look a bit like function calls. This is not coincidence; it is not just a declarative language, it's also a scripting language that allows you to programmatically create targets. For example, let's say you want to translate your documentation to a bunch of languages:


      for language in ['en_NZ', 'en_GB', 'it_IT', 'es_MX', 'el_GR']:
          lang_docs = genrule(
              name = 'txt_documentation_' + language,
              srcs = ['docs.txt'],
              outs = [f'docs_{language}.txt'],
              tools = ['//tools:translate_tool'],
              cmd = f'$TOOL -i $SRC --language {language} > "$OUT"',
          )

          genrule(
              name = 'documentation_' + language,
              srcs = [lang_docs],
              outs = [f'docs_{language}.html'],
              tools = ['//tools:html_doc_tool'],
              cmd = '$TOOL --src $SRC --out "$OUT"',
              visibility = ['PUBLIC'],
          )
      
Obviously this would be a bit repetitive if you redefined the rules separately for each language. Instead, we can take advantage of the build language to genericise them across languages; for each language we translate it and then format it as HTML.

The language itself shares some similarity to Python so it should feel familiar to many people. There is a complete description of it available if you're curious about more details.

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