Let’s Play a Text Game!

Sophie can hold an interactive conversation:

D:\GitHub\sophie\examples\games>sophie guess_the_number.sg
I have chosen a random number from 1 to 100.

What is your guess? 50
Too high. Try a lower number.
What is your guess? 25
Too low. Try a higher number.
What is your guess? 37
Too low. Try a higher number.
What is your guess? 45
You win after 4 guesses!

You can find the code for the game under examples/games/guess_the_number.sg, or it’s copied in full toward the end of this chapter.

Guided Tour of the Game 

Introducing the Console 

The main program gives an introduction and then begins a game with a random number. Here’s an excerpt for that part:

define:
    intro = ["I have chosen a random number from 1 to 100.", EOL, EOL];
    to game(r) is turn(1) where
        goal = int(r*100)+1;
        ...
    ...
begin:
    console ! echo(intro);
    console ! random(!game);

Already we see a few new concepts at work. Let’s work through this part from the bottom up.

console

This game uses the keyboard and text screen, called the console by ancient tradition, so that is the word Sophie defines for the purpose.

The console is considered a separate entity from your program. You might call it an actor but then the programming-language theorists would yell at me. So let’s call it an agent instead. You can send an agent a message, and the agent can do something in response whenever the message arrives. Your program will not stop to wait for that response, but will continue computing. Indeed you can have many agents operating concurrently, and they will not step on each other’s toes. This is what is meant by “asynchronous message-passing”.

console ! echo(intro)

This means to send the message echo, with single argument intro, to the agent called console. And to be clear, it means the idea of that action.

Here, console is the receiver. Any single receiver will handle its own incoming messages in the order they arrive. And messages sent from one agent to another will arrive in the order they were sent. However, messages from different sources may arrive at the same destination in any interleaving that respects the rules above. (The begin: block is considered a single source.)

Actions are values.

You can pass them around to functions, hold them in data structures, and select among them as you would with any other kind of value. The only way an action actually gets done is when an expression in the begin: section evaluates to an action – which can possibly kick off a cascade of consequences.

echo

The echo message asks the console to print some text. The text here is a list of strings. The reason echo takes a list rather than single string is applied laziness: You’ll often print several things together, such as some text and the EOL or end-of-line. It’s best by far if each distinct message expresses a single complete idea. (Violating this principle can lead to trouble down the line.)

random

The random message asks the console to:

Pick a real number at random in the half-open range [0, 1).
Send that number via message to … somewhere. I’ll explain.

to game(r) is ...

The thing called game is a procedure which takes a single argument. (In this case, that argument is a number that determines the goal of the game.)

There is a strict separation between functions and procedures. The point of a procedure is to encode some action that might happen. And in Sophie, the way to make things happen is to send messages around. In particular, we’ll soon see how game(r) represents the procedure of sending messages precisely orchestrated to implement a simple children’s game.

!game

The game procedure is to play the game. But the console’s random message does not expect a procedure. It expects to send a message.

We can use that same mark ! to construct the message to play the game. That message, !game, takes one parameter just like the game from which it is built. Sending such a message would result in a game getting played.

This message has no receiver, so it will not synchronize on any particular agent. Therefore in principle you could have many games operating at once. That would rapidly get confusing for a game like this, but sometimes such concurrent processing is just what the doctor ordered.

Asking for Input: Sequence 

The turn function is the main loop of the game. It’s job is to prompt for a guess and then interpret that guess as either too high, too low, or just right:

game(r) = turn(1) where
    goal = int(r*100)+1;
    to turn(score) is do
        console ! echo ["What is your guess? "];
        console ! read(!guess);
    end where
    ...
...

When it’s the player’s turn, we need to

ask our question, and
get an answer.

You can put a sequence of actions between do and end as shown here. That creates a single larger action. In this case, we have another echo and this time a read message.

