Playing Blackjack With Haskell

03 May 2012

Following a visit to Las Vegas a few months ago, I developed an interest in blackjack. According to Wikipedia, the house edge for a casino is typically around 1%. Rather than performing a statistical analysis, I decided to write a blackjack-playing bot and measure its losses.

This seemed like the perfect opportunity to get my feet wet with Haskell. A card game required minimal third party libraries and seemed well-suited to playing with Haskell’s type system.

The Haskell learning curve

Haskell is not an easy language to get started in. There are excellent online books to get you started (1, 2), and #haskell on Freenode were very helpful. However, the learning curve is difficult at the start.

Every Haskell program requires the programmer to have a moderate understanding of at least the type system and the IO monad. There is a substantial amount of syntax in just this. For example, you will want to learn the do notation (though some consider it harmful) which includes the unfortunately named return function.

For example:

greetingForUser = do
    name <- getLine
    let greeting = "Hello, " ++ name ++ "!"
    return greeting

greetingForUser is a value that, when evaluated, takes a line of input from stdin and returns a string containing that name wrapped in a friendly message.

So, what’s going here? getLine has type IO String, a string which is marked as having come from the outside, impure world. You can evaluate it twice and get different outputs. do notation allows us to use the string as a simple String, enabling us to keep IO actions out of the third line. return then wraps greeting in IO again.

Got that? Understanding IO is a conceptual hurdle you must overcome before you can start writing useful code.

There are a number of syntax gotchas which must be learned to write code effectively. For example, getting the type of array access can produce parse errors (:type !! is incorrect, :type (!!) should be used). To make matters worse, syntax between the REPL and source code files is slightly different. The excellent Hoogle search engine goes a long way to fix this. For example, you can search for functions like $ which are not Google-friendly.

Haskell offers very powerful, very high-level tools for coding. Once you’re comfortable with the normal syntax, you can even write macros. With such high-level tools, it’s often possible to greatly abstract concepts.

For example, every value in Haskell has a type. Every type has a kind, which is essentially a meta type. GHC (the most commonly used interpreter/compiler) even supports a number of extensions to this meta type system.

So the Haskell programmer has to really think about their code, up front. This is hard for a beginner.

Pedagogic effects

For the persistent learner, Haskell helps you think about the effects and assumptions of code in a rigorous way.

For instance, using only pure functions for the core of a program does produce more predictable, debug-friendly, testing friendly code. This is a perspective that has improved my code in impure languages.

Here’s an example from the bot. I originally dealt cards by randomly chosing cards one-by-one from the deck:

dealOneCard [] = error "No cards left in deck"
dealOneCard deck = do
  randomCardIndex <- randomRIO (0, length deck - 1)
  return (deck !! randomCardIndex)

This forced more of my functions to be impure. Any function whose output is dependent on a random number generator is a pain to test. With Haskell exposing this in the type system, this defect stares you in the face. I later refactored this into a superior function that reorders the whole deck in one go:

shuffleCards shuffled [] = return shuffled
shuffleCards shuffled unshuffled = do
  randomCardIndex <- randomRIO (0, length unshuffled - 1)
  let randomCard = unshuffled !! randomCardIndex
      unshuffledBefore = take randomCardIndex unshuffled
      unshuffledAfter = drop (randomCardIndex + 1) unshuffled
  
  shuffleCards (randomCard:shuffled) (unshuffledBefore ++ unshuffledAfter)

The combination of Haskell’s type system and syntax forces you to think about every situation that your function needs to deal with. Once the code is written, this is wonderful. Code resulting from this process is generally robust and reliable. However, I found writing the code to be less satisfying, as it’s hard to write a function piece-by-piece.

Code reliability

In any language, it’s well worth using lint tools to examine code to find potential errors or issues. This has saved me countless hours of debugging. In Python, I use the excellent Pyflakes to catch mistakes before I’ve even run the code.

It’s shocking how many of errors simply cannot occur in Haskell code. Haskell’s syntax and type system not only eliminated bugs that I could catch with lint tools, they prevent bugs that I can only detect at runtime in other languages. For example, the type system forces you to label any value that may be null with Maybe, eliminating null pointer exceptions entirely.

Blackjack conclusions

Originally, I would run the simulation with 10,000 hands, since it showed less variance in losses. I read later that a player typically plays 80 hands in an hour, so I tried simulating an hour’s play. 80 hands of blackjack produces a much larger range of outcomes, giving the game a completely different feel.

I had also intended to implement a perfect strategy inside my simulator. It became clear that a relatively simple strategy did not produce losses significantly less than a perfect strategy. I found that the opposite was generally true for house rules though. Small changes in house rules can substantially different losses.

Would I choose to play Blackjack if I visit Las Vegas again? I might have a clear idea of strategy now, but having lost over $100,000 in virtual money so far, it’s probably unwise.

Getting the code

My blackjack code is available on GitHub. The player’s tactics are not optimal, but losses are around 2%.