async tutorial 2020 (HaskellerZ talk)

by Niklas Hambüchen

This provides the files we will start with in the tutorial.

You can clone this repo and then follow along the talk.

Relevant links

• New async docs (to be part of next async release): https://nh2.me/async-docs
• Pooled concurrency in unliftio
• Concurrent stream processing: conduit-concurrent-map

Notes during the tutorial

Look at the live-tutorial branch to see how the code evolved.

• Tutorial template: https://github.com/nh2/haskellerz-async-tutorial-2020
• remind me to stop when following with code
• make a function that simulates downloading (getURL)
• go through https://nh2.me/async-docs
• aborting on failure by default -- silent failure is bad
  • introduce the important async properties
  • insert simulated connection failure for url1
• show how exceptions get raised on wait (switch to fail for url2)
• show higher-level functions (resume going through docs)
• exceptions
  • why we have to think about exceptions
  • synchronous and asynchronous, examples (timeout)
  • bracket
• how the RTS works (N-on-M green threading)
• plain forkIO
  • examples of why it is bad
• unliftio
• resource considerations
  • semaphore
  • pooled concurrency in unliftio: http://hackage.haskell.org/package/unliftio-0.2.12.1/docs/UnliftIO-Async.html#g:9
  • simple example: https://gist.github.com/nh2/1500af99cfe6a8f45848d8c1e92ad9ba
  • streaming processing
  • head-of-line blocking
  • conduit-concurrent-map: https://github.com/nh2/conduit-concurrent-map
• Alexey to metion starting overhead

Summary of what you should remember

• How does concurrency work in Haskell?
• What are the underlying primitives?
• What does async do?
• what functions to use in practice?
• What are common mistakes?