Step has two kinds of variables: local and global.  We've mostly discussed local variables, whose names start with a "?".  They're “local to” the particular method they appear in, meaning that if two different methods have variables named ?x, those are different variables in the same way that two people named “Jennifer” are still different people.  Local variables start without a value, acquire a value through matching, and then never change that value.

Global variables, whose names start with capital letters, are shared by all methods; hence the term "global."  We'll talk about these now.

Tasks are stored in global variables

Names like Siblings and Parent are global variables.  In this case, the values they refer to are tasks, and so we tend to just say “Parent is a task”, or “Parent is a predicate” rather than the pedantically correct "Parent is a global variable whose value is a task."  Technically, when we define a method, such as:

Siblings ?a ?b: [Parent ?a ?parent] [Parent ?b ?parent] [Different ?a ?b]

we're telling the system to find the task that Siblings refers to and then add a method to that task.  The first time you do that, there is no variable called Siblings.  So it makes one, along with a new task with just that one method, and sets Siblings to refer to that new task.

When you call [Siblings bart ?who], it looks up the value of Siblings, and calls that with bart and ?who as parameters.

Global variables don't have to be tasks

So global variables aren't wholly unfamiliar to you.  But it turns out you can give them values that aren't tasks; you can set a global variable to any value you like.  For example, in tracking discourse context, we'll show how you can use globals to keep track of what terms different pronouns refer to.

Making and updating global variables

To store a value into a global variable, you say:

[set Variable = value]

Note that set is lower-case.

After you run that, the variable will have that new value.¹  To give an initial value to a variable, just define a method for the task named initially, as in:

initially: [set X = 7]

We'll show a more substantive example shortly.  But for the moment, here's an example just to show you that setting values to variables works.  Note that Mention, just prints its parameter out.  Give it a try:

# Try: [Test]
initially: [set X = 0]

Test: [Mention X] [set X = X + 1] [Mention X] [set X = X + 1] [Mention X]

It should print:

0 1 2

Because first it prints X, then adds 1 to it, then prints it again, then adds 1 again, then prints it one last time.

Omake: printing global variables

You can print a local variable like ?x just by putting it in the list of things to output, as in our example:

Greet ?x: Hello, ?x

When Step sees ?x there, it treats it as a call to mention: [Mention ?x].  It can do that because it's pretty unambiguous what the author's intention is.²  However, if we wanted to print the value of the global variable X, we can't just say:

Greet: Hello, X

because Step wouldn't have any way of knowing that X was a global variable to print but Hello was just text to print.  That's why we had to say [Mention X] above.  But it's nice not to have to say that, so Step allows you to print global variables by putting a ^ before them.  That means we can write the code above like this:

# Try: [Test]
initially: [set X = 0]

Test: ^X [set X = X + 1] ^X [set X = X + 1] ^X

Notes