Writing a Haskell Program: The Guessing Game

We are going to write a moderately long Haskell program, consisting of multiple functions and I/O actions.

Guessing Game

The program is going to be a guessing game, called Starman. In this single-player, text-based game, there is a word which the player needs to guess. For each turn of the game, the player guesses a single letter. If that letter is correct, then the guessed letters are displayed in the correct places in the word. If that letter is incorrect, then the user loses a star. Once the user has no stars left, they have lost the game. However if the user guesses all the letters in the word, they have won the game.

Because this game is quite long, we should use a texteditor (like Notepad++ on Windows, TextEdit on Mac or Gedit on Linux). Well, I actually use emacs … if you’ve heard of it. Start by creating an empty text file called starman.hs — the hs extension is to indicate that this file contains Haskell source code.

Key Functions

The heart of the game involves checking the player’s guess.
We want to know whether the guess was right.
This outcome is a Bool value, either True or False.
We need to update the displayed word, if the guess was right, by replacing appropriate dashes in the displayed word with the correctly guessed character.
Therefore the result type of the function is a pair (Bool, String).
The first element of the pair is the guess outcome.
The second element is the String to display to the user for the next round.

Now, the checking function needs to know:

• • The secret word, a String

• • The current display, also a String

• • The character guessed by the player

These are the inputs to the checking function.
So now we can state the type of the function:

check :: String -> String -> Char -> (Bool,String)

Here is a great programming tip.
It’s always helpful to work out the type of a function first.
This focuses your attention on what the function is supposed to compute, and what data it needs to do it.
Good software engineers do specification before implementation.

What will the check function body look like?
The player’s guess is correct if and only if the guessed character c is in the target word.
So the guess is correct if

c `elem` word

The new displayed word will be:

[(if x==c then c else y) | (x,y) <- zip word display]

This is a list comprehension, where we select each letter from either the actual word or the old display. The word is plaintext, whereas the display starts with all dashed characters.

check :: String -> String -> Char -> (Bool, String)
check word display c
 = (c `elem` word, [if x==c
 then c
 else y | (x,y) <- zip word display])

The next function we will define is the turn function. This is the function that will be called each time it is the player’s turn to enter a guess.
First we need to check how many guesses the player has left:

if n == 0

If there are any guesses left, then we need to see whether the player is correct or not:

if word == display

So we will have two if checks, each followed by putStrLn status messages and the
end of the function calling sequence (since it is the end of the game).
However if neither of the if conditions is true, then the player can take a turn,
so we call another function to get another character from the user input.

turn :: String -> String -> Int -> IO ()
turn word display n =
 do if n==0
 then putStrLn "You lose"
 else if word==display
 then putStrLn "You win!"
 else mkguess word display n

Note that there is a neater way to write the turn function, using Haskell guards, but we won’t learn about guards for now.

mkguess word display n =
 do putStrLn (display ++ " " ++ take n (repeat '*'))
 putStr " Enter your guess: "
 q <- getLine
 let (correct, display') = check word display (q!!0)
 let n' = if correct then n else n-1
 turn word display' n'

What is the type of mkguess? Can you work it out and add it before the function definition?
We grab a line of user input, but only use the first character for the guess.
This will fail if the user just hits ENTER without typing any characters, since q will be an empty string.

OK, so now we just need a top-level function. Let’s call this
starman:

starman :: String -> Int -> IO ()
starman word n = turn word ['-' | x <- word] n

This function takes two arguments, the first is the word to be guessed, and the second is the number of incorrect guesses the player is allowed.

Running the Code

Let’s put all these four functions into a textfile, called starman.hs

Save the file, then start ghci perhaps by typing ghci into a DOS command prompt, running WinGHCi
or typing ghci in a terminal window (macOS or Linux).

If you are in the correct directory, i.e. the one where you saved
starman.hs, you should be able to type

:l starman.hs

and the program should load. It will either say something like:

[1 of 1] Compiling Main ( starman.hs, interpreted )
Ok, modules loaded: Main.

or report an error if you have made a mistake in the source code anywhere. Check and make corrections if necessary.
An error might look like this:

[1 of 1] Compiling Main ( starman.hs, interpreted )
(some error report)
Failed, modules loaded: none.

The error report should have a line number, so you can see where the mistake is. Try to fix it by following the instructions, or comparing your code with what’s written above.

When you get Ok from ghci, then you can run the program.
At the ghci prompt type

starman "functionally" 5

and start playing the game!
You will return to the GHCi prompt when the starman function completes.

We have provided the Haskell source code for starman.hs as a download below, along with some comments. You could use this, but it would be much better to type in the program yourself and try to understand it.

Possible Extensions

A real improvement to the game would be to generate a random word, perhaps from a list of words or a dictionary file.
If you are feeling ambitious, you might try this.
It would involve generating a random number i and read in the ith word from a dictionary.
You might import System.Random and use a Haskell random number generator.