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# Indy Code - Taking a Gamble With F#: Implementing Blackjack

Have you heard a lot about about functional programming, and aren’t sure how to get started? Have you tried learning the concepts, but still having a hard time applying them to a real problem? Then this talk is for you! In this presentation I’ll walk you through how to model the classic card game, Blackjack, using F#.

Intended for developers with little to no functional experience, my talk will introduce you to the basic concepts of functional programming while applying them to Blackjack. By the end of the presentation, you’ll know the basic concepts and how to apply them.

Have you heard a lot about about functional programming, and aren’t sure how to get started? Have you tried learning the concepts, but still having a hard time applying them to a real problem? Then this talk is for you! In this presentation I’ll walk you through how to model the classic card game, Blackjack, using F#.

Intended for developers with little to no functional experience, my talk will introduce you to the basic concepts of functional programming while applying them to Blackjack. By the end of the presentation, you’ll know the basic concepts and how to apply them.

## More Related Content

### Indy Code - Taking a Gamble With F#: Implementing Blackjack

1. 1. Taking a Gamble with F# Implementing Blackjack Cameron Presley @PCameronPresley http://Blog.TheSoftwareMentor.com
2. 2. Little About Myself  Senior Software Engineer at Pilot/Flying J  Musician and Board gamer  Co-organizer of @FunctionalKnox  “Developer Betterer”
3. 3. Goals  Explore the different types in F# and the when to use them  Demonstrate how to use these types to model Blackjack  Show how to deal with things that can fail
4. 4. Types Overview  How to hold data  Collections  Combining different types as one
5. 5. Know When to Hold Them Holding Data Source
6. 6. Holding Data Tuples  Provides a way to model all possible combinations of types  A tuple of int*string would represent all combinations of integer and strings  A triple of int*string*int would represent all combinations of integer, string, and integer
7. 7. Holding Data Tuples
8. 8. Holding Data Tuples  Advantages  Create on the fly  Easy to break down  Disadvantages  Parameters have to be in order (int*string is different from string*int)  Hard to keep values in the right order (int*int*int)  Doesn’t provide a lot of type safety
9. 9. Holding Data Tuples
10. 10. Holding Data Tuples  When Should You Use Tuples?  When working within a function  When there are few parameters  When Should You NOT Use Tuples?  Representing a domain model  Need more type safety  Representing the same data types (i.e. int*int*int)  Can be hard to remember which element represents which
11. 11. Holding Data Records  Like a tuple, provides a way to model all possible combinations of a value  Key Differences  Must define as a type  Provides labels (i.e. dot syntax)  Order doesn’t matter
12. 12. Holding Data Records
13. 13. Holding Data Records  Advantages  Order doesn’t matter  Named parameters  Easy to retrieve a value  Disadvantages  Can’t create on the fly  Verbose definition
14. 14. Holding Data Records  When Should You Use Records?  Representing domain models  Working with a lot of parameters  Providing better type safety  When Should You NOT Use Records?  Intermediate results for a function
15. 15. Holding Data Review  Tuples  Create on the fly  Easy to break down into separate values  Records  Representing domain models  Working with a lot of parameters  Providing better type safety
16. 16. Collections
17. 17. Collections Sequences  Provides a way to define the collection by using a generator function  Lazy by default  Only creates values when iterated  Similar to IEnumerable from C#
18. 18. Collections Sequences
19. 19. Collections Sequences  Advantages  Can model an infinite collection  Provides a way to generate all values  Only computes values when needed  Disadvantages  Expensive to add a value -> O(n)  Retrieving an item by index is slow -> O(n)  Can’t modify elements in place
20. 20. Collections Lists  Represents a collection as two parts, the first element, and the rest of the list  First element referred to as the head  Rest of the list is referred to as tail  Similar to linked lists  Typically used with recursion, pattern matching, and the :: (cons) operator
21. 21. Collections Lists
22. 22. Collections Lists  Advantages  Can define recursive functions to process element by element  Adding additional elements is fast -> O(1)  Can be defined with a generator function  Disadvantages  Can’t model infinite series  Indexing to an element is slow -> O(n)  Can’t modify elements in place
23. 23. Collections Arrays  Represents a collection as a dictionary such that the index finds the value  Long story short, similar to arrays in other languages  Typically used when lookup speed matters
24. 24. Collections Arrays
25. 25. Collections Arrays  Advantages  Quick lookup -> O(1)  Can change one element in the array with a different element -> O(1)  Can model multidimensional data (2D arrays)  Disadvantages  Can’t model infinite series  Adding additional elements is slow -> O(n)
