objects.md

Objects in Cell

It is possible to use the fact that functions carry their own values around with them (in so-called "closures") to construct trees of functions that return functions that work quite like objects from languages like C++ or Java.

For example, we can write a function open_account that we can use like this:

acc1 = open_account();
acc2 = open_account();

acc1("deposit")(10.25);
acc1("withdraw")(0.75);

acc2("deposit")(5.42);

print("acc1 balance=");
print(acc1("balance"));
print("");

print("acc2 balance=");
print(acc2("balance"));

and it will store the value of the account balance inside the acc1 and acc2 functions, so the output looks like this:

acc1 balance=
9.5

acc2 balance=
5.42

The magic to do this looks quite complicated, but once you understand the syntax, and the fact that we can pass functions around as values and call them later, it is reasonably simple:

open_account = {
    {
        bal = 0.0;
        {:(method_name)
            if(equals("deposit", method_name),
            {{:(amount)
                set("bal", bal + amount);
            }},
            {
                if(equals("withdraw", method_name),
                {{:(amount)
                    set("bal", bal - amount);
                }},
                {
                    if(equals("balance", method_name),
                        {bal;},
                        {print("Unknown method!");}
                    );
                });
            })
        };
    }();
};

bank_account is a function that creates a function containing a value called bal. It immediately calls that function and returns the answer.

The answer is itself a function (that takes an argument method_name). This function has a reference to the bal value, and can modify it, which is what happens when we call the functions returned when we supply a method name like "deposit".

If you can get your head around this, you understand closures, and, for good measure, you can understand that "objects" and "closures" are in some sense equivalent.