Just like with function calls, you don’t need parenthesis around a list if it’s the only argument to a message. The square brackets of the list itself are enough to make yourself clear.
The read message means to wait for a line of input text (ending with the ‘enter’ or ‘return’ key) and forward that on in similar manner to how random sent along a number. Naturally, read sends you a string because the player can type anything at all, not just numbers.
Speaking of random: the goal sub-function provides our number scaled up to between 1 and 100 inclusive, as promised in the introduction. The multiplication and addition should be self-explanatory. To get rid of any remaining fractional part, we apply the int function as shown here.

Analyzing Input: Selection 

Evidently, guess must analyze the input. Before we worry about comparing the guess to the goal, there’s another important possibility. The player might enter something which is not a number:

to guess(g) is case val(g) as v of
    this -> consider(int(v.item));
    nope -> do
        console ! echo ["I didn't grok that number.", EOL];
        turn(score);
    end;
esac;

The val function turns a string into a number. Or rather, it tries to do that. Not all strings make sense as numbers. So actually, the val function returns something called maybe[number].

What’s maybe about? Quite simply, maybe is about things that might or might not have an answer. What’s the number corresponding to "California"? The answer to that question is nope.

Notice also the as v in the top of the case expression. That is the closest thing Sophie has to assignment. Within the boundaries of the case expression, v here means the value of val(g). And that’s how we’re able to use v.item on the second line and for sure have a number at this point.

Note

Personally, I consider it a bit wordy to have to say v.item instead of just v. At some point I plan to change it, but that will take some nontrivial work.

Finally, notice how in the nope -> section, the action ends with turn(score). This decision says we do not charge the player a guess for having fat-fingered her number.

Evaluating a Guess 

Now let’s see what happens if the input actually is a number:

to consider(g:number) is case
    when g > goal then go_again('Too high. Try a lower number.');
    when g < goal then go_again('Too low. Try a higher number.');
    else win;
esac;

This procedure consider is probably about what you’d expect. Unsurprising code is virtuous code. But we do have two more words to define:

to go_again(text) is do console ! echo [text, EOL]; turn(score+1); end;
to win is console ! echo ["You win after ", str(score), " guesses!", EOL];

Things of particular interest here:

In contrast to the not-a-number case, when the player guesses wrong, the subsequent turn operates with the next higher score.
go_again crams multiple actions on one line. This can get hard to read. Use your discretion.
The str function turns a number into a string. That helps here because in Sophie, the members of any given list are all the same type of thing as each other.
Notice the blank spaces after after and before guesses. If they were absent, then you might see something like You win after4guesses! in the output. That would be hard to read. The point is that Sophie takes you exactly at your word, and does not blithely insert spaces between bits of what you echo.

The Full Game 

Without further ado, here’s the completed game:

# Classic guess-a-number game to show working with the console

define:
intro = ["I have chosen a random number from 1 to 100.", EOL, EOL];
to game(r) is turn(1) where
    goal = int(r*100)+1;
    to turn(score) is do
        console ! echo ["What is your guess? "];
        console ! read(!guess);
    end where
        to guess(g) is case val(g) as v of
            this -> consider(int(v.item));
            nope -> do
                console ! echo ["I didn't grok that number.", EOL];
                turn(score);
            end;
        esac;
        to consider(g:number) is case
            when g > goal then go_again('Too high. Try a lower number.');
            when g < goal then go_again('Too low. Try a higher number.');
            else win;
        esac;
        to go_again(text) is do console ! echo [text, EOL]; turn(score+1); end;
        to win is console ! echo ["You win after ", str(score), " guesses!", EOL];
    end turn;
end game;

begin:
    console ! echo(intro);
    console ! random(!game);

Concluding Remarks 

In principle, you now have what it takes to make a wide variety of interactive programs. In practice, it will take practice!

Also in practice, Sophie will benefit from a greater variety of interactive capabilities. These new abilities will come in time.

Let’s Play a Text Game!

Guided Tour of the Game

Introducing the Console

Asking for Input: Sequence

Analyzing Input: Selection

Evaluating a Guess

The Full Game

Concluding Remarks