26. 26. Collections Summary  Sequences  IEnumerable from C#  Should use when all values can be generated by a function or when modeling an infinite series  Lists  Linked lists  Great for when the number of elements can change or when processing element by element  Arrays  Similar to traditional arrays  Best used when certain elements need to change or for quick access
27. 27. Combining Different Types Discriminated Unions
28. 28. Combining Different Types Discriminated Unions  Discriminated Unions provides a way to specify that a type can be one of many different types  The Difference between Tuples/Records and Discriminated Unions (DUs)  Tuples/Records -> int*string (all combinations of integer and string)  DUs -> int | string (either it’s an integer or a string)  Typically used with pattern matching
29. 29. Combining Different Types Discriminated Unions
30. 30. Combining Different Types Discriminated Unions
31. 31. Combining Different Types Discriminated Unions
32. 32. Combining Different Types Discriminated Unions  Now I needed to create a new coordinate and I do this:  What’s to stop me from making this error?
33. 33. Combining Different Types Discriminated Unions  Problem is that Lat and Long are part of the domain for Coordinate  Instead of modeling them as floats, they should have their own type  But float is the correct data type, so how I create a type that wraps around a float?  Single Case Discriminated Unions!
34. 34. Combining Different Types Discriminated Unions
35. 35. Combining Different Types Discriminated Unions  When a primitive needs to be modeled as part of the domain  Single Case  When different type signatures actually model the same thing  Combining different types  When there are a finite number of possibilities  Enum Style
36. 36. Data Type Recap  Holding Data  Tuples, Records  Collections  Sequences, Lists, Arrays  Combining Different Types  Discriminated Unions
37. 37. Modeling The Domain
38. 38. Modeling The Domain  Goals  Identify the different models  Implementation
39. 39. Modeling The Domain Goals  Has to be easy to read and understand  Easier to maintain  Defines a ubiquitous language  Provides a way for developers, QA, and users to communicate  Make illegal states unrepresentable  Why allow bad data to be created?
40. 40. Identifying Models  Usually start big and then break down to smaller pieces  How do we represent the entire game?  Define a Game type
41. 41. Identifying Models
42. 42. Identifying Models  What’s included in a game?  Game has  Deck  Players  Dealer
43. 43. Identifying Models
44. 44. Identifying Models
45. 45. Identifying Models  What’s a Deck?  Represents all of the Cards that will be used in the Game  What’s a Card?  Combination of Rank and Suit  What’s a Rank  One of 13 possible values  A, 2, 3, 4, 5, 6, 7, 8, 9, 10, J, Q, K  What’s a Suit  One of 4 possible values  Hearts, Clubs, Spades, Diamonds
46. 46. Identifying Models
47. 47. Identifying Models
48. 48. Identifying Models  What are Players?  Collection of Player  What’s a Player  Someone playing against the Dealer  Has both a Hand and a Status  What’s a Hand?  Represent the Cards a Player has
49. 49. Identifying Models
50. 50. Identifying Models  Represents the state of the Player  Blackjack -> 2 cards worth with 21  Busted -> Has a Score over 21  Stayed -> Not Blackjack and a Score of 21 or less  Cards Dealt -> Hasn’t taken a turn yet
51. 51. Identifying Models
52. 52. Identifying Models  Represents the final result of a Player’s Hand  Result is based on whether the Player Busted or Stayed  Can only be of one value (integer)  Important to know when comparing who won
53. 53. Identifying Models
54. 54. Identifying Models  Competes against every Player  Similar to Player  Dealer has a Hand and a Status  Unlike Player, there’s only one, so no ID is needed
55. 55. Identifying Models
56. 56. Identifying Models  Talked about the components of the game  What about points?  What about the actions that a player can take?
57. 57. Identifying Models
58. 58. Identifying Models  Cards are worth Points based on their Rank  Face cards are worth 10 points  Point cards are worth that many  (2’s worth 2, 3’s worth 3 …, 10’s worth 10)  Aces can be counted as 1 or 11
59. 59. Identifying Models
60. 60. Identifying Models  During the game, a participant can take one of two different actions  Draw Cards (Hit)  Adds a single card to their Hand  Stay  Stops drawing cards
61. 61. Identifying Models
62. 62. Implementing the Domain  We’ve broken down the domain into smaller pieces and identified dependencies amongst the pieces  Time to implement the pieces in a bottom-up order
63. 63. Implementing the Domain
64. 64. Implementing the Domain
65. 65. Implementing the Models Rank  Can be one of 13 possible values  A, 2, 3, 4, 5, 6, 7, 8, 9, 10, J, Q, K  What type is great for modeling a finite set of values?
66. 66. Implementing the Domain
67. 67. Implementing the Models Suit  Can be one of 4 possible values  Hearts, Clubs, Spades, Diamonds  How should we model this?
68. 68. Implementing the Domain
69. 69. Implementing the Models Card  Contains both a Rank and Suit  What type is great for holding data?
70. 70. Implementing the Domain
71. 71. Implementing the Models Deck  Contains 1 or more Cards  What are some properties of a Deck?  Length will change throughout the game  Which collection type should we use to model this?
72. 72. Implementing the Domain
73. 73. Implementing the Models Hand  Contains the Cards for a Player  Initially starts with two cards, but can draw more during the game  How should we model this?
