Either Error Success

Transcript

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2. Either Error Success
   FP-SYD 2018
   

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3. Introduction
   • who, what, why, where, how questions
   
   • who -> Tim McGilchrist @lambda_foo
   
   • what -> exciting area of error handling
   
   • where -> in Haskell mostly
   
   • how -> with functions and data types
   

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4. –Mark H
   “I have people sitting around solutionizing”
   

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5. Examples
   • Code
   

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6. –Nick H
   “I will admit human error played a large part”
   

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7. The Async Problem
   • Asynchronous exceptions are a feature of Haskell
   
   • No fancy effect systems or modular IO effects
   
   • Deal with it!
   
   • Code in IO can fail at any point with an exception
   

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8. Exception Pattern
   • Code
   

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9. Exception Pattern
   • type signature doesn’t indicate errors
   
   • exceptions compose without effort
   
   • no stack traces by default
   
   • dealing with exceptions properly is hard
   

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10. EitherT Pattern
    • Code
    

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11. EitherT Pattern
    • pattern matching has your back
    
    • composition is explicit (you need to think)
    
    • application stack traces
    
    • constrained exception handling
    
    • extra work
    
    • interfacing with exception heavy code
    

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12. Transformers Either
    • Code
    

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13. Either Examples
    • mismi - wrapper around Amazonka
    
    • mafia - cabal build tool
    
    • boris - continuous integration / builds
    

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14. Exception Examples
    • http-client - http library
    
    • stack - Haskell build tool ++
    
    • hs-tls - Native Haskell TLS / SSL implementation
    

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15. Conclusion
    • make error behaviour explicit
    
    • use appropriate tool for the
    job
    
    • EitherT yields more
    maintainable code
    
    • similar or less effort to
    exceptions
    

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16. Resources
    • https://simonmar.github.io/posts/2017-01-24-
    asynchronous-exceptions.html
    
    • https://haskell-lang.org/library/safe-exceptions
    
    • https://www.fpcomplete.com/blog/2016/11/exceptions-
    best-practices-haskell
    
    • https://simonmar.github.io/bib/papers/async.pdf
    

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