RPN Calc Part 5 – Eliminating the Globals

Throughout the last four parts of this series, the common theme has been that the state of the calculator program has been managed globally. This requires the main command loop to directly update global data after each command, to prepare the state for the next command. While this works, it would be nice to remove the need for the global update. This post talks about how that’s done in the functional version of rpncalc.

The last installment in the series updated the calculator by adding a Java class to model State. This allows the entire state of the calculator to be managed as a first-class entity within the program. That’s evident from the fact that the calcualtor now models state through a single global reference:

public class RpnCalc extends Calculator
    // ...

    private State state = null;

Before I continue, I should clarify by saying that state isn’t truly a global variable. Obviously state is an instance variable within a single instance of RpnCalc. However, because of the fact that the calculator program only runs with a
single instance of RpnCalc, any instance variables within that class are effectively global within the program, and suffer many of the same problems associated with true globals. This parallel is also true for instance variables within singletons managed by Spring. They’re instance variables, but they act like globals and should be viewed with the same kind of suspicion.

To remove the global state, we’re going to take advantage of the fact that the last update to rpncalc changed the signature for a command, so that it returns a new copy of the state:

abstract class Command
        State execute(State in)

This implies that the main command loop has direct access to the updated state, and can manage it as a local variable:

public void main()
        throws Exception
        State state = new State();

        while(state != null) {
            // ...read and gather command... 
            state = cmd.execute(state);

This eliminates the global variable, moving the same data to a single variable contained within a function definition. This hugely reduces the variable’s footprint within the code. The global variable can be read and updated by 200 lines of code, compared to 20 lines of code for the local variable. For a developer, this makes the management of state more evident within the code, and makes it easier to write and maintain correct code.

Unfortunately, there’s a problem. For most of the commands, storing state in a local variable works well: a command like addition only needs access to the current state to produce the next state. Where it breaks down is in the undo command. In contrast to every other command, undo doesn’t need access to the current state, it needs access to the previous state. Now that we’ve moved the state management entirely within the command loop function, there’s no way for the undo command to get access to the history of states. Given that undo is a requirement, we need to find a solution.

One approach would be to extend the command loop to maintain a full history of states, and then broaden the command function signature to accept both the current and previous states. Each command would then be a function on a history of states. This approach has some appeal,
but a simpler way to achieve the same result is to acknowledge the fact that each state is a function of the previous state, and store a back reference within each state:

private class State
    State prev;
    // ...

        prev = null;
        // ...

    State(State original)
        prev = original;
        // ...

This makes undo easy:

cmds.put("undo", new Command() {
        public State execute(State in) {
            // skip past the copy made for 'undo', and get to the previous.
            return in.prev.prev;

This design is also closely related to how git stores commits: each commit stores a back link to the previous commit in the history. It does have
potentially unbounded memory usage, but our state in rpncalc is small enough in size and our memory is large enough that this shouldn’t present a problem. If it ever did present a memory consumption problem, adding a bound on the length number of retained states is as
simple as walking down the previous links and nulling the previous link of the last history element to retain.

Next up, we’ll start taking a bit more advantage of the newly functional nature of rpncalc, and start using functional techniques to decompose the main command loop into component modules.

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