dtonhofer

dtonhofer

Functional Programming in Java, Second Edition: All the code for Chapter 8, "Using Tail-Call Optimization", in one class

All the code for Chapter 8, Using Tail-Call Optimization, in one class. But without the part where we later need to “fix the arithmetic overflow”, an ancillary problem that is solely due to the fact that the example recursive function we use is the factorial. Here we use a simple recursive sum instead.

package chapter8.tailcalls;

import org.junit.jupiter.api.Test;

import java.util.function.Function;
import java.util.stream.Stream;

public class Chapter8_TailCallOptimization {

    private final static int limit = 70000;

    // ---
    // The "very slick stack simulator" (VSSS) for optimizable (not necessarily recursive) tail calls
    // This is the code "recur/fpij/TailCall.java" on p.142
    // with a fix: the .get() on the Stream has been replaced by .orElseThrow()
    // If we only use get(), the linter tells us: "Optional.get() without "isPresent()" check".
    // ---

    @FunctionalInterface
    public interface TailCall<T> {

        TailCall<T> apply();

        default boolean isComplete() {
            return false;
        }

        default T result() {
            throw new Error("not implemented");
        }

        default T invoke() {
            return Stream.iterate(this, TailCall::apply)
                    .filter(TailCall::isComplete)
                    .findFirst()
                    .orElseThrow()
                    .result();
        }
    }

    // "recur/fpij/TailCalls.java" on p.143

    public static class TailCalls {

        // call() simply exists so that usage has a symmetric look
        // (...but I'm not sure this improves understanding)

        public static <T> TailCall<T> call(final TailCall<T> nextCall) {
            return nextCall;
        }

        public static <T> TailCall<T> done(final T value) {
            return new TailCall<T>() {
                @Override
                public boolean isComplete() {
                    return true;
                }

                @Override
                public T result() {
                    return value;
                }

                @Override
                public TailCall<T> apply() {
                    throw new Error("not implemented");
                }
            };
        }
    }

    // ---
    // Summing using a recursive call that is not a tail call (avoid if possible, though
    // it is not always possible)
    // Replaces "recur/fpij/Factorial.java" on page 140.
    // ---

    private static class SumRecursivelyNaively {

        public static long sum(final int number) {
            return (number == 1) ? 1 : (number + sum(number - 1));
        }
    }

    // ---
    // Summing using a recursive call that is a proper tail call.
    // This approach uses an accumulator while going "down/into" the recursion.
    // On the way "back up/outo of" the recursion there are only "returns".
    // This can be optimized so that only 1 stack frame is used. However, the Java compiler
    // (and/or the JVM?) does not do that fully (maybe because it needs to keep track of
    // stack frames for debugging?), so we still get stack overflow after some time.
    // ---

    private static class SumRecursivelyUsingTailCalls {

        public static long sum(final int number) {
            return sum_inner(1, number);
        }



        private static long sum_inner(final long accumulator, final int number) {
            if (number == 1) {
                return accumulator;
            } else {
                return sum_inner(accumulator + number, number - 1);
            }
        }
    }

    // ---
    // An application of VSSS - the very sly stack simulator
    // Replaces "recur/fpij/Factorial.java" on p.141
    // ---

    private static class SumRecursivelyWithVSSS {

        public static long sum(final int number) {
            return sum_inner(1, number).invoke();
        }

        // Note that this method is *never* called recursively!

        public static TailCall<Long> sum_inner(final long accumulator, final int number) {
            if (number == 1) {
                // Return the "TailCalls" instance that ends the stream
                return TailCalls.done(accumulator);
            } else {
                // "call()" does nothing, and we could just return the closure directly, but it looks nice
                return TailCalls.call(
                        // When called by Stream.iterate(), this closure is supposed to generate&return the
                        // "next TailCall instance" of the stream
                        () -> sum_inner(accumulator + number, number - 1)
                );
            }
        }
    }

    // === TESTING SUPPORT BEGINS ===

    private static boolean callSum(final boolean skip, final String name, int n, Function<Integer, Long> sum) {
        boolean skipNextTime = skip;
        if (!skip) {
            try {
                long res = sum.apply(n);
                // Properly, this test should be in the caller
                if (n == limit - 1) {
                    System.out.println("Reached the end: " + name + "(" + n + ") = " + res);
                }
            } catch (StackOverflowError err) {
                System.out.println("Stack overflow for " + name + "(" + n + ")");
                skipNextTime = true;
            }
        }
        return skipNextTime;
    }

