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Introducing

Type Guard

to Steep

Takesh KOMIYA (@tk0miya)

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self.introduction

  • Takeshi KOMIYA (@tk0miya)
  • CTO of TIME INTERMEDIA Inc.
  • My first Ruby: 1.4.2 (maybe)
  • My first RubyKaigi: 2011
  • Member of asakusa-bashi_rbs, Gotanda.rb
  • OSS Developer
      • Sphinx (Doc. generator for python)
      • RBS ecosystem contributor

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self.introduction

  • Takeshi KOMIYA (@tk0miya)
  • CTO of TIME INTERMEDIA Inc.
  • My first Ruby: 1.4.2 (maybe)
  • My first RubyKaigi: 2011
  • Member of asakusa-bashi_rbs, Gotanda.rb
  • OSS Developer
      • Sphinx (Doc. generator for python)
      • RBS ecosystem contributor

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My works for RBS ecosystem

  • Update types
      • ruby-core and stdlibs (ruby/rbs)
      • 3rd party gems (ruby/gem_rbs_collection)
  • Development type generators
      • A maintainer of rbs_rails
      • The author of type generators
          • Rails (ActiveRecord, ActiveSupport, ActiveModel, etc.)
          • 3rd party gems (devise, discard, draper, config, etc.)

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My works for RBS ecosystem

  • Adjust Rubocop cops
      • Update official cops to allow RBS::Inline style comments
      • rubocop-rbs_inline gem
  • Develop reviewdog for Steep
  • Develop VSCode extension (rbs-helper)
      • Run RBS::Inline on save
      • Get quick feedback cycle with Steep

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My works for RBS ecosystem

  • Adjust Rubocop cops
      • Update official cops to allow RBS::Inline style comments
      • rubocop-rbs_inline gem
  • Develop reviewdog for Steep
  • Develop VSCode extension (rbs-helper)
      • Run RBS::Inline on save
      • Get quick feedback cycle with Steep

​

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My works for RBS ecosystem

  • Contribute to RBS, Steep, RBS::Inline and IRB (katakata_irb)
      • Feature requests
          • steep:ignore
      • Implementations
          • silent mode
          • completion for lonely operator
          • completion for keyword arguments
      • Bug fixes

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My works for RBS ecosystem

  • Contribute to RBS, Steep, RBS::Inline and IRB (katakata_irb)
      • Feature requests
          • steep:ignore
      • Implementations
          • silent mode
          • completion for lonely operator
          • completion for keyword arguments
      • Bug fixes

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We’re hiring

We’re system integrator in Japan. Visit https://timedia.co.jp/

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Big thanks to my sponsors

Kazamori LLC.

Gold sponsors

Tea sponsors

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Introducing Type Guard to Steep

    • What’s Type Guard?
    • Type Guard for Union Types
    • User-defined Type Guard
    • Conclusion

​

​

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Introducing Type Guard

to Steep

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Introducing Type Guard to Steep

    • What’s Type Guard?
    • Type Guard for Union Types
    • User-defined Type Guard
    • Conclusion

​

​

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Could you find a bug?

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Could you find a bug?

To find a bug, let’s analyze this by Steep

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Could you find a bug?

Steep detects a bug in the code

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Could you find a bug?

Steep detects a bug in the code

Type `(::User | nil)` does not have method `name`

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Could you find a bug?

Steep detects a bug in the code

Type `(::User | nil)` does not have method `name`

Steep recognizes user’s type is (::User | nil)

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How do you fix?

Next, let’s fix the bug. How do you fix it?

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How do you fix?

Next, let’s fix the bug. How do you fix it?� My answer is nil-check

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How do you fix?

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How do you fix?

We confirm

user is not nil

by nil-check

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How do you fix?

We confirm

user is not nil

by nil-check

user is User | nil

user is User

user is nil

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How do you fix?

This behavior is called as “Type narrowing”

user is User | nil

user is User

user is nil

We confirm

user is not nil

by nil-check

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How do you fix?

