Called “downcase” on nil? Forgot to return the right object in one of your logic branches? Called “first” on a String instead of an Array and spent half an hour trying to figure out why a single character was getting passed around everywhere? At Grailed, these situations were not uncommon. We are the largest marketplace for luxury men's fashion, with over 7 million users, and a growing Rails codebase that spans hundreds of thousands of lines. Before typing, changes to core interfaces often required creative grepping, modification of type checking unit tests, and updating brittle type documentation. Ever since we started gradually typing our codebase with Sorbet, we’ve been able to make intrusive changes faster and confidently. In this talk, we’ll walk you through our prior art, challenges, learnings, and big benefits of typing our codebase.

1. Sorbet at Grailed
Typing a Large Rails Codebase to Ship with Confidence

2. About me
> $ Hi, I’m Jose Rosello!
> $ Live in Brooklyn, NY
> $ Staff Engineer at Grailed
> $ Worked on infrastructure, security, and payments over the past 4 years

3. About this talk
> $ About Grailed
> $ Motivation for Static Types
> $ Intro to Sorbet
> $ Rolling out Sorbet at Grailed
> $ Current Challenges
> $ Parting Thoughts

4. About Grailed
> $ Peer-to-peer marketplace focusing on style and fashion
> $ Grown to support 7+ million users
> $ We use Rails to power our API
> $ 30k lines of code in 2017 - 150k lines of code in 2021
> $ Typescript for our client and Next.js code
> $ Introduced Sorbet in early 2020 by an engineer who shipped a null
pointer bug and said “nevermore”

5. Why static typing in Ruby?

6. > $ Refactoring widely used interfaces was scary
Good grepping / code finding skills required
> $ Type enforcement subject to human error
Unit tests could miss it
Null pointers find a way (especially ActiveRecord ones)
Changes

7. > $ Stale documentation around types
# Converts the object into textual markup given a specific format.
#
# @param format [Symbol] the format type, `:text` or `:html`
# @return [String] the object converted into the expected format.
def to_format(format = :html)
# format the object
end
> $ Better enforce documentation to begin with (especially for a big
codebase!)
Legibility

8. Intro to Sorbet

9. What can Sorbet do?
> $ Sorbet will help address these (with some tradeoffs)
> $ Research is split on the overall benefit of static typing, but researchers
aren’t looking at your codebase
(not everyone agrees!)

10. > $ Gradual typing
> $ Sigils:
typed: ignore -- Sorbet won’t look at this file
typed: false -- (Default) Checks for missing constants and syntax errors
typed: true -- All of the above + methods and their arity and any existing signatures
typed: strict -- All methods and constants must have types declared
typed: strong -- No dynamic typing at all, all call issued within must also return typed values
> $ We aim to have true for existing files, and strict for all new code

11. Signature Types
# typed: strict
class PaymentTransaction < ApplicationRecord
extend T::Sig # Exposes sig and other Sorbet helpers
sig { returns(T::Boolean) } # Declare signature, return T::Boolean
def committed?
status == "success"
end
end

12. Sigils
# typed: true
class Photo
COMPANY = "Acme"
PHOTOGRAPHERS = {
principal: "Jenna",
backup: "Aaron",
}
def watermark
"#{COMPANY - #{PHOTOGRAPHERS[:principal]}"
end
end
# typed: strict
class Photo
extend T::Sig
COMPANY = T.let("Acme", String)
PHOTOGRAPHERS = T.let({
principal: "Jenna",
backup: "Aaron",
}, T::Hash[Symbol, String])
sig { returns(String) }
def watermark
"#{COMPANY} -
#{PHOTOGRAPHERS[:principal]}"
end
end

13. > $ Runtime checks
Important, since not all code is typed
We don’t treat them as exceptions (for now)
> $ RBI’s (Ruby Interface files)
Used to declare types for dependencies or dynamically generated code
Separate files
Not “real” code
Valid Ruby

14. RBI’s (Ruby Interface Files)
class Pry
extend ::Forwardable
extend ::Pry::Forwardable
def initialize(options = T.unsafe(nil)); end
def add_sticky_local(name, &block); end
def backtrace; end
def backtrace=(_arg0); end
def binding_stack; end
end

15. RBI’s (Ruby Interface Files)
module Designer::GeneratedAttributeMethods
sig { returns(T.nilable(T::Boolean)) }
def basic?; end
sig { params(value: T.nilable(T::Boolean)).void }
def basic=(value); end
sig { params(value: T.nilable(T::Boolean)).void }
def classify_enabled=(value); end
sig { returns(T::Boolean) }
def classify_enabled?; end
end

16. Our Favorite Sorbet Features

17. T.nilable to enforce nil checks
sig {params(label:
T.nilable(String)).returns(T.nilable(String))}
def label_id(label)
"#{label.upcase}-#{item_id}"
end
sig {params(label:
T.nilable(String)).returns(T.nilable(String))}
def label_id(label)
if label
"#{label.upcase}-#{item_id}"
end
end
type error, label can be nil

18. Exhaustiveness checking with Enum
class FeeCalculator
extend T::Sig
sig {params(fee: Fee).returns(Integer)}
def self.calculate_percentage(fee)
case fee
when Fee::None
0
when Fee::StandardFee
2
when Fee::NewMemberFee
3
else
T.absurd(fee)
end
end
end
class Fee < T::Enum
extend T::Sig
enums do
None = new
StandardFee = new
NewMemberFee = new
end
end
Sorbet will complain
if this code is reachable!

