Overview of functional programming and functional programming in C#.

1. λ Functional Programming

2. Who am I? Ryan Riley Catapult Systems Houston Blog: http://wizardsofsmart.net GitHub: http://github.com/panesofglass Twitter: @panesofglass

3. Agenda Functional Programming Concepts Func’y C# Resources

4. 1. Functional Programming Concepts

5. Functional programming is just a style.

6. Treat functions as values.

7. What is a function? A function, as in math. (In particular, as in lambda calculus.) ((Ask Jeff Odell.))

8. Follow the Arrows add x y has a type of int -> int -> int In C#, this could be Func<int, int, int> (Actually, that’s Func<int,<Func<int, int>>>. We’ll see why in a moment….)

9. Functions are values, too. .NET 1.0 => delegates .NET 2.0 => anonymous delegates + generics .NET 3.0 => lambdas Higher-order functions accept a function as a parameter.

10. Immutability NO side effects! NO state! NO I/O! Functions take an input and return a result Image from One Year Bible Blog

11. Implications Recursion Persistent data structures

12. Paradigm-shift Declarative vs. Imperative What? How?

13. Advantages Very easy to reason about Concurrency friendly Easy to compose:doubleSq x = square(double(x))doubleSq = double . square Foundation of Domain Specific Languages

14. Disadvantages Too much recursion can overflow the stack (without tail-call optimization, or TCO) High memory consumption from creating so many objects …

15. However,

16. Referential transparency Replace a function with its value Efficiency with caching (a.k.a. memoization) Easier for programmers to understand ∴ Easier to reuse and test

17. So what? Moore’s Law is failing. Multiple CPUs / cores is the future. Controlling mutable state in concurrency is very hard …unless you have no mutable state!

18. Also, “Mock objects and dependency injection are just for people who don’t know math.”* ~ Erik Meijer, Microsoft * See the Silverlight Toolkit for test examples.

19. 2. Play that Func’yMusic C#

20. Func & Action Define your functions. Generics are important. Type-safety over many types.

21. Lists everywhere! Functional languages like Lisp build entirely on lists and maps (i.e. Dictionaries). Feel like you’ve seen this before?

22. Also by Erik Meijer! LINQ! Implementation of common functional programming functions over lists via IEnumerable. Select => map Where => filter Aggregate => fold / reduce SelectMany => bind / fmap, or “map then flatten”

23. This means you are probably already using FP!

24. Take it further Functions as data Closures => Capture external state for later Currying + Partial application =>Reduce dependencies Memoization => Run it again later

25. Demo Currying and Partial application

26. Make it concurrent Task Parallel Library (PFx) Reactive Extensions (Rx) PLinq => Parallel LINQ

27. Demo Concurrency

28. What didn’t we cover? Many more functional operators (LINQ) Managing mutable state from the environment LINQ to Observables, Continuations, etc. The “M” word

29. 3. Resources

30. Books ‘n Blogs Real-World Functional Programming PFx Team Blog Rx Team Blog Matthew Podwysocki’s Weblog

31. Libraries Functional C# Reactive Extensions (Rx)

32. Videos Functional Programming Fundamentals(Channel 9 Lecture Series)

33. Languages More F# => http://c4fsharp.groups.live.com More Clojure and IronScheme => http://hdlug.groups.live.com

34. Questions?