This behavior is called as “Type narrowing”

user is User | nil

user is User

user is nil

Expression for type narrowing is called as “Type guard”

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Steep and Type narrowing

Steep supports Type narrowing

        • if/else
        • ternary operator
        • logical operators (and, or)
        • case-when
        • while, until

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Steep and Type narrowing

Steep supports some type guards

        • nil-check
        • built-in methods
            • Object#is_a?, #kind_of?
            • Object#instance_of?
            • Object#nil?
            • Object#!
            • Object#===, Module#===
            • Module#<, Module#<=

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Steep and Type narrowing

Steep supports type narrowing by built-in methods�(e.g. is_a?)

​

​

​

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Steep and Type narrowing

Steep supports type narrowing by built-in methods�(e.g. is_a?)

​

​

​

Type guard found.

Let’s narrowing

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Steep and Type narrowing

Steep supports type narrowing by built-in methods�(e.g. is_a?)

​

​

​

user is narrowed

into User

Type guard found.

Let’s narrowing

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Interim Summary

    • Type narrowing is the process of making a variable’s type more specific
    • Type Guard is an expression for type narrowing
    • Steep supports type narrowing
    • Steep considers some Ruby built-in methods as type guards

​

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Introducing Type Guard to Steep

    • What’s Type Guard?
    • Type Guard for Union Types
    • User-defined Type Guard
    • Conclusion

​

​

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Narrowing by #present?

    • Do you know #present? method of ActiveSupport?
    • It checks the presence of object. It can be used like nil-check

​

​

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Narrowing by #present?

    • Do you know #present? method of ActiveSupport?
    • It checks the presence of object. It can be used like nil-check

​

​

We confirm

the user is present

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Narrowing by #present?

    • Do you know #present? method of ActiveSupport?
    • It checks the presence of object. It can be used like nil-check

​

​

We expect

user is narrowed to User

We confirm

the user is present

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Narrowing by #present?

    • Do you know #present? method of ActiveSupport?
    • It checks the presence of object. It can be used like nil-check

​

​

We confirm

the user is present

We expect

user is narrowed to User

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Narrowing by #present?

    • Do you know #present? method of ActiveSupport?
    • It checks the presence of object. It can be used like nil-check

​

​

Steep considers #present? is not

a type guard

We confirm

the user is present

We expect

user is narrowed to User

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Narrowing by #present?

    • We know #present? narrows the type runtime
    • But Steep doesn’t narrow the type by #present?
        • Steep considers #present? is not a Type Guard
        • Because Steep now supports built-in methods only
    • To resolve that, I proposed “Type Guard for Union Types”

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Type Guard for Union Types

    • Expression satisfies followings is recognized as Type Guard
        • Method call
        • The receiver of the call is a variable or pure method call
        • The type of receiver is a union type
    • It narrows the receiver’s type by the return type of the call

​

​

(※)

(※) Pure method is a method with no side effects. See the document of Steep.

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Type Guard for Union Types

As a preparation, set the types of #present? up

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Type Guard for Union Types

Let’s check it again

​

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Type Guard for Union Types

Let’s check it again

​

Steep considers

this expression is a Type Guard

​

Because

…

​

​

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Type Guard for Union Types

Let’s check it again

​

Steep considers

this expression is a Type Guard

​

Because

  1. The expression is a method call

​

​

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Type Guard for Union Types

Let’s check it again

​

Steep considers

this expression is a Type Guard

​

Because

  • The expression is a method call
  • the receiver of the call is a variable

​

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Type Guard for Union Types

Let’s check it again

​

Steep considers

this expression is a Type Guard

​

Because

  • The expression is a method call
  • the receiver of the call is a variable
  • The type of receiver is a Union type