19. Enforce data shape with T::Struct
class Reply < T::Struct
const :valid, T::Boolean
const :title, T.nilable(String)
const :body, T.nilable(String)
const :actions, T::Array[Action], default: []
const :takeover, T.nilable(String)
end

20. Void return types
sig { void }
def send_messages!
shipping_address = Grailed::ShippingAddress.find_through_order(listing: @listing)
Conversations::GrailedBot::PurchaseConfirmation.call(
listing: @listing,
shipping_address: shipping_address,
shipping_label: @shipping_label,
)
end

21. Aid in discovery and refactoring code (like improving callsite to only take symbols and not string, or getting rid of symbol and
string altogether!)
class AutomaticAction < T::Enum
extend T::Sig
enums do
Disburse = new("disburse")
Refund = new("refund")
end
sig { returns(ActiveSupport::Duration) }
def shipment_required_period
case self
when Disburse then 5.days
when Refund then 7.days
else T.absurd(self)
end
end
end
Aid in discovery

22. Rolling out Sorbet

23. First steps
> $ Big initial PR to perform `srb init`
> $ Some things Sorbet did not like:
No dynamic imports (we used DryMonad)
Undefined constants and dead code
> $ Issues with Rails in general

24. Tooling
> $ sorbet-rails to create RBI’s for all the dynamic methods Rails generates
(on models, etc)
> $ Tapioca to create RBI’s for gems
> $ rubocop-sorbet to enforce sigils and other linting errors
> $ spoom for reporting and auto-bumping the sigil strictness across files if
possible

25. Evangelize and Enforce
> $ Finally ready to start using typed: true and typed: strict around our codebase
as code is refactored
> $ Evangelize Sorbet and encourage folks to type code as things went along
> $ Enforce it on CI

26. Type your Dynamic Code: Service Objects at Grailed
class SaveService
# This lets us use SaveService.call(...)
instead of SaveService.new(...).call
include ServiceObject
sig { params(sales_tax: SalesTax, order:
Order).void }
def initialize(sales_tax, order)
@sales_tax = sales_tax
@order = order
end
sig { returns(SalesTax) }
def call
# ...
end
end
module ServiceObject
def self.included(klass)
klass.extend(ClassMethods)
end
module ClassMethods
def call(*args, **kwargs)
if kwargs.present?
new(*args, **kwargs).call
else
new(*args).call
end
end
end
end

27. Type your Dynamic Code: Breaking Things
class SaveService
include ServiceObject
sig { params(sales_tax: SalesTax, order:
Order).void }
def initialize(sales_tax, order)
@sales_tax = sales_tax
@order = order
end
sig { returns(SalesTax) }
def call
# ...
end
sig { params(sales_tax: SalesTax, order:
Order).returns(SalesTax) }
def self.call
# ...
end
end
We broke
ServiceObject.call(...)
because we only statically declared
ServiceObject.new(...).call

28. Type your Dynamic Code: Parlour to the Rescue
ParlourHelper.traverse_rbi_tree(source_location) do |current_child| # source_location is our service object
if ParlourHelper.instance_method?(current_child)
if current_child.name == "initialize"
init_method = current_child
elsif current_child.name == "call"
call_method = current_child
end
end
# Creates an RBI for our class with the correct `call` class method signature
generator.root.create_class(service_object.to_s) do |klass|
klass.create_method(
"call",
class_method: true,
parameters: init_method.parameters,
return_type: call_method.return_type,
)
end
end

29. Removing Dynamic Code
> $ Removed dynamic imports
> $ Moved away from DryMonads as a whole - wasn’t really possible to
enforce types

30. Replace Result Monad with Type union
ErrorType = T.type_alias {
T.any(InvalidPermissionsError, NoListingError)
}
sig { returns(T.any(Listing, ErrorType)) }
def call
err = validate
return err if err.present?
# ...
@listing
end
def listing
result = Service.call(
listing: @listing,
)
case result
when Listing
render_json(result)
when Service::InvalidPermissionsError
render_error(...)
else
render_error(...)
end
end

31. Challenges We Still Face
> $ Manual refreshing of RBI’s for gems and Rails models
> $ Not confident in runtime checks yet
> $ Gradual typing still lets some type errors through
> $ Hacks for testing (RSpec ignore)
> $ Impedance mismatch with Rails
> $ Learning curve for new developers
> $ It’s very much in beta, you will almost certainly run into bugs and
limitations

32. Parting Words
> $ Almost 2 years after the initial rollout, we’re still very invested in Sorbet
> $ Has met our goals in preventing a whole class of type errors, has aided in
documenting our code

33. Grailed is Hiring!
● From 50 to 105 employees in 2021, and we expect to
continue that trend
● Excellent work/life balance, benefits, fully remote
● Engineers value empathy as much as they do good
code
● Tackling a lot of interesting problems in the peer to
peer marketplace place
● Honestly, my favorite employer in my career
See more at grailed.com/jobs or reach out to me at
jose@grailed.com

34. Appendix
Some papers on the impact of static typing
> $ An empirical study on the impact of static typing on software
maintainability
> $ Gradual Typing of Erlang Programs: A Wrangler Experience
> $ Unit testing isn't enough. You need static typing too.

35. Thank You!