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Stdlib Signatures Guide

This is a guide for contributing to rbs by writing/revising stdlib signatures.

The typical steps of writing signatures will be like the following:

  1. Generate a prototype
  2. Import RDoc document
  3. Give correct types to the prototype
  4. Add tests


Signatures for builtin libraries are located in core directory. Also, signatures for standard libraries are located in stdlib directory.

To write signatures see syntax guide.

Generating prototypes

rbs provides a tool to generate a prototype of signatures, rbs prototype. It provides several options, rbi from Sorbet RBI files, rb from Ruby code, and runtime from runtime API. runtime should be the best option for standard libraries because they may be implemented in C, no Ruby source code.

The tool requires all of the libraries specified with -r option, and then use introspection APIs like instance_methods to know the structure of the class. The commandline receives the name of classes you want to prototype, exact class name (like Pathname) or pattern with * (like IO::*).

$ bundle exec rbs prototype runtime --require pathname Pathname
class Pathname
  def self.getwd: () -> untyped

  def self.glob: (*untyped) -> untyped

  def self.pwd: () -> untyped

  def +: (untyped other) -> untyped

  alias / +

  def <=>: (untyped) -> untyped

  # snip

# snip

The prototype includes:

  • Instance method definitions,
  • Singleton method definitions,
  • Includes, and
  • Constants

It generate a simple prototype in the sense that all of the types included are untyped. But it will help you to have an overview of the signatures you are trying to write.

What to do with existing RBS files

Generating prototypes will override everything, so the problem is if there is a RBS files already. You can try to find missing parts, or you can start from the scratch.

One non-trivial but absolutely better solution is to make a tool:

  1. To load type definitions from existing RBS file, and
  2. Generate prototypes only for missing methods.

Import RDoc document

The next step should be importing RDoc documents.

$ bin/annotate-with-rdoc stdlib/pathname/0/pathname.rbs
Loading store from /Users/soutaro/.rbenv/versions/2.7.0-dev/share/ri/2.7.0/system...
Loading store from /Users/soutaro/.rbenv/versions/2.7.0-dev/share/ri/2.7.0/site...
Opening stdlib/pathname/pathname.rbs...
  Importing documentation for Pathname...
    Processing glob...
    Processing +...
    # snip
Writing stdlib/pathname/pathname.rbs...

The annotate-with-rdoc command adds annotations to RBS files.

  1. Query RDoc documents to annotate classes, modules, methods, and constants,
  2. Put annotations on RBS AST, and
  3. Update the given .RBS files

We recommend using the command to annotate the RBS files.

Writing types

The next step is to replace untyped types in the prototype. See syntax guide for the detail of the syntax.

We can show some of the guides for writing types.

  1. Use bool for truth values, truthy or falsey. More specific types like TrueClass | FalseClass may be too strict.
  2. Use void if the return value is useless.
  3. Use nil instead of NilClass.
  4. The most strict types may not be the best types. Use untyped if you cannot find the best one.

Add Tests

We support writing tests for stdlib signatures.

Writing tests

First, execute generate:stdlib_test rake task with a class name that you want to test.

$ bundle exec rake 'generate:stdlib_test[String]'
Created: test/stdlib/String_test.rb

It generates test/stdlib/[class_name]_test.rb. The test scripts would look like the following:

class StringSingletonTest < Test::Unit::TestCase
  include TypeAssertions

  testing "singleton(::String)"

  def test_initialize
    assert_send_type "() -> String",
                     String, :new
    assert_send_type "(String) -> String",
                     String, :new, ""
    assert_send_type "(String, encoding: Encoding) -> String",
                     String, :new, "", encoding: Encoding::ASCII_8BIT
    assert_send_type "(String, encoding: Encoding, capacity: Integer) -> String",
                     String, :new, "", encoding: Encoding::ASCII_8BIT, capacity: 123
    assert_send_type "(encoding: Encoding, capacity: Integer) -> String",
                     String, :new, encoding: Encoding::ASCII_8BIT, capacity: 123
    assert_send_type "(ToStr) -> String",
                     String, :new,"")
    assert_send_type "(encoding: ToStr) -> String",
                     String, :new, encoding:'Shift_JIS')
    assert_send_type "(capacity: ToInt) -> String",
                     String, :new, capacity:

class StringTest < Test::Unit::TestCase
  include TypeAssertions

  # library "pathname", "set", "securerandom"     # Declare library signatures to load
  testing "::String"

  def test_gsub
    assert_send_type "(Regexp, String) -> String",
                     "string", :gsub, /./, ""
    assert_send_type "(String, String) -> String",
                     "string", :gsub, "a", "b"
    assert_send_type "(Regexp) { (String) -> String } -> String",
                     "string", :gsub, /./ do |x| "" end
    assert_send_type "(Regexp) { (String) -> ToS } -> String",
                     "string", :gsub, /./ do |x|"") end
    assert_send_type "(Regexp, Hash[String, String]) -> String",
                     "string", :gsub, /./, {"foo" => "bar"}
    assert_send_type "(Regexp) -> Enumerator[String, self]",
                     "string", :gsub, /./
    assert_send_type "(String) -> Enumerator[String, self]",
                     "string", :gsub, ""
    assert_send_type "(ToStr, ToStr) -> String",
                     "string", :gsub,"a"),"b")

You need include TypeAssertions which provide useful methods for you. testing method call tells which class is the subject of the class. assert_send_type method call asserts to be valid types and confirms to be able to execute without exceptions. And you write the sample programs which calls all of the patterns of overloads.

Note that the instrumentation is based on refinements and you need to write all method calls in the unit class definitions. If the execution of the program escape from the class definition, the instrumentation is disabled and no check will be done.

Running tests

You can run the test with:

$ bundle exec rake stdlib_test                # Run all tests
$ bundle exec ruby test/stdlib/String_test.rb # Run specific tests