User | nil

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Type Guard for Union Types

Next, do narrowing

​

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Type Guard for Union Types

Next, do narrowing

​

This type guard contains

two method calls

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Type Guard for Union Types

Next, do narrowing

​

This type guard contains

two method calls

User#present?: () -> true

NilClass#present?: () -> false

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Type Guard for Union Types

Next, do narrowing

​

This type guard contains

two method calls

User#present?: () -> true

NilClass#present?: () -> false

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Type Guard for Union Types

Next, do narrowing

​

This type guard contains

two method calls

User#present?: () -> true

NilClass#present?: () -> false

Calls having truthy return type

is used to if-block narrowing

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Type Guard for Union Types

Next, do narrowing

​

This type guard contains

two method calls

User#present?: () -> true

NilClass#present?: () -> false

Calls having truthy return type

is used to if-block narrowing

Calls having falsey return type

is used to else-block narrowing

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Type Guard for Union Types

Next, do narrowing

​

Narrowed to User

because User#present? returns true

Narrowed to nil

because NilClass#present? returns false

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Type Guard for Union Types

Next, do narrowing

​

🎉🎉🎉🎉🎉

Narrowed to User

because User#present? returns true

Narrowed to nil

because NilClass#present? returns false

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Case of String#present?

String#present? returns both true and false

        • Returns false if empty, Otherwise true
        • The return type of String#present? becomes bool

​

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Case of String#present?

String#present? returns both true and false

        • Returns false if empty, Otherwise true
        • The return type of String#present? becomes bool

​

​

​

​

​

        • String will be applied to both if/else block

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Case of String#present?

​

​

​

IO#gets returns String | nil

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Case of String#present?

​

​

​

IO#gets returns String | nil

String#present?: () -> bool

NilClass#present?: () -> false

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Case of String#present?

​

​

​

String#present?: () -> bool

NilClass#present?: () -> false

Narrowed to String

because String#present? returns bool

Narrowed to String | nil

because String#present? returns bool

and NilClass#present? returns false

IO#gets returns String | nil

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Current status of

Type Guard for Union Types

    • I proposed Type Guard for Union Types to Steep
        • soutaro/steep#1497

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Current status of

Type Guard for Union Types

    • I proposed Type Guard for Union Types to Steep
        • soutaro/steep#1497
        • Released as Steep-1.10

🎉

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Current status of

Type Guard for Union Types

    • I proposed Type Guard for Union Types to Steep
        • soutaro/steep#1497
        • Released as Steep-1.10
    • I also modified the type of #present? and #blank?
        • ruby/gem_rbs_collection#815

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Current status of

Type Guard for Union Types

    • I proposed Type Guard for Union Types to Steep
        • soutaro/steep#1497
        • Released as Steep-1.10
    • I also modified the type of #present? and #blank?
        • ruby/gem_rbs_collection#815
        • Merged now

🎉

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Interim Summary

I introduced Type Guard for Union Types to Steep

        • Some kinds of method calls for Union are considered as Type Guard
        • It narrows the type of receiver using type definition
        • Now available in Steep-1.10
        • Types of ActiveSupport is available

​

​

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Introducing Type Guard to Steep

    • What’s Type Guard?
    • Type Guard for Union Types
    • User-defined Type Guard
    • Conclusion

​

​

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Narrowing by own method

In real world apps, we often have own methods that determine the class or a state of the object

​

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Narrowing by own method

In real world apps, we often have own methods that determine the class or a state of the object

​

​

​

​

​

​

It would be great if Steep supports them. Let’s go!

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Narrowing by own method

    • Let’s talk about User#admin?
    • There are three classes
        • 1 superclass
        • 2 subclasses
    • User has a checker method�named `#admin?`

​

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Narrowing by own method

This code calls #admin? to classify user to admin or not

​

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Narrowing by own method

This code calls #admin? to classify user to admin or not

​

We confirm

the user is an admin

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Narrowing by own method

This code calls #admin? to classify user to admin or not

​

We expect

user is narrowed to AdminUser

We confirm

the user is an admin

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Narrowing by own method

This code calls #admin? to classify user to admin or not

​

We expect

user is narrowed to AdminUser

We confirm

the user is an admin

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Narrowing by own method

This code calls #admin? to classify user to admin or not

​

We expect

user is narrowed to AdminUser

Steep considers #admin? is not

a type guard

We confirm

the user is an admin

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Narrowing by own method

    • We often have own methods that determine the class or a state of the object
    • But Steep considers own method is not a Type Guard
        • Steep now supports built-in methods only
    • To resolve that, I proposed “User-defined Type Guard”

​

​

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User-defined Type Guard

User-defined Type Guard is declared as annotated method

​

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User-defined Type Guard

User-defined Type Guard is declared as annotated method

        • The annotation starts with “guard:”

