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    // with a fix: the .get() on the Stream has been replaced by .orElseThrow()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
jeffmcompsci
Title: Design and Build Great Web APIs - typo “https://company-atk.herokuapp.com/2258ie4t68jv” (page 19, third bullet in URL list) Typo:...
New
ianwillie
Hello Brian, I have some problems with running the code in your book. I like the style of the book very much and I have learnt a lot as...
New
JohnS
I can’t setup the Rails source code. This happens in a working directory containing multiple (postgres) Rails apps. With: ruby-3.0.0 s...
New
New
Chrichton
Dear Sophie. I tried to do the “Authorization” exercise and have two questions: When trying to plug in an email-service, I found the ...
New
dsmith42
Hey there, I’m enjoying this book and have learned a few things alredayd. However, in Chapter 4 I believe we are meant to see the “&gt;...
New
s2k
Hi all, currently I wonder how the Tailwind colours work (or don’t work). For example, in app/views/layouts/application.html.erb I have...
New
Henrai
Hi, I’m working on the Chapter 8 of the book. After I add add the point_offset, I’m still able to see acne: In the image above, I re...
New
jwandekoken
Book: Programming Phoenix LiveView, page 142 (157/378), file lib/pento_web/live/product_live/form_component.ex, in the function below: d...
New

Other popular topics Top

Devtalk
Hello Devtalk World! Please let us know a little about who you are and where you’re from :nerd_face:
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
AstonJ
I ended up cancelling my Moonlander order as I think it’s just going to be a bit too bulky for me. I think the Planck and the Preonic (o...
New
Exadra37
I am asking for any distro that only has the bare-bones to be able to get a shell in the server and then just install the packages as we ...
New
Margaret
Hello everyone! This thread is to tell you about what authors from The Pragmatic Bookshelf are writing on Medium.
1147 29994 760
New
PragmaticBookshelf
Author Spotlight: VM Brasseur @vmbrasseur We have a treat for you today! We turn the spotlight onto Open Source as we sit down with V...
New
PragmaticBookshelf
Author Spotlight: Peter Ullrich @PJUllrich Data is at the core of every business, but it is useless if nobody can access and analyze ...
New
First poster: bot
zig/http.zig at 7cf2cbb33ef34c1d211135f56d30fe23b6cacd42 · ziglang/zig. General-purpose programming language and toolchain for maintaini...
New
New
PragmaticBookshelf
Use advanced functional programming principles, practical Domain-Driven Design techniques, and production-ready Elixir code to build scal...
New

Latest in Functional Programming in Java, Second Edition

Functional Programming in Java, Second Edition Portal

Sub Categories: