Functional programming in C#

Functional Programming in C#
NCrafts 21-22 May, Paris 2015
@tjaskula

What is functional programming ?
• Functions as building blocks
• Function returns values only based on the passed input
• Recursion
• HOF (Higher Order Functions)
• Creation of anonymous functions, in-line, lambda expressions
• Closures
• Immutability

What REALY is FP ?
“Natural way of telling the computer what it should do, by
describing properties of a given problem in a concise language”

Programmers responsibility in FP ?
Specify functions to describe a given set of problems

Object Oriented Programming
• Encapsulation
• Combining data and behavior into classes and objects
• Modeling Real-World

Relationship FP to OOP
• No silver bullet
• Programmer needs to understand different techniques

Why to talk about ?
• Concurrency programming models
• Writing software it’s a complex process
• Managing side effects

Part 1 – Make it simple
Composing dependencies

The OOP way
IoC container granted from the start.
Developers argue about which framework to chose and not the
problem to solve.

public class EnrollmentCommandHandler
{
private readonly StudentRepository _studentRepository;
private readonly ClassRepository _classRepository;
private readonly StudentArchiveRepository _studentArchiveRepository;
public EnrollmentCommandHandler(StudentRepository studentRepository,
ClassRepository classRepository,
StudentArchiveRepository studentArchiveRepository)
{
_studentRepository = studentRepository;
_classRepository = classRepository;
_studentArchiveRepository = studentArchiveRepository;
}
public void Enroll(StudentEnrollCommand command)
{
var student = _studentRepository.GetById(command.StudentId);
var @class = _classRepository.GetById(command.ClassId);
try
{
student.TryEnrollIn(@class);
@class.TryEnroll(student);
student.Enroll(@class);
@class.Enroll(student);
}
catch (Exception e)
{
// log
}
}

New requirements
Log, Security, Audit, Cache…Cross Cutting Concerns

{
private readonly UnitOfWork _unitOfWork;
private readonly EnrollementNotificationService _notificationService;
private readonly ILogger _logger;
private readonly AuthorizationService _authorizationService;
private readonly CalendarService _calendarService;
private readonly ServiceFoo _serviceFoo;
private readonly ServiceBlah _serviceBlah;
private readonly FactoryFoo _facoFactoryFoo;
private readonly FactoryBlah _factoryBlah;
StudentArchiveRepository studentArchiveRepository,
UnitOfWork unitOfWork,
EnrollementNotificationService notificationService,
ILogger logger,
AuthorizationService authorizationService,
CalendarService calendarService,
ServiceFoo serviceFoo,
ServiceBlah serviceBlah,
FactoryFoo facoFactoryFoo,
FactoryBlah factoryBlah
)
{
_unitOfWork = unitOfWork;
_notificationService = notificationService;
_logger = logger;
_authorizationService = authorizationService;
_calendarService = calendarService;
_serviceFoo = serviceFoo;
_serviceBlah = serviceBlah;
_facoFactoryFoo = facoFactoryFoo;
_factoryBlah = factoryBlah;
}
}
public void Handles(StudentEnrollCommand command)
{
try
{
_unitOfWork.BeginTransaction();

…or better… AOP to the rescue

[Logable]
[Authorizable]
[Cachable]
[ExceptionPolicy]
[Blablable]
{
{
}
public void Handles(StudentEnrollCommand command)
{
try
{
}
catch (Exception e)
{
// log
}
}

Yes, but container can do more !

AOP with interception
var calculator = new Calculator();
var calculatorProxy = Intercept.ThroughProxy<ICalculator>(calculator,
new InterfaceInterceptor(), new[] { new LogBehavior() });

but one must :
• know Dynamic Proxy pattern
• know the difference of Instance and Type Interceptors
• know Interception behaviors
• not forget VIRTUAL keyword on methods
• wire up IoC container correctly

Really ?!!! Is this…
SIMPLE ?

DI flavors
IoC with conventions...
Scan(x =>
{
x.TheCallingAssembly();
x.ExcludeNamespaceContainingType<IEvent>();
x.ExcludeNamespaceContainingType<SearchModel>();
x.ExcludeNamespaceContainingType<AuthenticationService>()
x.ExcludeNamespaceContainingType<DovetailController>();
x.AddAllTypesOf<IDomainMap>();
x.WithDefaultConventions();

DI flavors
IoC with manual configuration...
var container = new UnityContainer();
container.RegisterType<IService, Service>(“Service”);
container.RegisterType<IService,ServiceDecorator>(
new InjectionConstructor(new ResolvedParameter(typeof(IS
Func<IUnityContainer, object> factoryFunc = c => new ServiceDecorator(ne
c.Resolve<ISubServiceProvider();
));
container.AddNewExtension<DecoratorContainerExtension>();
container.RegisterType<IService, ServiceDecorator>();

