An Introduction to Static Typing in Ruby 3

An Introduction to Static Typing
in Ruby 3
Soutaro Matsumoto 
@soutaro

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Soutaro Matsumoto
• One of the Ruby core committers

• Develops RBS, Steep, and rbs_protobuf

• Works for Square

• @soutaro on GitHub/Twitter

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Ruby 3.0
• Released Dec 25, 2020 🎉🎄🎅

• Key updates

• Performance with MJIT

• Concurrency with Ractor and Fiber Scheduler

• Static typing with RBS and TypeProf ← 👀
https://www.ruby-lang.org/en/news/2020/12/25/ruby-3-0-0-released/

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Static Typing in Ruby 3
• Ruby 3 ships with RBS

• RBS gem and type de
fi
nitions of standard libraries

• RBS is not a type checker, but a language,
fi
les, or a library

• Several RBS-based tools are available

• You choose the best tools for your projects

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# The conference object represents a conference.

#

# conf = Conference.new("RubyConf 2020")

# conf.talks << talk1


#

class Conference

# The name of the conference.

attr_reader name: String

# Talks of the conference.

attr_reader talks: Array[Talk]

def initialize: (String name) -> void

# Yields all speakers of the conference.

# Deduped, and no order guaranteed.

def each_speaker: { (Speaker) -> void } -> void

| () -> Enumerator[Speaker, void]

end

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Static Type Checking
conference = Conference.new("RubyConf 2020")

talks = [...]

conference.talks.push(*talks)

conference.each_speaker do |speaker|

puts "%s (%s)" % [speaker.name, speaker.email]

end

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talks = [...]




end
Does new method accept a String argument?

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talks = [...]




end
What is the type of the value of #talks?

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talks = [...]




end
What is the type of the value of #talks?
What is the type of the speaker?

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# The conference object represents a conference.

#

# conf = Conference.new("RubyConf 2020")



#

class Conference

# The name of the conference.

attr_reader name: String

# Talks of the conference.

attr_reader talks: Array[Talk]

def initialize: (String name) -> void

# Yields all speakers of the conference.

# Deduped, and no order guaranteed.

def each_speaker: { (Speaker) -> void } -> void

| () -> Enumerator[Speaker, void]

end

talks = [...]




end

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RBS Features
Union types
Generics
Interface types untyped type
String | Symbol

Array[String | Integer]

Integer? #== Integer | nil
Array[Integer]

Hash[String, Array[Integer]]
def eval: (String) -> untyped
interface _IntegerConvertible

def to_int: () -> Integer

end

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RBS Based Tools
• Type checkers

• Steep

• rbs-test
• RBS generators

• rbs-prototype

• TypeProf,

• rbs_rails

• Sord

• rbs_protobuf
• Others

• gem_rbs_collection

• rbs2ts

• (Sorbet)

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Steep
• Static type checker for Ruby written in Ruby based on RBS

• You write types of your Ruby program in RBS, and Steep checks the
consistency between the code and RBS

• https://github.com/soutaro/steep
$ steep check

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Type Checking with Steep
• VSCode integration

• On the
fl
y error reporting

• Completion

• Hover to show meta-info

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rbs-prototype
$ rbs prototype rb lib/goodcheck/import_loader.rb

$ rbs prototype runtime -rgoodcheck 'Goodcheck::ImportLoader'
• Detect classes and methods and generate RBS for them

• All types are left untyped
module Goodcheck

class Analyzer

attr_reader rule: untyped

attr_reader trigger: untyped

attr_reader buffer: untyped

def initialize: (rule: untyped rule, trigger: untyped trigger, buffer: untyped buffer) -> untyped

def scan: () { (untyped) -> untyped } -> untyped

def scan_simple: (untyped regexp) { (untyped) -> untyped } -> untyped

def scan_var: (untyped pat) { (untyped) -> untyped } -> untyped

end

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rbs_rails
• Generates RBS
fi
les for Active Record models and URL helpers

• It knows how associations and attributes are represented as Ruby
objects in Rails app

• https://github.com/pocke/rbs_rails
$ rake rbs_rails:generate_rbs_for_models \
 
rbs_rails:generate_rbs_for_path_helpers

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class Logfile < ApplicationRecord

extend _ActiveRecord_Relation_ClassMethods[Logfile, Logfile::ActiveRecord_Relation]

attr_accessor id (): Integer

def id_changed?: () -> bool

def id_change: () -> [Integer?, Integer?]

def id_will_change!: () -> void

def id_was: () -> Integer?

def id_previously_changed?: () -> bool

def id_previous_change: () -> Array[Integer?]?

def id_previously_was: () -> Integer?

