Either Error Success

Transcript

1. 1.

   None
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   Either Error Success FP-SYD 2018

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   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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   –Mark H “I have people sitting around solutionizing”

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   Examples • Code

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   –Nick H “I will admit human error played a large

   part”
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   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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   Exception Pattern • Code

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   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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    EitherT Pattern • Code

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    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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    Transformers Either • Code

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    Either Examples • mismi - wrapper around Amazonka • mafia

    - cabal build tool • boris - continuous integration / builds
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    Exception Examples • http-client - http library • stack -

    Haskell build tool ++ • hs-tls - Native Haskell TLS / SSL implementation
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    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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    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