DI flavors
IoC with XML configuration...
<unity>
<typeAliases>
<typeAlias alias="string" type="System.String, mscorlib" />
<typeAlias alias="ILogger" type="UnitySamples.ILogger, UnitySamples" />
<typeAlias alias="ConsoleLogger" type="UnitySamples.ConsoleLogger, UnitySamples" />
<typeAlias alias="DebugLogger" type="UnitySamples.DebugLogger, UnitySamples" />
<typeAlias alias="IContext" type="UnitySamples.IContext, UnitySamples" />
<typeAlias alias="UnityContext" type="UnitySamples.UnityContext, UnitySamples" />
<typeAlias alias="CustomerTasks" type="UnitySamples.CustomerTasks, UnitySamples" />
</typeAliases>
<containers>
<container>
<types>
<type type="ILogger" mapTo="ConsoleLogger" name="defaultLogger"/>
<type type="ILogger" mapTo="DebugLogger" name="debugLogger"/>
<type type="IContext" mapTo="UnityContext">
<typeConfig extensionType="Microsoft.Practices.Unity.Configuration.TypeInjectionElement, Microsoft.Practi
<constructor>
<param name="logger" parameterType="ILogger">

What problem do we try to solve ?
• Decoupling ?
• Testing ?
• Modular Design ?
• Dependencies management ?

Dependencies are bad....
{
public EnrollmentCommandHandler(StudentRepository studentReposit
ClassRepository classReposit
StudentArchiveRepository stu
EnrollementNotificationServi
ILogger logger,
AuthorizationService authori
CalendarService calendarServ

Cyclic dependencies are evil....
{
public EnrollmentCommandHandler(StudentRepository studentReposit
ClassRepository classReposit
StudentArchiveRepository stu
EnrollementNotificationServi
ILogger logger,
AuthorizationService authori
CalendarService calendarServ

{
{
}
{
}
}
Code we have....

{
{
}
{
}
}
Code that matters…

Functional way
int Multiply(int a, int b)
{
return a * b;
}
Partial application
Func<int, int> multiplyBy10 = b => Multiply(10, b);

Functional way int a = 3;
int b = a + 7;
int c = b * 10;
Composition
Func<int, int> calcCFromA = a => CalcC(CalcB(a));
int CalcCFromA(int a)
{
return (a + 7) * 10;
}
int CalcCFromA(int a)
{
return CalcC(CalcB(a));
}
let calcCFromA = calcB >> calcC

{
{
}
{
}
}
Refactor this…

public void Enroll(StudentRepository studentRepository,
StudentEnrollCommand command)
{
var student = studentRepository.GetById(command.StudentId);
var @class = classRepository.GetById(command.ClassId);
}
to this…

var studentRepository = new StudentRepository();
var classRepository = new ClassRepository();
var pipeline = c =>
handlers.Enroll(studentRepository, classRepository, c);
Bootstrap in one place…

_handlers.Dispatch(new StudentEnrollCommand(1));
=
pipeline(new StudentEnrollCommand(1));
Handle in another…

var pipeline
= c =>
handlers.Log(c, c1 =>
handlers.Enroll(studentRepository, classRepository, c1));
Want to log ?

var studentEnrollPipeline
= c =>
handlers.Audit(c, c1 =>
handlers.Log(c1, c2 =>
handlers.Enroll(studentRepository,classRepository, c2));
Audit ?

var lifeTime = new LifeTime();
var studentRepositoryFactory = () => new StudentRepository();
var classRepositoryFactory = () => new ClassRepository();
var studentEnrollPipeline
= c =>
handlers.Audit(c, c1 =>
handlers.Log(c1, c2 =>
handlers.Enroll(lifeTime.PerThread(studentRepositoryFactory),
lifeTime.PerThread(classRepositoryFactory), c2));
Lifetime management ?

Why to do it ?
• 99% of time you don’t have a problem for IoC container
• IoC makes easier things you shouldn’t be doing anyway
• Feel the pain and think

To many dependencies ?
• You have another serious problem

Is this…
SIMPLE ?
Is is just pure C# code

Part 2 – Control complexity
Composition strikes again…but in different shape

Writing a program using functions
How to order sequence of functions ?
ParseCopyright(string text) { return text + “©” }
ParseAppendix(string text) { return text.Remove(“APPENDIX”); }

Solution
By composing them
ParseCopyright(ParseAppendix(text));

But this can fail
ParseAppendix function may throw an exception

Solution
Return two kind of things from ParseAppendix function
string string
Instead of Let’s allow
string
string
Error
or

But function can return one thing
And only one thing

Solution
Put in in the box
string ParseResult

What happens to this now ?
ParseCopyright(ParseAppendix(text));
string ParseResultParseAppendix ParseCopyright

is success
Solution
Let’s have a special “link”/”connect”/”compose” function
string ParseResultParseAppendix ParseCopyrightstring
Error
Connect

What we have seen is a…
M***D

Composition
Composition is the key to controlling complexity in software.
“In our study of program design, we have seen that expert
programmers control the complexity of their designs with the
same general techniques used by designers of all complex
systems. They combine primitive elements to form
compound objects, they abstract compound objects to
form higher-level building blocks, and they preserve
modularity by adopting appropriate large-scale views of system
structure.”

Should I do functional programming
in C# ?

To take away
• Be stupid! Don’t waste your brain on complicated code
• Don’t waste your time to understand Rube Goldberg machines
• Simplicity. Write only the code that matters.
• Readability
• Less bugs

Functional programming in C#

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