Monadic Approach To Ruby Error Handling

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MONADIC APPROACH TO RUBY ERROR HANDLING Huy Du | exponentdev.com RubyConf Taiwan Taipei, 17th December 2023

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EXCEPTION IN RUBY Exception is an unique data structure representing an error or unexpected condition in the Ruby program. Exception objects carry information about: • Type • Message • Backtrace

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EXCEPTION IN RUBY Exception is a unique data structure representing an error or unexpected condition in the Ruby program. Exception objects carry information about: • Type • Message • Backtrace

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EXCEPTION IN RUBY Exception is a unique data structure representing an error or unexpected condition in the Ruby program. Exception objects carry information about: • Type • Message • Backtrace

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As a User, I want to buy a Ticket for attending Ruby Conference. EXAMPLE PROBLEM

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User Frontend API Server Database Payment Gateway Infrastructure HIGH LEVEL SYSTEM DESIGN Buy a ticket Send API request Store data Submit payment

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Model Controller Database Payment Gateway APPLICATION COMPONENTS Application

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NAIVE IMPLEMENTATION Ticket not found? User not found? Order created? User buy ticket NO NO

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NAIVE IMPLEMENTATION Ticket not found? User not found? Order created? User buy ticket Insufficient balance? Payment success? NO NO YES YES Render success YES

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NAIVE IMPLEMENTATION Ticket not found? User not found? Order created? User buy ticket Insufficient balance? Render Error Raise Exception Raise Exception Render Error Payment success? YES YES NO NO NO YES YES Render success Render Error Raise Exception YES Render Error NO NO Raise Exception Render Error

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Can we make it better? NAIVE IMPLEMENTATION • Chaotic • Error Prone • Poor extendibility • Poor readability • Not reusable

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Huy Dư Software Engineer @ @dugiahuy https://exponentdev.com

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PROGRAMMING f(x) = y • f() is solution • x is input • y is expected output Programming, in simple term, involves solving problem by providing instructions to a computer.

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WHAT ARE ERRORS? Software errors are defined as results in unexpected outcomes from a computer program or cause it to act in ways that were not intended.

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EXPECTED ERROR These are errors that developers anticipate and expect as part of normal program execution • Part of business logic ⚬ Ticket has been sold out. ⚬ Purchase ticket cannot process after midnight. • External dependencies ⚬ Wrong library usage ⚬ Connection error

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UNEXPECTED ERROR

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UNEXPECTED ERROR Errors are not expected to occur. They are interrupted our program or also known as bugs. • Database corrupted • Memory exceeded • Developer mistake

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As a User, I want to buy a Ticket for attending Ruby Conference. EXAMPLE PROBLEM

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Model Controller Database Payment Gateway APPLICATION COMPONENTS Application

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Model Controller Database Payment Gateway BUSINESS LOGIC LAYER Application Business Logic

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REFACTOR V1

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REFACTOR V1 Ticket not found? User not found? Order created? User buy ticket Render Error Raise Exception Raise Exception Render Error YES YES NO NO NO Render Error Raise Exception

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REFACTOR V1 Ticket not found? User not found? Order created? User buy ticket Insufficient balance? Render Error Raise Exception Raise Exception Render Error Payment success? YES YES NO NO NO YES YES Render Error Raise Exception

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Can we improve it? REFACTOR V1 • Better code flow • Poor extendibility • Better readability • Not reusable • Poor encapsulation

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REFACTOR V2

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REFACTOR V2

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REFACTOR V2 User buy ticket Create Order failed? Render Error Checkout Order failed? NO Insufficient balance? Render Error YES NO YES YES NO Payment failed? YES Render Error NO Render success

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REFACTOR V2 User buy ticket Create Order failed? Render Error Checkout Order failed? NO Insufficient balance? Render Error YES NO YES YES NO Payment failed? YES Render Error NO Render success Leaky error details • Inconsistent return value • No standardize

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RETURN MONADS AS RESULT

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MONADS Monads is structure that wrap return value of function in monadic way

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MONAD RESULT Result Success, Value Failure, Error

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MONAD RESULT

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SERVICE OBJECT Service Object are Ruby objects, that are designed to execute one single action in the business logic, and do it well.

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SERVICE OBJECT

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MONADIC HANDLING Monadic handling is the combine between Service Object and Monads Result. Monadic handling involves using Monad Result to manage the flow of execution within the Service Object

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MONADIC HANDLING

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MONADIC HANDLING Monadic Handling Refactor V2

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MONADIC HANDLING Monadic Handling Refactor V2

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MONADIC HANDLING User buy ticket Create Order failed? Render Error Checkout Order failed? NO Render success YES NO YES Render Error

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COMING TO REALITY

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MONADIC HANDLING • Provide standardize and consistency way error handling • Explicit and reduce complexity of conditional cases • Making code more robust and maintainable • Completely encapsulate logic and reusable everywhere

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MONADIC HANDLING GEMS https://github.com/dry-rb/dry-monads https://github.com/kaligo/stimpack

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ERROR HANDLING f(x) = y Input x must meet the specific criteria of validity to ensure f() can process it properly. • f() is solution • x is input • y is expected output Solution f() is designed to take a valid input and process it into our expected output, y

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Huy Du ”Error handling is ensuring valid inputs and predictable outputs.”

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VALIDITY VS EXECUTION f(x) = y Validity errors are directly tied to the appropriateness of the input x. • f() is solution • x is input • y is expected output Execution errors emerge even with a valid input, arising from issues within the function f(x) itself during the execution.

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MONADIC VS TRADITIONAL Monadic Approach (Validity Errors) Traditional Approach (Execution Errors) They are not being mutually exclusive • Encapsulates and validates input data. • Ensures errors are part of the expected flow. • Ideal for catching issues before they escalate. • Catches unexpected operational issues. • Acts as safety net through ruby exception mechanisms. • Handles disruptions in the program’s flow.

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BEFORE RAISING EXCEPTION • Is this action truly unexpected? • Am I going to disrupt the execution flow? • Will I have a heart attack when this exception thrown?

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Huy Du ”The correct way of error handling will stop your lifespan for a couple of seconds."

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WRAP UP • Understand different type of errors in software application • Understand how Exception in Ruby • Create boundaries for Business Logic layer and encapsulate with Service Object • Use monadic error handling by combining Service Object and Monad Result • You have the practical strategies for error handling

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Huy Du "Pray is not prevented your system from crash. You need monadic handling."

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SALUTE! Huy Du https://exponentdev.com/talks https://github.com/dugiahuy/talks