74. 74. Implementing the Domain
75. 75. Implementing the Models Score  Represents the final value of a Hand  Can only be one value (integer)  What’s a way to wrap primitives as their own type?
76. 76. Implementing the Domain
77. 77. Implementing the Models Status  Blackjack  Player has two cards and their score is 21  Busted  Player went over 21 and that’s their final score  Stayed  Player decided to not take any more Cards and that’s their final score  Cards Dealt  Player hasn’t taken their turn
78. 78. Implementing the Models Status  Can be one of a finite number of choices  Some of the type require Score and other ones don’t  How should we model this?
79. 79. Implementing the Domain
80. 80. Implementing the Models Dealer  A Dealer contains  Hand  Status  How should we model this?
81. 81. Implementing the Domain
82. 82. Implementing the Models Player  A Player contains  Hand  Status  ID  How should we model this?
83. 83. Implementing the Domain
84. 84. Implementing the Models Players  Represents all the players in the Game  Number of Players won’t change during the game  Each player must finish their turn before the next player can start  Which collection type should we use?
85. 85. Implementing the Domain
86. 86. Implementing the Models Game  Contains  Deck  Players  Dealer  How should we model this?
87. 87. Implementing the Domain
88. 88. Implementing the Models Points  Every Card is worth a value based on the Rank  One of two possible values  Aces can be worth 1 or 11  Everything else can be worth one value  Need to combine two different types as one, how should we model this?
89. 89. Implementing the Domain
90. 90. Implementing the Models Actions  Represents what a participant can do during the Game  One of two possible values  Hit  Stay  Finite number of choices, how to model?
91. 91. Implementing the Domain
92. 92. Full Implementation
93. 93. Implementing the Models Summary  Determined the models by using domain terms (ubiquitous language)  Started with the biggest model (Game) and broke it down into smaller pieces and then repeating the process with the smaller pieces  After finding the models, implemented the models in a bottom-up fashion  Able to define the domain in a way such that bad states are unrepresentable
94. 94. Playing the Game
95. 95. Playing the Game  Sometimes, pure functions can fail  How can we handle this?  We’ll look at  Drawing a Card  Setting up a Player
96. 96. Playing the Game Drawing a Card  Throughout the course of the Game, Cards will be drawn from the Deck  Let’s create a function, drawCard that takes a Deck as input and returns the top Card and the rest of the Deck as output
97. 97. Drawing a Card  Are there any issues with this implementation?  What about an empty deck?  Match Failure Exception -> The match cases were incomplete  How can we model a type that may have a value or not?
98. 98. Playing the Game Drawing a Card  Introducing the Option type!  Special kind of Discriminated Union that handles when there’s a value and when there isn’t.  Let’s rewrite the drawCard function to handle an empty deck
99. 99. Playing the Game Drawing a Card
100. 100. Playing the Game Setting up a Player  Now that we have a way to draw cards, we can now setup a Player  Remember that a Player starts off with a Hand of two cards and a Status of CardsDealt
101. 101. Playing the Game Setting up a Player  Remember, drawCard returns an option, so we need to handle that.
102. 102. Playing the Game Setting up a Player  This code is not nearly as readable as the first solution  What are we really trying to accomplish?  Try to draw two cards, if so, return Some Player, otherwise, None  Wouldn’t it be nice if we could have the short-circuit logic of the second solution, but still have the readability of the happy path?  How can we work around all the pattern matching?
103. 103. Playing the Game Setting up a Player  By using the Maybe Builder pattern!  Known in other FP languages as Maybe.  Two parts  Provides a way to short-circuit logic if the input is None (called Bind)  A way to wrap a value as an option (called Return)
104. 104. Playing the Game Setting up a Player
105. 105. Playing the Game Setting up a Player
106. 106. Playing the Game Setting up a Player  Problem  Creating a Player can fail because there’s not enough cards  Putting in the error handling code makes it cluttered and hard to read  Solution  Using the Maybe pattern, we can short-circuit our function flow to return None before calling anything else  When to Use  Lots of nested pattern matching on options
107. 107. Wrapping Up  Overview of the different F# types and the advantages of each  Modeled and implemented the domain for Blackjack using the different types  Explored how to handle functions that can fail
108. 108. Additional Resources  Learning more about F#  F# for Fun and Profit -> http://fsharpforfunandprofit.com/  Code Solution  Blackjack Repo -> https://github.com/cameronpresley/Blackjack  How to find me  Twitter ->@pcameronpresley  Blog -> http://blog.thesoftwarementor.com

### Editor's Notes

• Software Engineer at Pilot Flying J (headquartered in Knoxville, TN) – always looking
• Ever had a method that should return an abstraction, but there wasn’t a clear contract?
• Suppose we had the following definition?
• What are we trying to accomplish?