Mike Schaeffer's Blog

June 1, 2014

In the last installation of this series, we started using Java iterators to decompose the monolithic REPL (read-eval-print-loop) into modular compoments. This let us start decoupling the semantics of the REPL from the mechanisms that it uses to implement read, evaluate, and print. Unfortunately, the last version of rpncalc only modularized the command prompt itself: the ‘R’ in REPL. The evaluator and printer are still tightly bound to the main command loop. In this post I’ll use another kind of custom iterator to further decompose the main loop, breaking out the evaluator and leaving only the printer itself in the loop.

Going back to the original command loop from the stateobject version of rpncalc, the loop traverses two sequences of values in parallel.

state = new State();
while(running) {
    System.out.print("> ");
    String cmdLine = System.console().readLine();
    if (cmdLine == null)
    Command cmd = parseCommandString(cmdLine);
    State initialState = state;
    state = cmd.execute(state);
    lastState = initialState;

Neither of the two sequences this loop traverses are made explicit within the code, both are implicit in the sequence of values taken on by variables managed by the loop. The first sequence the loop traverses is the sequence of commands that the user enters at the console. This sequence manifests in the code as the sequence of values taken on by cmd through each iteration of the loop. The second sequence is similarly implicit: the sequence of states that state takes on through each iteration. Last post, when we added the CommandStateIterator, the key to that refactoring was that we took one of the implicit loop sequences and made it explicitly a sequence witin the code. Having an explicit structure within the code for the sequence of commands provided a place for the loop to invoke the reader that wasn’t in the body of the loop itself.

// Set initial state
State state = new State();
// Loop over all input commands
for(Command cmd : new ConsoleCommandStream()) {
    // Evaluate the command and produce the next state.
    state = cmd.execute(state);
    if (state == null)
    // Print the current state

Looking forward, the next refactoring for the REPL is to make explicit the implicit sequence of result states in the same way we transformed the sequence of input commands. This will let us take our current loop, which loops over input commands, and turn it into a loop over states. The call to evaluate will be pushed into an iterator in the same way that we pushed the reader into an iterator in the last post. This leaves us with a main loop that simply loops over states and prints them out:

for(State state : new CommandStateReduction(new State(), new CommandStream()))

This code is short, but it’s dense: most of the logic is now outside the text of the loop, and within CommandStateReduction and CommandStream. The command stream is the same stream of commands used in the last version of rpncalc. The ‘command state reduction’ stream is the stream that invokes the commands to produce the sequence of states. I’ve given it the name ‘reduction’ because of the way it relates to reduce in funcional programming. To see why, look back at abstract class we’re using to model a command:

abstract class Command
    abstract State execute(State in);

Given a state, applying a command results in a new state, returned from the execute method. A second command can then be applied to the new state giving an even newer state, and there’s no inherent bound on the number of times this can happen. In this way, a sequence of commands applied to an initial state produces a corresponding sequence of output states. The sequence of output states is the sequence of command results that the REPL needs to print for each entered command. Each time a command is executed, the result state needs to be printed and stored for the next command.

The relationship between this and reduction comes from the fact that reduction combines the elements of a sequence into an aggregate result. Reducing + over a list of numbers gives the sum of those numbers. Applying a sequence of commands combines the effects of those commands into a single final result. The initial value that gets passed into the reduction is the initial state. The sequence over which the reduction is applied is the sequence of commands from the console. The combining operator is command application. The most significant difference between this and traditional reduce is that we need more than just the final result, we also need each intermediate result. (This makes our reduction more like Haskell’s scan operator.)

Practically speaking CommandStateReduction is implemented as an Iterable. The constructor takes two arguments: the initial state before any commands are executed, and a sequence of commands to be executed.

class CommandStateReduction implements Iterable<State>
    CommandStateReduction(State initialState, Iterable<Command> cmds)

Note that the only property that the command state reduction requires of the sequence of commands is that it be Iterable and produce Commands. There’s nothing about the signature of the reduction iterator that requires the sequence of commands to be concrete and known ahead of time. This is useful, because our current command source is CommandStream, which lazily produces commands. Both the command stream and the command state reduction are lazily evaluated, and only operate when a caller makes a request. The command stream doesn’t read until the evaluator requests a command, the evaluator doesn’t evaluate until the printer makes a request for a value. Despite the fact that it’s hidden behind a pipeline of iterable object, the REPL still operates as it did before: first it reads, then it evaluates, then it prints, and then it loops back.

As with the command state iterator, most of the logic in command state reduction is handled with a single advanceIfNecessary method. The instance variable state is used to maintain the state between command applications:

private State state = initialState;
private boolean needsAdvance = true;
Iterator<Command> cmdIterator = cmds.iterator();
private void advanceIfNecessary()
    if (!needsAdvance)
    needsAdvance = false;
    if (cmdIterator.hasNext())
        state = cmdIterator.next().execute(state);
        state = null;

Looking back at the code, the Java version of the RPN calculator has come a long way. From heavily procedural origins, we’ve added command pattern based undo logic, switched over to a functional style of implementation, and redesigned our main loop so that it operates via lazy operations on streams of values. We’ve taken a big step in the direction of functional programming. The downside has been in the size of the code. The functional style has many benefits, but it’s not a style that’s idiomatic to Java (at least before Java 8). Our code side has more than doubled from 150 to 320 LOC. In the next few entries of this series, we’ll continue evolving rpncalc, but switch over to Clojure. This will let us continue this line of development without getting buried in the syntax of Java.