Domain Modeling with FP (DDD Europe 2020)

Domain Modeling
With Functional Programming
(DDD Europe 2020)
@ScottWlaschin
fsharpforfunandprofit.com

Functional
Programming
Domain
Driven
Design

FP is good for:
• Interactive & collaborative domain modeling
• Representing a domain model accurately

Part I
The importance of design

Input Output
Process
The software development process

Input Output
Process
Garbage in Garbage out

Input Output
Process
(reduce) Garbage in (reduced) Garbage out

• Agile contribution:
– Rapid feedback during design
• DDD contribution:
– Stakeholders have a shared mental model
– …which is also represented in the code
How can we do design right?

Domain Modeling with FP (DDD Europe 2020)

Can you really make code
represent the domain?

What non-developers think source code looks like

module CardGame =
type Suit = Club | Diamond | Spade | Heart
type Rank = Two |Three | Four | Five | Six | Seven | Eight
| Nine |Ten | Jack | Queen | King
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
Sharedlanguage What DDD source code should look like

module CardGame =
* means a pair. Choose one from each type
list type is built in

Deal
(original)
Deck
(remaining)
Deck
(on table)
Card
Modeling an action with a function
type Deal = Deck -> (Deck * Card)
Input Output

Pickup Card
(updated)
Hand
(original)
Hand
(on table)
Card
Modeling an action with a function
type PickupCard = (Hand * Card) –> Hand
Input Output

module CardGame =

module CardGame =
Can non-programmers provide
useful feedback?

Rapid feedback during
the design stage

Building a shared mental model is an
interactive process

...

...
type Deal = ShuffledDeck –› (ShuffledDeck * Card)

...
type ShuffledDeck = Card list

...
type Shuffle = Deck –› ShuffledDeck

In the real world
Suit
Rank
Card
Hand
Deck
Player
Deal
In the code
Suit
Rank
Card
Hand
Deck
Player
Deal

In the real world
Suit
Rank
Card
Hand
Deck
Player
Deal
In the code
Suit
Rank
Card
Hand
Deck
Player
Deal
ShuffledDeck
Shuffle
ShuffledDeck
Shuffle


In the real world
Suit
Rank
Card
Hand
Deck
Player
Deal
In the code
Suit
Rank
Card
Hand
Deck
Player
Deal
PlayerController
DeckBase
AbstractCardProxyFactoryBean


The process of building the shared
mental model is critical!
Collaboration!

Key DDD principle:
Communicate the design
in the code

type Contact = {
FirstName: string
MiddleInitial: string
LastName: string
EmailAddress: string
IsEmailVerified: bool
} // true if ownership of
// email address is confirmed

type Contact = {
FirstName: string
LastName: string
}
Prologue: which values are optional?

type Contact = {
FirstName: string
LastName: string
}
Prologue: what are the constraints?

type Contact = {
FirstName: string
LastName: string
}
Prologue: domain logic?

Prologue: F# can help
type Contact = {
FirstName: string
LastName: string
}

Part II
Understanding FP type systems

FP principle:
Composition everywhere

Composition = Make big things from small things

New types are built from smaller types by:
Composing with “AND”
Composing with “OR”

FruitSalad = One each of and and
Compose with “AND”
type FruitSalad = {
Apple: AppleInfo
Banana: BananaInfo
Cherry: CherryInfo
}

Snack = or or
Compose with “OR”
type Snack =
| Apple of AppleInfo
| Banana of BananaInfo
| Cherry of CherryInfo

Part III
Domain modeling
with composable types

type Contact = {
FirstName: string
LastName: string
} // true if ownership of
// email address is confirmed
This looks suspiciously like database-driven design...

type Contact = {
Name: PersonalName
Email: EmailContactInfo }
"A contact has a name AND email address"
"Like this?..."

type Contact = {
Name: PersonalName
"A contact has a name AND email address"
We have two new concepts already!

type PersonalName = {
FirstName: string
LastName: string
}
"What's a personal name?"

FirstName: string
LastName: string
}
required
required
optional
"What's required or optional?"

type Option<'T> =
| Some of 'T
| None

FirstName: string
MiddleInitial: Option<string>
LastName: string
}

FirstName: string
MiddleInitial: string option
LastName: string
}

Modeling simple values and
constrained values

Modeling simple values
• Avoid "Primitive Obsession"
• Simple values should not be modelled with
primitive types like "int" or "string" or "float"
"Does 'float' have
something to do
with water?"

Modeling constrained values
• It's rare to have an unconstrained int or string:
– An EmailAddress must not be empty,
it must match a pattern
– A PhoneNumber must not be empty,
it must match a pattern
– A CustomerId must be a positive integer

Is an EmailAddress just a string? No!
Is a CustomerId just a int? No!

type EmailAddress = EmailAddress of string
Use wrapper types to keep domain concepts
distinct from their representation
type CustomerId = CustomerId of int
type String50 = String50 of string

type EmailAddress = EmailAddress of string
type PhoneNumber = PhoneNumber of string
type CustomerId = CustomerId of int
type OrderId = OrderId of int
Two benefits:
- Clearer domain modelling
- Can't mix them up accidentally

The "Contact" challenge,
after first refactor

type Contact = {
FirstName: string
LastName: string
}

type Contact = {
FirstName: string
MiddleInitial: string option
LastName: string
}
Use option type for
potentially missing values

type Contact = {
FirstName: String50
MiddleInitial: String1 option
LastName: String50
EmailAddress: EmailAddress
}
Use wrapper types
instead of primitives

FirstName : String50
MiddleInitial : String1 option
LastName : String50 }
type EmailContactInfo = {
EmailAddress : EmailAddress
IsEmailVerified : bool }
Aggregates a.k.a. "consistency boundaries"
2 different
domain concepts

What about this?

• Rule 1: If the email is changed, the verified flag
must be reset to false.
• Rule 2: The verified flag can only be set by a
special verification service

"Email contact info is either Verified OR Unverified"
Listen closely to what the domain expert says...
type EmailContactInfo =
| Unverified of EmailAddress
| Verified of VerifiedEmail

"Email contact info is either Verified OR Unverified"

Q: Is a Verified email different?
Are there different business rules?
A: Yes, it must not be mixed up with unverified.
type VerifiedEmail = VerifiedEmail of EmailAddress
"there is no problem that can’t be
solved by wrapping it in another type"

type VerificationService =
(EmailAddress * VerificationHash) –› VerifiedEmail option
Q: Where do we get Verified emails from?
A: A special verification process

type VerifiedEmail =
VerifiedEmail of EmailAddress
Q: Are the business rules clear now?

Those business rules are automatically enforced by the design!

The "Contact" challenge,
completed

type EmailAddress = ...
FirstName: String50
MiddleInitial: String1 opt
LastName: String50 }
type Contact = {
Name: PersonalName

FirstName: String50
MiddleInitial: String1 opt
LastName: String50 }
type Contact = {
Name: PersonalName
type EmailAddress = ...

Summary
• Represent the shared mental model in code
– The developers should become domain experts too
– Write code collaboratively to build the shared
mental model
• Use the power of a composable type system
– Options instead of null
– Wrappers for constrained types
– Choice types rather than inheritance or flags
– Static types give you confidence when refactoring

More "Domain Modeling Made Functional" at
– fsharpforfunandprofit.com/ddd
Thanks!
Twitter: @ScottWlaschin

Domain Modeling with FP (DDD Europe 2020)

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