Steep - Gradual Typing for Ruby
Install via RubyGems.
$ gem install steep
Steep requires Ruby 2.6 or later.
Steep does not infer types from Ruby programs, but requires declaring types and writing annotations. You have to go on the following three steps.
steep init to generate a configuration file.
$ steep init # Generates Steepfile
target :app do check "lib" signature "sig" library "set", "pathname" end
1. Declare Types
Declare types in
.rbs files in
class Person @name: String @contacts: Array[Email | Phone] def initialize: (name: String) -> untyped def name: -> String def contacts: -> Array[Email | Phone] def guess_country: -> (String | nil) end class Email @address: String def initialize: (address: String) -> untyped def address: -> String end class Phone @country: String @number: String def initialize: (country: String, number: String) -> untyped def country: -> String def number: -> String def self.countries: -> Hash[String, String] end
- You can use simple generics, like
- You can use union types, like
Email | Phone.
- You have to declare not only public methods but also private methods and instance variables.
- You can declare singleton methods, like
- There is
niltype to represent nullable types.
2. Write Ruby Code
Write Ruby code with annotations.
class Person # `@dynamic` annotation is to tell steep that # the `name` and `contacts` methods are defined without def syntax. # (Steep can skip checking if the methods are implemented.) # @dynamic name, contacts attr_reader :name attr_reader :contacts def initialize(name:) @name = name @contacts =  end def guess_country() contacts.map do |contact| # With case expression, simple type-case is implemented. # `contact` has type of `Phone | Email` but in the `when` clause, contact has type of `Phone`. case contact when Phone contact.country end end.compact.first end end class Email # @dynamic address attr_reader :address def initialize(address:) @address = address end def ==(other) # `other` has type of `untyped`, which means type checking is skipped. # No type errors can be detected in this method. other.is_a?(self.class) && other.address == address end def hash self.class.hash ^ address.hash end end class Phone # @dynamic country, number attr_reader :country, :number def initialize(country:, number:) @country = country @number = number end def ==(other) # You cannot use `case` for type case because `other` has type of `untyped`, not a union type. # You have to explicitly declare the type of `other` in `if` expression. if other.is_a?(Phone) # @type var other: Phone other.country == country && other.number == number end end def hash self.class.hash ^ country.hash ^ number.hash end end
3. Type Check
steep check command to type check.
$ steep check lib/phone.rb:46:0: MethodDefinitionMissing: module=::Phone, method=self.countries (class Phone)
You now find
Phone.countries method is not implemented yet.
You can use
rbs prototype command to generate a signature declaration.
$ rbs prototype rb lib/person.rb lib/email.rb lib/phone.rb class Person @name: untyped @contacts: Array[untyped] def initialize: (name: untyped) -> Array[untyped] def guess_country: () -> untyped end class Email @address: untyped def initialize: (address: untyped) -> untyped def ==: (untyped) -> untyped def hash: () -> untyped end class Phone @country: untyped @number: untyped def initialize: (country: untyped, number: untyped) -> untyped def ==: (untyped) -> void def hash: () -> untyped end
It prints all methods, classes, instance variables, and constants. It can be a good starting point to writing signatures.
Because it just prints all
defs, you may find some odd points:
- The type of
- There are no
Generally, these are by our design.
rbs prototype offers options:
rbi to generate prototype from Sorbet RBI and
runtime to generate from runtime API.
You can find examples in
Steep implements some of the Language Server Protocol features. You can use Steep with VSCode and its plugin.
Other LSP supporting tools may work with Steep where it starts the server as
After checking out the repo, run
bin/setup to install dependencies. Then, run
rake test to run the tests. You can also run
bin/console for an interactive prompt that will allow you to experiment.
To install this gem onto your local machine, run
bundle exec rake install. To release a new version, update the version number in
version.rb, and then run
bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the
.gem file to rubygems.org.
Bug reports and pull requests are welcome on GitHub at https://github.com/soutaro/steep.