TechDaysNL 2015 - DDD with F#

"Focus on the domain and
domain logic rather than
technology”
Eric Evans

“First, it’s almost certainly true that functional programming
is the next big thing. “
Robert C. Martin

“Object oriented programming makes code understandable
by encapsulating moving parts. Functional programming makes
code understandable by minimizing moving parts.“
Michael Feathers

“F# is excellent at concisely expressing business and domain logic.
Developers trying to achieve explicit business logic within an
Application may opt to express their domain in F# with the
majority of plumbing code in C#.”

Functional programming
• Declarative
• Clearity
• Composability
• Immutability

Characteristic Imperative approach Functional approach
Programmer focus
How to perform tasks (algorithms)
how to track changes in state.
What information is desired and what
transformations are required.
State changes Important. Non-existent.
Order of execution Important. Low importance.
Primary flow control
Loops, conditionals, and function
(method) calls.
Function calls, including recursion.
Primary manipulation
unit
Instances of structures or classes.
Functions as first-class objects
data collections.

Characteristic C# F#
Programmer focus
How to perform tasks (algorithms)
how to track changes in state.
What information is desired and what
transformations are required.
State changes Important. Non-existent.
Order of execution Important. Low importance.
Primary flow control
Loops, conditionals, and function
(method) calls.
Function calls, including recursion.
Primary manipulation
unit
Instances of structures or classes.
Functions as first-class objects
data collections.

Why F# ?
• Functional-first
• Very strong type inference
• Integrate with other .NET technologies
• Windows, Web, Xamarin iOS & Android, Linux
The functional language in .NET

DDD
But first a small introduction to functions in F#

Functions
Everything is a function

let a = 10
int a () {
return 10;
}

let sum a b = a + b
int sum (int a, int b) {
return a + b;
}

Chaining
Chaining functions makes a service,
Chaining services makes an use-case

public static class Calc
{
public static decimal Square(decimal n)
{
return n * n;
}
public static decimal MultiplyByPi(decimal n)
{
return n*3.14m;
}
public static decimal AreaOfCicle(decimal r)
{
return MultiplyByPi(Square(r));
}
}

module Calc
let square n = n ** 2.0
let multiplyByPi n = n * 3.14
let areaOfCircle r =
multiplyByPi (square (r))

module Calc
let square n = n ** 2.0
let multiplyByPi n = n * 3.14
let areaOfCircle r =
r |> square |> multiplyByPi

DDD
It’s all about modelling types

Types
type Email = string
type SimplePerson = Name * Email
type AnotherPerson =
{ Name: string; Age: int }

public class AnotherPerson
{
public AnotherPerson(string name, int age)
{
Name = name;
Age = age;
}
public string Name { get; private set; }
public int Age { get; private set; }
public override int GetHashCode()
{
return Name.GetHashCode() + Age.GetHashCode();
}
public override bool Equals(object obj)
{
return Equals(obj as AnotherPerson);
}
public bool Equals(AnotherPerson other)
{
if (other == null)
return false;
return Name == other.Name && Age == other.Age;
}
}

module CardGameBoundedContext =
type Suit = Club | Diamond | Spade | Heart
type Rank = Two | Three | Four | Five | Six
| Seven | Eight | Nine | Ten
| Jack | Queen | King | Ace
type Card = Suit * Rank
type Hand = Card list
type Deck = Card list
type Player = {Name:string; Hand:Hand}
type Game = {Deck:Deck; Players: Player list}
type Deal = Deck -> (Deck * Card)
type PickupCard = (Hand * Card)-> Hand

module MoreTypes
type IPrintable =
abstract member Print : unit -> unit
type SomeClass () =
interface IPrintable
with
member this.Print () = printfn “Hello”
type AnotherClass (i: int) =
member this.Value = i
member val this.Mutable: int with get, set

Ronald Harmsen
ronald@nforza.nl
www.nforza.nl
@ronaldharmsen
Simple. Clear. Software.

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TechDaysNL 2015 - DDD with F#

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