Either Error Success

Transcript

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