Through the lens of Haskell - Exploring new ideas for library design

Haskell is very different from Python, and provide different tools to library and framework designers. As a result, its ecosystem is filled with libraries and frameworks that solve the same problems we try to solve in our favorite programming languages, but with a very different approach.

This talk is an exploration of the Haskell ecosystem, from the point of view of a Python developer. We will review various popular Haskell libraries and frameworks, focusing on the library design. The goal is to provide the audience a sneak peak of some different ways to tackle problems, and hopefully to inspire library authors to explore some design space that we don’t usually explore in Python.

This talk should be interesting to any intermediate Python programmer who is curious about other ways to solve problems. No Haskell knowledge is required from the audience.

Transcript

1. Through the lens of Haskell Exploring new ideas for library design
2. @georgesdubus
3. Haskell, the language
4. Haskell, the ecosystem
5. Design space
6. There should be one — and preferably only one — obvious way to do it. (Python)
7. There should be one — and preferably only one — obvious way to do it. Let’s keep looking for it! (Python) (Haskell)
9. Some Haskell libraries
10. JSON Packaging HTTP
11. JSON Packaging HTTP ??? ??? ???
12. Some Haskell libraries Part 1: attoparsec
13. “Real life” use case : A slack bot that answers movie quotes
14. subtitleParser Full package definition Name Type
15. subtitleParser All I need
16. attoparsec All I need to know : parseOnly :: Parser Subtitles -> ByteString -> Either ErrorMessage Subtitles
17. attoparsec All I need to know : parseOnly :: Parser Subtitles -> ByteString -> Either ErrorMessage Subtitles parseOnly :: Parser a -> ByteString -> Either ErrorMessage a
18. attoparsec Or : incremental parsing parse :: Parser a -> ByteString -> Result a feed :: Result a -> ByteString -> Result a (Result can be Partial, Failed or Done)
19. attoparsec Part of a bigger parser many :: Parser a -> Parser [a] or :: Parser a -> Parser b -> Parser (Either a b)
20. Parsers everywhere parseCSV :: Parser CSV in attoparsec-csv json :: Parser JSONValue in aeson crontab :: Parser Crontab in cron emailAddress :: Parser String in email-header toml :: Parser TOMLValue in toml ...
21. A good library simplifies the implementation
22. A good library simplifies the interface
23. General solution Specific building blocks
24. General solution Specific building blocks attoparsec all parsers
25. Some Haskell libraries Part 2: conduit
26. Conduit Streaming library Producers Consumers Conduits that both consume and produce
27. Lot of libraries sourceSocket socket =$= ungzip =$= sinkFile "/tmp/output" producer from Data.Conduit.Network conduit from Data.Conduit.Zlib consumer from Data.Conduit.Binary
28. conduit + attoparsec = High-performance subtitles streaming for free ! sourceFile “something.srt” =$= conduitParser parseSubtitleLine =$= ircConsumer Parser of Subtitle Lines Parser ➡ Conduit
29. General solution Specific building blocks conduit all conduits
30. Some Haskell libraries Part 3: lens
31. BlogPost { title = “Made-up examples considered harmful” , author = Person {name=“Alice”} , comments = [ Comment { author = “Bob” , content = “Great insight!” } , Comment { author = “Carol” , content = “I completely disagree” } ] } Data manipulation
32. >>> view title blogpost “Made-up examples considered harmful” Getters Lens
33. >>> view title blogpost “Made-up examples considered harmful” >>> view (author . name) blogpost "Alice" Getters Lens Lens Lens
34. >>> view title blogpost “Made-up examples considered harmful” >>> view (author . name) blogpost "Alice" >>> set (speaker . name) “Alicia” blogpost BlogPost { title = “Made-up examples considered harmful” , author = “Alicia” , ... } Getters Setters Lens Lens Lens
35. >>> toListOf (comments . each . author) blogpost [“Bob”, “Carol”] Getters/setters with multiple values ?!? Lens Lens Traversal
36. >>> let commentContents = comments . each . content >>> toListOf commentContents blogpost [“Great insight!”, “I completely disagree”] >>> set commentContents “Blah blah blah” blogpost BlogPost { comments = [ Comment { author = “Bob” , content = “Blah blah blah” } , Comment { author = “Carol” , content = “Blah blah blah” } ] , ... } Getter / setter pairs are values
37. [{“id”: “1”, “name”: “georges”}, {“id”: “2”, “name”: “lucie”}] >>> input & (values . key “name”) %~ capitalize [{“id”: “1”, “name”: “Georges”}, {“id”: “2”, “name”: “Lucie”}] Libraries provide lenses: JSON
38. titles = allNamed (only "h2") . contents Libraries provide lenses: HTML Traversal into all tags with a given name Their content
39. General solution Specific building blocks lens all lenses
40. “Borrowing” ideas
41. Python ➡ Haskell wreq = requests + lens
42. Haskell ➡ Python hypothesis
43. Conclusion
44. Explore the design space
45. Explore the design space Factorize library interfaces
46. Explore the design space Factorize library interfaces Bonus : DIY conclusion