Either Error Success

Transcript

1. None

2. Either Error Success FP-SYD 2018

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

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

5. Examples • Code

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

   part”

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

8. Exception Pattern • Code

9. Exception Pattern • type signature doesn’t indicate errors • exceptions

   compose without effort • no stack traces by default • dealing with exceptions properly is hard

10. EitherT Pattern • Code

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

12. Transformers Either • Code

13. Either Examples • mismi - wrapper around Amazonka • mafia

    - cabal build tool • boris - continuous integration / builds

14. Exception Examples • http-client - http library • stack -

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

15. Conclusion • make error behaviour explicit • use appropriate tool

    for the job • EitherT yields more maintainable code • similar or less effort to exceptions

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