Mixing Functional and Object Oriented Approaches to Programming in C#, Michael Wagg, Mark Needham

1. Mixing Functional and Object Oriented approaches to programming in C# Mike Wagg & Mark Needham

2. C# 1.0

3. http://www.impawards.com/2003/posters/back_in_the_day.jpg

4. int[] ints = new int[] {1, 2, 3, 4, 5}

5. int[] Filter(int[] ints) { ArrayList results = new ArrayList(); foreach (int i in ints) { if (i % 2 == 0) { results.Add(i); } } return results.ToArray(typeof(int)); }

6. int[] ints = new int[] {1, 2, 3, 4, 5}

7. int[] Filter(int[] ints) { ArrayList results = new ArrayList(); foreach (int i in ints) { if (i >3) { results.Add(i); } } return results.ToArray(typeof(int)); }

8. int[] Filter(int[] ints) { ArrayList results = new ArrayList(); foreach (int i in ints) { if ( i % 2 == 0 ) { results.Add(i); } } return results.ToArray(typeof(int)); }

9. int[] Filter(int[] ints) { ArrayList results = new ArrayList(); foreach (int i in ints) { if ( i > 3 ) { results.Add(i); } } return results.ToArray(typeof(int)); }

10. interface IIntegerPredicate { bool Matches(int value); }

11. class EvenPredicate : IIntegerPredicate { bool Matches(int value) { return value % 2 == 0; } }

12. class GreaterThanThreePredicate : IIntegerPredicate { bool Matches(int value) { return value > 3; } }

13. int[] Filter(int[] ints, IIntegerPredicate predicate) { ArrayList results = new ArrayList(); foreach (int i in ints) { if (predicate.Matches(i)) { results.Add(i); } } return results.ToArray(typeof(int)); }

14. int[] ints = new int[] {1, 2, 3, 4, 5 }; int[] even = Filter(ints, new EvenPredicate()); int[] greaterThanThree = Filter(ints, new GreaterThanThreePredicate());

15. interface IIntegerPredicate { bool Matches(int value); }

16. bool delegate IntegerPredicate(int value);

17. bool Even(int value) { return value % 2 == 0; }

18. bool GreaterThanThree(int value) { return value > 3; }

19. int[] Filter(int[] ints, IntegerPredicate predicate) { ArrayList results = new ArrayList(); foreach (int i in ints) { if (predicate(i)) { results.Add(i); } } return results.ToArray(typeof(int)); }

20. int[] ints = new int[] {1, 2, 3, 4, 5 }; int[] even = Filter(ints, new IntegerPredicate(Even)); Int[] greaterThanThree = Filter(ints, new IntegerPredicate(GreaterThanThree));

21. C# 2.0

22. Inference int[] ints = new int[] {1, 2, 3, 4, 5 }; int[] even = Filter(ints, new IntegerPredicate(Even)); Int[] greaterThanThree = Filter(ints, new IntegerPredicate(GreaterThanThree));

23. Inference int[] ints = new int[] {1, 2, 3, 4, 5 }; int[] even = Filter(ints, Even); Int[] greaterThanThree = Filter(ints, GreaterThanThree); The compiler can infer what the type of the delegate is so we don’t have to write it.

24. Generics delegate bool IntegerPredicate(int value);

25. Generics delegate bool Predicate<T> (T value);

26. Generics int[] Filter(int[] ints, IntegerPredicate predicate) { ArrayList results = new ArrayList(); foreach (int i in ints) { if (predicate(i)) { results.Add(i); } } return results.ToArray(typeof(int)); }

27. Generics T[] Filter<T>(T[] values, Predicate<T> predicate) { List<T> results = new List<T>(); foreach (T i in value) { if (predicate(i)) { results.Add(i); } } return results.ToArray(); }

28. Generics IEnumerable<T> Filter<T>(IEnumerable<T> values, Predicate<T> predicate) { List<T> results = new List<T>(); foreach (T i in value) { if (predicate(i)) { results.Add(i); } } return results; }

29. Iterators IEnumerable<T> Filter<T>(IEnumerable<T> values, Predicate<T> predicate) { List<T> results = new List<T>(); foreach (T i in value) { if (predicate(i)) { results.Add(i); } } return results; }

30. Iterators IEnumerable<T> Filter<T>(IEnumerable<T> values, Predicate<T> predicate) { foreach (T i in value) { if (predicate(i)) { yield return i; } } }

31. Anonymous Methods IEnumerable<int> greaterThanThree = Filter(ints, GreaterThanThree);

32. Anonymous Methods IEnumerable<int> greaterThanThree = Filter(ints, delegate(int value) { return value > 3; });

33. Anonymous Methods int minimumValue = 3; IEnumerable<int> greaterThanThree = Filter(ints, delegate(int value) { return value > minimumValue; }); Anonymous methods add support for closures. The delegate captures the scope it was defined in.