def restore_id!: () -> void

class Logfile < ApplicationRecord

belongs_to :report

validates :report_id, :presence => true

validates :ext,

:uniqueness => { :scope => :report_id },

:inclusion => { :in => %w[log.txt diff.txt log.html diff.html],

:message => "%{value} is not a valid ext" }

validates :data, :presence => true

def uri

r = report

t = r.datetime.strftime('%Y%m%dT%H%M%SZ')

"#{r.server.uri}ruby-#{r.branch}/log/#{t}.#{ext}.gz"

end

end

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gem_rbs_collection
• RBS
fi
les for Ruby gems

• RBS supports loading RBS
fi
les from a gem, but this is for gems
without RBS
fi
les

• == De
fi
nitelyTyped in TypeScript, typeshed in Python
https://github.com/ruby/gem_rbs_collection

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Only 14 gems... 㱺 Chance for contribution! 🤩

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Work
fl
ow
1. Generate RBS with rbs_rails

2. Choose a class from your project

3. Write RBS for the class (or generate with rbs-prototype)

4. Run Steep to con
fi
rm your RBS and Ruby code are consistent

5. Goto 2

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Start Small
• I don't recommend generating RBS and running the tool for everything

• Many type errors will be reported and you will give up

• You can type check gradually
# This won't work well ⚠

$ rbs prototype rb lib/**/*.rb

$ steep check

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RBS Based Tools
• Type checkers

• Steep

• rbs-test
• RBS generators

• rbs-prototype

• TypeProf,

• rbs_rails

• Sord

• rbs_protobuf
• Others

• gem_rbs_collection

• rbs2ts

• (Sorbet)

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rbs-test
• Runtime type checking (not static)

• Hooks method calls and type checks if arguments/return values have
expected types

• Pros: Easier to use compared to Steep 
Cons: Slow, unsupported features, incompatibilities
$ rbs test --target='Goodcheck::*' \

ruby -Ilib:test test/config_loader_test.rb

...

ERROR["test_load_config"] test_load_config#ConfigLoaderTest (0.03s)

Minitest::UnexpectedError:

TypeError: [Goodcheck::Pattern::Regexp#initialize]

ArgumentTypeError: expected `bool` but given `nil`

...

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rbs-test

expected types



...





...
Class patterns to type check

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rbs-test

expected types



...





...
Class patterns to type check
Command line to run Ruby program

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TypeProf
• Generates RBS
fi
les from Ruby code.

• Much smarter than rbs-prototype.

• Execute the Ruby script at type level, and print the result.
Ruby script to start execution
$ typeprof exe/goodcheck

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module Goodcheck

class Analyzer



attr_reader buffer: untyped

def initialize: (rule: untyped rule, trigger: untyped trigger, buffer: untyped buffer) -> untyped

def scan: () { (untyped) -> untyped } -> untyped

def scan_simple: (untyped regexp) { (untyped) -> untyped } -> untyped

def scan_var: (untyped pat) { (untyped) -> untyped } -> untyped

end

end

$ rbs prototype rb
module Goodcheck

class Analyzer



attr_reader buffer: Buffer

def initialize: (rule: untyped, trigger: untyped, buffer: Buffer) -> Buffer

def scan: ?{ (Issue) -> Array[bot]? } -> ((Array[Issue] | Enumerator[Issue, untyped] | Enumerator

def scan_simple: (Regexp regexp) ?{ (Issue) -> Array[bot]? } -> Array[Issue]?

def scan_var: (untyped pat) ?{ (Issue) -> untyped } -> nil

end

end
$ typeprof

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Sord and rbs_protobuf
• Sord reads YARD docs and generate RBS
(and RBI)
fi
les using the types included in
the comments

• https://github.com/AaronC81/sord
$ sord defs.rbs
• rbs_protobuf runs as a plugin of protoc and
translates .proto to RBS type de
fi
nition

• https://github.com/square/rbs_protobuf
$ protoc --rbs_out=sig/protos \

protos/a.proto
Refer types written by human

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rbs2ts
• Converts RBS type de
fi
nitions to TypeScript types

• https://github.com/mugi-uno/rbs2ts
$ rbs2ts convert type.rbs

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Sorbet
• The most widely used static type checker for Ruby

• They plan to support reading (a subset of) RBS

• RBI will continue to be the primary type de
fi
nition format in Sorbet

• Supporting RBS will help using gems even without RBI
fi
les

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Recap
• Ruby 3 ships with RBS and there are tools based on RBS

• Waiting for your contribution

• Writing RBS of gems and push it to gem_rbs_collection

• Generating API docs from RBS

• I want more Steep users

• Let me help you to start type checking your Ruby code using Steep

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@soutaro

[email protected]

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An Introduction to Static Typing in Ruby 3

An Introduction to Static Typing in Ruby 3

Soutaro Matsumoto

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