​

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User-defined Type Guard

User-defined Type Guard is declared as annotated method

        • The annotation starts with “guard:”
        • The annotation has predicate after the prefix

​

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User-defined Type Guard

User-defined Type Guard is declared as annotated method

        • The annotation starts with “guard:”
        • The annotation has predicate after the prefix
            • It describes how it narrows the type when the method call is evaluated as truthy

​

The type of receiver �will be narrowed to AdminUser

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User-defined Type Guard

The predicate consists of subject, verb and type

        • Subject indicates the target of narrowing
        • Verb describes how to narrow the type
        • Type describes the type after narrowing

Subject verb Type

“self”

argument name

“is”

“is_a”

Type name

“self”

argument name

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User-defined Type Guard

The predicate consists of subject, verb and type

        • Subject indicates the target of narrowing
        • Verb describes how to narrow the type
        • Type describes the type after narrowing

​

​

        • e.g.
            • self is String
            • arg1 is Integer

​

    • self is arg1
    • self is_a arg2

Subject verb Type

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Narrowing by

User-defined Type Guard

Let’s analyze the code with User-defined Type Guard

​

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Narrowing by

User-defined Type Guard

Let’s analyze the code with User-defined Type Guard

​

This call is considered as type guard

by declaration

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Narrowing by

User-defined Type Guard

Let’s analyze the code with User-defined Type Guard

​

🎉🎉🎉

This call is considered as type guard

by declaration

Narrowed to AdminUser

based on the predicate

“self is AdminUser”

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Interim Summary

    • User-defined Type Guard can narrow the type
    • You can change any method to type guard by annotation
    • At present, it’s under development
        • soutaro/steep#1501
        • “self is TYPE” style predicate is available

​

​

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User-defined Type Guard

Limitations (1)

User-defined Type Guard cannot “change” the type

        • It can be used only for “narrowing”
        • The new type must be a subtype of the original type

​

​

Invalid.

Object is not

a subtype of User

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User-defined Type Guard

Limitations (2)

The type in else-block could not be narrowed well

        • Steep attempts to calculate the type in the else-block,�But the processing sometimes fails
        • In AdminUser’s case, it failed to calculate the type �“User but not AdminUser”
            • As a result, its type is remained unchanged as “User”
        • Narrowing goes well if calculation succeeded

​

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Advanced usage

Conditional Types

User-defined Type Guard can represent conditional types

        • e.g. title is optional by default, but required if published

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Advanced usage

Conditional Types

    • User-defined Type Guard can represent conditional types

​

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Advanced usage

Conditional Types

    • User-defined Type Guard can represent conditional types
        • Add a virtual constraint module for “published” state

​

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Advanced usage

Conditional Types

    • User-defined Type Guard can represent conditional types
        • Add a virtual constraint module for “published” state
        • Use it in type guard predicate

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Advanced usage

Conditional Types

    • User-defined Type Guard can represent conditional types
        • Add a virtual constraint module for “published” state
        • Use it in type guard predicate

article is narrowed to

Article & Published

#title is also narrowed

String from String?

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Advanced usage

Conditional Types

    • Apps usually uses conditional types technique
    • ActiveRecord provides conditional columns
        • Delegated types
        • validations (#valid?)
    • I’ll propose supporting conditional types to rbs_rails after merging User-defined Type Guard into Steep

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Introducing Type Guard to Steep

    • What’s Type Guard?
    • Type Guard for Union Types
    • User-defined Type Guard
    • Conclusion

​

​

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Conclusion

    • Type Narrowing helps you to write Ruby code more safe
    • I proposed two Type Guards to Steep
        • Type Guard for Union Types
        • User-defined Type Guard
    • Please enjoy types with Steep!

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Acknowledges

    • soutaro
    • pocke
    • ksss
    • tompng
    • #types channel in ruby-jp slack
    • and all types fans

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Current status of

Type Guard for Union Types

    • We choose bool for the return value of Object#present?
        • Almost subclasses will return true
        • But some subclasses will return both true and false
            • Developers can change the behavior via overriding #empty? method
    • Type narrowing will not work well in else-block
        • Please override the type if you need it

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