Monads in .NET

Transcript

1. Monads in .NET Mateusz Stasch 2018.11.06 | 4Developers Katowice
2. Mateusz Stasch https://mattstasch.net | @mattstasch
5. Screenshot from: https://www.slideshare.net/mariofusco/monadic-java
6. Screenshot from: https://www.slideshare.net/mariofusco/monadic-java
7. Maybe different?
8. Duty duty = null; var cm = bad.FindCrewMember(crewMemberId); if (null != cm?.MusterListId) { var musterList = bad.FindMusterList(cm.MusterListId); if (null != musterList) { duty = bad.FindDuty(musterList.DutyId); } }
9. public class Maybe<T> { private readonly T value; public bool HasValue { get; } public Maybe(T value) { this.value = value; this.HasValue = true; } public Maybe() { this.HasValue = false; } public Maybe<U> Bind<U>(Func<T, Maybe<U>> f) { if (HasValue) return f(value); return Maybe.None<U>(); } public T Get() { if(!HasValue) throw new InvalidOperationException(); return value; }
10. Maybe!
11. null T
12. T U V X
13. T null V X
14. T null V X null null
15. • Lack of NULLs in code! → better! • NULL is always a bug → cleaner! • Explicite expression that something can have no value • Easier code to reason about - even with monad :)
16. Maybe? What is the generalization of
17. IEnumerable<T>
18. svc.FindCrewMemberAsync(cmId) .Then(cm => svc.FindMusterListAsync(cm) .Then(ml => svc.FindDutyAsync(ml)) Task<T>* * Requires some tuning ;)
19. • Nullable<T> — represents a T that could be null (almost Maybe<T>) • Func<T> — represents a T that can be computed on demand • Lazy<T> — represents a T that can be computed on demand once, then cached • Task<T> — represents a T that is being computed asynchronously and will be available in the future, if it isn’t already https://ericlippert.com/2013/02/25/monads-part-two/
20. public static Nullable<U> Bind<T,U>(Nullable<T> source, Func<T, Nullable<U>> func) where T: struct where U: struct { if(source == null || !source.HasValue) { return new Nullable<U>(); } return func(source.Value); }
21. Any Pattern?
22. svc.FindCrewMember(cmId) .Bind(cm => cm.MaybeMusterListId) .Bind(id => svc.FindMusterList(id)) .Bind(ml => svc.FindDuty(ml.DutyId));
23. svc.FindCrewMembers() .SelectMany(cm => svc.FindMusterLists(cm) .SelectMany(ml => svc.FindDuties(cm, ml)))
24. svc.FindCrewMemberAsync(cmId) .Then(cm => svc.FindMusterListAsync(cm) .Then(ml => svc.FindDutyAsync(ml))
25. svc.FindCrewMember(cmId) .Bind(cm => cm.MaybeMusterListId ).Bind(id => svc.FindMusterList(id)) .Bind(ml => svc.FindDuty(ml.DutyId) );
26. svc.FindCrewMembers() .SelectMany(cm => svc.FindMusterLists(cm) .SelectMany(ml => svc.FindDuties(cm, ml) ))
27. svc.FindCrewMemberAsync(cmId) .Then(cm => svc.FindMusterListAsync(cm) .Then(ml => svc.FindDutyAsync(ml) )
28. T U V U V T => U => operation operation
29. Monad is an “Amplifier”. An “amplifier” is something that increases the representational power of their “underlying” type. - Wes Dyer
30. byte - 256 possible values
31. byte - 256 possible values Task<IEnumerable<Nullable<byte>>>
33. • Maybe • Either (or ValidationMonad) • Try • State, Writer, Reader • Monadic Parsers • ... https://github.com/louthy/csharp-monad
34. T V X R U Either<L, R>
35. T V X ERR U Try<T>
36. Monad Laws
37. Return & Bind
38. Return & Bind I can create
39. Return & Bind I can transform
40. Left Identity M(b).Bind(f) == f(b)
41. Right Identity f(b).Bind(id) == f(b)
42. Associativity M(b).Bind(f).Bind(g) == M(b).Bind(v => f(v).Bind(g))
43. var duty = from cm in better.FindCrewMember(crewMemberId) from id in cm.MaybeMusterListId from ml in better.FindMusterList(id, cm) from dt in better.FindDuty(ml.DutyId) select dt;
44. Thanks! Monads in .NET Mateusz Stasch
45. Links • https://www.slideshare.net/mariofusco/monadic-java • https://mikhail.io/2018/07/monads-explained-in-csharp-again/ • https://channel9.msdn.com/shows/Going+Deep/Brian-Beckman-The-Zen-of-Expressing-State-The-State-Monad/ • https://lorgonblog.wordpress.com/2007/12/02/c-3-0-lambda-and-the-first-post-of-a-series-about-monadic-parser-c ombinators/ • https://miklos-martin.github.io/learn/fp/2016/03/10/monad-laws-for-regular-developers.html • https://fsharpforfunandprofit.com/rop/ • https://ericlippert.com/category/monads/ • https://stackoverflow.com/questions/2704652/monad-in-plain-english-for-the-oop-programmer-with-no-fp-backgr ound