34. C# 3.0

35. Lambdas int minimumValue = 3; IEnumerable<int> greaterThanThree = Filter(ints, delegate(int value) { return value > minimumValue; });

36. Lambdas int minimumValue = 3; IEnumerable<int> greaterThanThree = Filter(ints, value => value > minimumValue);

37. More Type Inference int minimumValue = 3; IEnumerable<int> greaterThanThree = Filter(ints, value => value > minimumValue);

38. More Type Inference int minimumValue = 3; var greaterThanThree = Filter(ints, value => value > minimumValue);

39. Extension Methods int minimumValue = 3; var greaterThanThree = Filter(ints, value => value > minimumValue);

40. Extension Methods int minimumValue = 3; var greaterThanThree = ints.Filter(value => value > minimumValue);

41. Anonymous Types var anonymous = new { Foo = 1, Bar = “Bar” }

42. LINQ

43. LINQ New delegates in System namespace Action<T>, Action<T1, T2>, Func<TResult>, Func<T1, TResult> etc.

44. LINQ New delegates in System namespace Action<T>, Action<T1, T2>, Func<TResult>, Func<T1, TResult> etc. System.Linq Extension methods Where, Select, OrderBy etc.

45. LINQ New delegates in System namespace Action<T>, Action<T1, T2>, Func<TResult>, Func<T1, TResult> etc. System.Linq Extension methods Where, Select, OrderBy etc. Some compiler magic to translate sql style code to method calls

46. LINQ var even = ints.Where(value => value % 2 == 0) var greaterThanThree = ints.Where(value => value > minimumValue) or var even = from value in ints where value % 2 == 0 select value var greaterThanThree = from value in ints where value > minimumValue select value

47. A (little) bit of theory

48. http://www.flickr.com/photos/stuartpilbrow/2938100285/sizes/l/

49. Higher order functions

50. Immutability

51. Lazy evaluation

52. Recursion & Pattern Matching

53. Transformational Mindset We can just pass functions around instead in most cases - find an example where it still makes sense to use the GOF approach though.

54. Input -> ??? -> ??? -> ??? -> Output

55. http://www.emt-india.net/process/petrochemical/img/pp4.jpg

56. So why should you care?

57. Functional can fill in the gaps in OO code

58. Abstractions over common operations means less code and less chances to make mistakes

59. So what do we get out of the box?

60. Projection

63. Restriction

65. Partitioning

70. Set

75. Ordering and Grouping

78. Aggregation

87. We can just pass functions around instead in most cases - find an example where it still makes sense to use the GOF approach though.

104. So what could possibly go wrong? http://icanhascheezburger.files.wordpress.com/2009/06/funny-pictures-cat-does-not-think-plan-will-fail.jpg

105. Hard to diagnose errors

106. var people = new [] { new Person { Id=1, Address = new Address { Road = &quot;Ewloe Road&quot; }}, new Person { Id=2}, new Person { Id=3, Address = new Address { Road = &quot;London Road&quot;}} }; people.Select(p => p.Address.Road);

107. Null Reference Exception on line 23

108. http://www.flickr.com/photos/29599641@N04/3147972713/

109. public T Tap(T t, Action action) { action(); return t; }

110. people .Select(p => Tap(p, logger.debug(p.Id)) .Select(p => p.Address.Road);

111. Readability

112. Lazy evaluation can have unexpected consequences

116. Encapsulation is still important

118. Linq isn't the problem here, it's where we have put it

119. Company naturally has the responsibility so encapsulate the logic here

121. Sometimes we need to go further

122. If both Company and Division have employees do we duplicate the logic for total salary?

123. IEnumerable<T> and List<T> make collections easy but sometimes it is still better to create a class to represent a collection

125. In conclusion…

127. Mike Wagg mikewagg.blogspot.com [email_address] Mark Needham markhneedham.com [email_address]