    // Running the three approaches at summing recursively till stack overflow occurs!
    // Works best if one reduces the maximum stack size of the JVM,
    // options "-Xss1m" or "-XX:ThreadStackSize=1024" (the latter in KiB)
    // See https://docs.oracle.com/en/java/javase/17/docs/specs/man/java.html#advanced-runtime-options-for-java
    //
    // Example output:
    //
    // Stack overflow for SumRecursivelyNaively.sum(38919)
    // Stack overflow for SumRecursivelyUsingTailCalls.sum(58375)
    // Reached the end: SumRecursivelyWithVSSS.sum(69999) = 2449965000

    @Test
    public void runThem() {
        boolean skipNaiveVersion = false;
        boolean skipTailCallVersion = false;
        boolean skipVsssVersion = false;
        for (int n = 1; n < limit && !(skipTailCallVersion && skipNaiveVersion && skipVsssVersion); n += 1) {
            skipNaiveVersion = callSum(skipNaiveVersion, "SumRecursivelyNaively.sum", n, SumRecursivelyNaively::sum);
            skipTailCallVersion = callSum(skipTailCallVersion, "SumRecursivelyUsingTailCalls.sum", n, SumRecursivelyUsingTailCalls::sum);
            skipVsssVersion = callSum(skipVsssVersion, "SumRecursivelyWithVSSS.sum", n, SumRecursivelyWithVSSS::sum);
        }
    }

}

Where Next?

Popular Pragmatic Bookshelf topics Top

jimschubert
In Chapter 3, the source for index introduces Config on page 31, followed by more code including tests; Config isn’t introduced until pag...
New
mikecargal
Title: Hands-On Rust (Chap 8 (Adding a Heads Up Display) It looks like ​.with_simple_console_no_bg​(SCREEN_WIDTH*2, SCREEN_HEIGHT*2...
New
lirux
Hi Jamis, I think there’s an issue with a test on chapter 6. I own the ebook, version P1.0 Feb. 2019. This test doesn’t pass for me: ...
New
herminiotorres
Hi! I know not the intentions behind this narrative when called, on page XI: mount() |&gt; handle_event() |&gt; render() but the correc...
New
gilesdotcodes
In case this helps anyone, I’ve had issues setting up the rails source code. Here were the solutions: In Gemfile, change gem 'rails' t...
New
New
digitalbias
Title: Build a Weather Station with Elixir and Nerves: Problem connecting to Postgres with Grafana on (page 64) If you follow the defau...
New
Charles
In general, the book isn’t yet updated for Phoenix version 1.6. On page 18 of the book, the authors indicate that an auto generated of ro...
New
andreheijstek
After running /bin/setup, the first error was: The foreman' command exists in these Ruby versions: That was easy to fix: gem install fore...
New
SlowburnAZ
Getting an error when installing the dependencies at the start of this chapter: could not compile dependency :exla, "mix compile" failed...
New

Other popular topics Top

Exadra37
Please tell us what is your preferred monitor setup for programming(not gaming) and why you have chosen it. Does your monitor have eye p...
New
DevotionGeo
I know that -t flag is used along with -i flag for getting an interactive shell. But I cannot digest what the man page for docker run com...
New
Rainer
Not sure if following fits exactly this thread, or if we should have a hobby thread… For many years I’m designing and building model air...
New
AstonJ
In case anyone else is wondering why Ruby 3 doesn’t show when you do asdf list-all ruby :man_facepalming: do this first: asdf plugin-upd...
New
Exadra37
Oh just spent so much time on this to discover now that RancherOS is in end of life but Rancher is refusing to mark the Github repo as su...
New
PragmaticBookshelf
Build highly interactive applications without ever leaving Elixir, the way the experts do. Let LiveView take care of performance, scalabi...
New
foxtrottwist
A few weeks ago I started using Warp a terminal written in rust. Though in it’s current state of development there are a few caveats (tab...
New
mafinar
This is going to be a long an frequently posted thread. While talking to a friend of mine who has taken data structure and algorithm cou...
New
PragmaticBookshelf
Build efficient applications that exploit the unique benefits of a pure functional language, learning from an engineer who uses Haskell t...
New
hilfordjames
There appears to have been an update that has changed the terminology for what has previously been known as the Taskbar Overflow - this h...
New

Latest in Functional Programming in Java, Second Edition

Functional Programming in Java, Second Edition Portal

Sub Categories: