Taking a Gamble with Functional Domain Modeling

@pcameronpresley
Taking a Gamble With
Functional Domain Modeling
Cameron Presley
Cameron@TheSoftwareMentor.com

@pcameronpresley
https://blog.TheSoftwareMentor.com/Presentations
No Need To Take Notes!

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What Are We Covering?
Introduction to Blackjack
Defining the Domain
Implementing the Domain
Implementing common workflows
4

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Overview
Game played with a standard deck of cards
Objective is to get closest to 21 points
without going over
Involves one or more players playing against
the dealer
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Set Up
Each player is dealt two cards face-up to
form their hand
Dealer is dealt two cards (one face-up) to
form their hand
7

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Game Play
On their turn, a player can take an action
Once all players have taken their turn, dealer
takes their turn
After dealer completes their turn, each
player compares their hand with the dealer
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Player Turn
Players can either “Hit” or “Stand”
◦ Stand – Ends the turn, staying in
◦ Hit – Draw a card
If their hand goes over 21, they “bust”
ending their turn and leaving the round
9

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Scoring
Aces can be worth 1 or 11 points
Face cards are worth 10 points
Number cards are worth that many points
If a player or dealer is dealt a hand worth 21
points (an Ace and a 10 point card), they
have a Blackjack, winning automatically
10

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What are the nouns?
Game played with a standard deck of cards
Objective is to get closest to 21 points
without going over
Involves one or more players playing against
the dealer
13

@pcameronpresley
Each player is dealt two cards face-up to
form their hand
Dealer is dealt two cards (one face-up) to
form their hand
15

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On their turn, a player can take an action
Once all players have taken their turn, dealer
takes their turn
After dealer completes their turn, each
player compares their hand with the dealer
17

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Players can either “Hit” or “Stand”
◦ Stand – Ends the turn, staying in
◦ Hit – Draw a card
If their hand goes over 21, they “bust”
ending their turn and leaving the round
19

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Building Up the Dependency Chart
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Determining Dependencies using
“has-a”
Game has a Deck, a Dealer, and Players
Deck has one or more Cards
Dealer has a Hand
Player has a Hand
Hand has one or more Cards
Card has a Rank and Suit
22

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Rank
Represents one of 13 possible values
Rank can be an Ace OR a Two OR a Three…
How to Model?
28

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Introducing The Sum Type!
Type that can be one of a finite number of
choices
Each choice is a different constructor
When a type can be A or B or C
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Suit
Represents one of four possible values
Suit can be Hearts or Clubs or Spades or
Diamonds
How can we model this?
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Card
Represents the combination of any Rank
and Suit
13 Ranks x 4 Suits = 52 possibilities
How can we model combinations?
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Introducing the Product Type!
Type that specifies the combination of other
types
It’s the product of the possible values for all
types it combines
36

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Deck
Represents the Cards to be used in the game
Players and Dealers will be drawing from this
during the Game
How to model?
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Introducing the Type Alias!
40
Any code that can work on a Card
List will work on a Deck

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Hand
Represents one or more Cards that a Player
or Dealer has
How to model this?
Leveraging a Type alias!
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Any issue with Hand and Deck being
interchangeable?

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Making Deck Different
44
In order to create this type, a Card list must be specified
Deck != Card list

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Player
Represents someone playing the game
Has a Hand and an ID
How to model?
47

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Dealer
Represents who every player is competing
against
Has a Hand
How to model?
50

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Game
It’s what’s being played!
Consists of a Deck, a Dealer, and 1 or more
Players
How to Model?
53

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Status
Represents the Player or Dealer during the
game
Can be one of 3 possible values
- Blackjack, Busted, Stayed
How to model?
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Status
57
To construct Stayed or Busted, an integer is needed
No dependencies to construct the Blackjack type

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Points
Represents how much a Card is worth
Ace = 1 or 11 points
Face Cards = 10 points
Number Cards = That number of points
How to model?
59

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Points
60
To construct the Hard type, need an int
To construct the Soft type, need a tuple of int and int

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Action
Represents what a Player can do during the
Game
Can either be Hit or Stay
How to model?
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Domain Defined
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Scoring a Hand
How Would You Score a Hand?
Calculate how many points each card is
worth
Sum them up!
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Scoring Rules
Aces can be worth 1 or 11 points
Face cards are worth 10 points
Number cards are worth that many points
71

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How to add points?
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Scoring a Hand
74

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Drawing A Card!
76
What do we do if the deck is empty?

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Introducing the Maybe Type!
Allows us to model the absence of a value
Compile time checked
Forces us to do the right thing
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78
Most languages have this construct
In F#, it’s called Option

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79
By using Option, we’re signaling that this function may return
a value and that callers will need to deal with the absence of
value

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Let’s Draw Multiple Cards!
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82
Where did maybe come from?
Where did let! come from?
Where did return come from?

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What Did We Cover?
Introduction to Blackjack
Defining the Domain
Implementing the Domain
Implementing common workflows
85

@pcameronpresley
Resources
◦ Thinking Functionally (series) by Scott Wlaschin
◦ Functional Architecture : a definition by Mark Seemann
◦ Functional Architecture Is Ports and Adapters by Mark Seemann
◦ Algebraic Data Types Aren't Numbers on Steroids by Mark
Seemann
◦ Domain Modeling Made Functional by Scott Wlaschin

@pcameronpresley
88
Thanks!
Any questions?
◦ Cameron@TheSoftwareMentor.com

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Taking a Gamble with Functional Domain Modeling