Mixing functional and object oriented approaches to programming in C#

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

C# 1.0

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

int[] ints = new int[] {1, 2, 3, 4, 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)); }

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

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

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)); }

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

interface IIntegerPredicate { bool Matches(int value); }

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

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

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)); }

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

interface IIntegerPredicate { bool Matches(int value); }

bool delegate IntegerPredicate(int value);

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

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

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)); }

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

C# 2.0

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

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.

Generics delegate bool IntegerPredicate(int value);

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

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)); }

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(); }

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; }

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; }

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

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

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

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.

C# 3.0

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

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

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

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

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

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

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

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

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.

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

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

A (little) bit of theory

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

Higher order functions

Immutability

Lazy evaluation

Recursion & Pattern Matching

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.

Input -> ??? -> ??? -> ??? -> Output

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

So why should you care?

Functional can fill in the gaps in OO code

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

So what do we get out of the box?

Projection

people.Select(person => person.Name)

people.SelectMany(person => person.Pets)

Restriction

people.Where(person => person.HasPets)

Partitioning

people.Take(5)

people.Skip(5)

people.TakeWhile(person =>
person.Name != "David")

people.SkipWhile(person =>
person.Name != "David")

people.Select(person => person.Name)
.Distinct()

people.Union(someOtherPeople)

people.Intersect(someOtherPeople)

people.Except(someOtherPeople)

Ordering and Grouping

people.OrderBy(person => person.Name)

people.GroupBy(person => person.Name)

Aggregation

people.Count()

people.Select(person => person.Age)
.Sum()

.Min()

.Max()

.Average()

Things can get more complex

.Aggregate(0, (totalAge, nextAge) =>
nextAge % 2 == 0
? nextAge + totalAge
: totalAge)

people.Join(addresses,
person => person.PersonId,
address => address.PersonId,
(person, address) => new {
person, address})

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

public class SomeObject { private readonly IStrategy strategy; public SomeObject(IStrategy strategy) { this.strategy = strategy; } public void DoSomething(string value) { strategy.DoSomething(value); } }

public class Strategy : IStrategy { public void DoSomething(string value) { // do something with string } }

public class SomeObject { private readonly Action<string> strategy; public SomeObject(Action<string> strategy) { this.strategy = strategy; } public void DoSomething(string value) { strategy(value); } }

Hole in the middle pattern

public class ServiceCache<Service> { protected Res FromCacheOrService <Req, Res>(Func<Res> serviceCall, Req request) { var cachedRes = cache.RetrieveIfExists( typeof(Service), typeof(Res), request); if(cachedRes == null) { cachedRes = serviceCall(); cache.Add(typeof(Service), request, cachedRes); } return (Res) cachedRes; } }

public class CachedService : ServiceCache<IService> { public MyResult GetMyResult(MyRequest request) { return FromCacheOrService(() => service.GetMyResult(request), request); } }

Passing functions around

private void AddErrorIf<T>(Expression<Func<T>> fn, ModelStateDictionary modelState, Func<ModelStateDictionary, Func<T,string, string, bool>> checkFn) { var fieldName = ((MemberExpression)fn.Body).Member.Name; var value = fn.Compile().Invoke(); var validationMessage = validationMessages[fieldName]); checkFn.Invoke(modelState)(value, fieldName, validationMessage); } AddErrorIf(() => person.HasPets, modelState, m => (value, field, error) => m.AddErrorIfNotEqualTo(value,true, field, error)); AddErrorIf(() => person.HasChildren, modelState, m => (value, field, error) => m.AddErrorIfNull(value, field, error));

Continuation Passing Style

static void Identity<T>(T value, Action<T> k) { k(value); }

Identity("foo", s => Console.WriteLine(s));

Identity("foo", s => Console.WriteLine(s)); as compared to var foo = Identity(“foo”); Console.WriteLine(foo);

public ActionResult Submit(string id, FormCollection form) { var shoppingBasket = CreateShoppingBasketFrom(id, form); return IsValid(shoppingBasket, ModelState, () => RedirectToAction("index", "ShoppingBasket", new { shoppingBasket.Id} ), () => LoginUser(shoppingBasket, () => { ModelState.AddModelError("Password", "User name/email address was incorrect - please re-enter"); return RedirectToAction("index", ""ShoppingBasket", new { Id = new Guid(id) }); }, user => { shoppingBasket.User = user; UpdateShoppingBasket(shoppingBasket); return RedirectToAction("index", "Purchase", new { Id = shoppingBasket.Id }); } )); }

private RedirectToRouteResult IsValid(ShoppingBasket shoppingBasket, ModelStateDictionary modelState, Func<RedirectToRouteResult> failureFn, Func<RedirectToRouteResult> successFn) { return validator.IsValid(shoppingBasket, modelState) ? successFn() : failureFn(); } private RedirectToRouteResult LoginUser(ShoppingBasket shoppingBasket, Func<RedirectToRouteResult> failureFn, Func<User,RedirectToRouteResult> successFn) { User user = null; try { user = userService.CreateAccountOrLogIn(shoppingBasket); } catch (NoAccountException) { return failureFn(); } return successFn(user); }

http://www.thegeekshowpodcast.com/home/mastashake/thegeekshowpodcast.com/wp-content/uploads/2009/07/wtf-cat.jpg

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

Hard to diagnose errors

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

Null Reference Exception on line 23

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

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

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

Readability

Lazy evaluation can have unexpected consequences

IEnumerable<string> ReadNamesFromFile() { using(var fileStream = new FileStream("names.txt",
FileMode.Open)) using(var reader = new StreamReader(fileStream)) { var nextLine = reader.ReadLine(); while(nextLine != null) { yield return nextLine; nextLine = reader.ReadLine(); } } }

IEnumerable<Person> GetPeople() { return ReadNamesFromFile()
.Select(name => new Person(name)); }

IEnumerable<Person> people = GetPeople(); foreach (var person in people) { Console.WriteLine(person.Name); }
Console.WriteLine("Total number of people: " +
people.Count());

Encapsulation is still important

Total salary for a company

company.Employees.Select(employee =>
employee.Salary)
.Sum()
This could lead to duplication
What if we add rules to the calculation?
Who should really have this responsibility?
.Sum()

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

Company naturally has the responsibility so encapsulate the logic here

class Company
{
public int TotalSalary
{
get
{
return employees.Select(e =>
e.Salary).Sum();
}
}
}

Sometimes we need to go further

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

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

class EmployeeCollection
{
private List<Employee> employees;

public int TotalSalary
{
get
{
return employees.Select(e => e.Salary).Sum();
}
}
}

In conclusion…

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

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Mixing functional and object oriented approaches to programming in C#

by Mark Needham on Apr 02, 2010

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