Kerflyn's Blog

EDIT 2012-02-23: simplified getFrom method + corrected the returned type from getFromCountry method. · Bird Of a Feather

import scala.util.parser.combinator.RegexParsers

object ReallySimpleParser1 extends RegexParsers {
  def hello = "hello"
}

import org.specs2.mutable._
import org.specs2.matcher.ParserMatchers

class ReallySimpleParser1Test extends Specification with ParserMatchers {
  val parsers = ReallySimpleParser1

  import ReallySimpleParser1.hello

  "hello" should {
    "succeed to recognize 'hello'" {
      hello must succeedOn("hello").withResult("hello")
    }
    "fail to recognize 'world'" {
      hello must failOn("world")
    }
}

object NumberParser extends RegexParsers {
  def number: Parser[Int] = """-?\d""".r ^^ { _.toInt }
}

number must succeedOn("42").withResult(42)
number must succeedOn("-24").withResult(-24)
number must failOn("hello")

object ReallySimpleParser2 extends RegexParsers {
  def sentence = hello ~ world
  def hello = "hello"
  def world = "world"
}

sealed abstract class ExpressionSymbol
case class Const(value: Int) extends ExpressionSymbol
case object X extends ExpressionSymbol
case class Add(x: ExpressionSymbol, y: ExpressionSymbol) extends ExpressionSymbol
case class Mul(x: ExpressionSymbol, y: ExpressionSymbol) extends ExpressionSymbol

object ExpressionParser extends RegexParsers {
  def expression = operation1
  def operation1: Parser[ExpressionSymbol] = operation2 ~ rep("+" ~ operation2) ^^ {
    case op ~ list => list.foldLeft(op) {
      case (x, "+" ~ y) => Add(x, y)
    }
  }
  def operation2: Parser[ExpressionSymbol] = operand ~ rep("*" ~ operand) ^^ {
    case op ~ list => list.foldLeft(op) {
      case (x, "*" ~ y) => Mul(x, y)
    }
  }
  def operand: Parser[ExpressionSymbol] = (constant | variable)
  def variable: Parser[ExpressionSymbol] = "x" ^^ { _ => X }
  def constant: Parser[ExpressionSymbol] = """-?\d+""".r ^^ { s => Const(s.toInt) }

  def apply(input: String): Option[ExpressionSymbol] = parseAll(expression, input) match {
    case Success(result, _) => Some(result)
    case NoSuccess(_, _) => None
  }
}

val myHouse = new House with Garage with Garden

class ForkingUDPServer(ForkingMixIn, UDPServer): pass
class ForkingTCPServer(ForkingMixIn, TCPServer): pass

class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass
class ThreadingTCPServer(ThreadingMixIn, TCPServer): pass

public interface Collection<T> extends Iterable<T> {

    <R> Collection<R> filter(Predicate<T> p)
        default { return Collections.<T>filter(this, p); }

}

public interface SwitchableMixin {

    boolean isActivated() default { return Switchables.isActivated(this); }

    void setActivated(boolean activated) default { Switchables.setActivated(this, activated); }

}

public final class Switchables {

    private static final Map<SwitchableMixin, SwitchableDeviceState> SWITCH_STATES = new HashMap<>();

    public static boolean isActivated(SwitchableMixin device) {
        SwitchableDeviceState state = SWITCH_STATES.get(device);
        return state != null && state.activated;
    }

    public static void setActivated(SwitchableMixin device, boolean activated) {
        SwitchableDeviceState state = SWITCH_STATES.get(device);
        if (state == null) {
            state = new SwitchableDeviceState();
            SWITCH_STATES.put(device, state);
        }
        state.activated = activated;
    }

    private static class SwitchableDeviceState {
        private boolean activated;
    }

}

private static class Device {}

private static class DeviceA extends Device implements SwitchableMixin {}

private static class DeviceB extends Device implements SwitchableMixin {}

DeviceA a = new DeviceA();
DeviceB b = new DeviceB();

a.setActivated(true);

assertThat(a.isActivated()).isTrue();
assertThat(b.isActivated()).isFalse();

interface FakeBrokenMixin {
    static Map<FakeBrokenMixin, String> backingMap
        = Collections.synchronizedMap(new WeakHashMap<FakeBrokenMixin, String>());

    String getName() default { return backingMap.get(this); }
    void setName(String name) default { backingMap.put(this, name); }
}

interface X extends Runnable, FakeBrokenMixin {}

X makeX() { return () -> { System.out.println("X"); }; }

    X x1 = makeX();
    X x2 = makeX();
    x1.setName("x1");
    x2.setName("x2");

    System.out.println(x1.getName());
    System.out.println(x2.getName());

x1
x2

MyTest$1@30ae8764
MyTest$1@123acf34

x2
x2

MyTest$$Lambda$1@5506d4ea
MyTest$$Lambda$1@5506d4ea

private static void lambda$0() {
    System.out.println("X");
}

interface Switchable {    boolean isActive();
    void setActive(boolean active);
}

public interface SwitchableView extends Switchable {
    Switchable getSwitchable();


    boolean isActive() default { return getSwitchable().isActive(); }
    void setActive(boolean active) default { getSwitchable().setActive(active); }
}

public class SwitchableImpl implements Switchable {


    private boolean active;


    @Override
    public boolean isActive() {
        return active;
    }


    @Override
    public void setActive(boolean active) {
        this.active = active;
    }
}

public class Device {}


public class DeviceA extends Device implements SwitchableView {
    private Switchable switchable = new SwitchableImpl();


    @Override
    public Switchable getSwitchable() {
        return switchable;
    }
}


public class DeviceB extends Device implements SwitchableView {
    private Switchable switchable = new SwitchableImpl();


    @Override
    public Switchable getSwitchable() {
        return switchable;
    }
}

DeviceA a = new DeviceA();DeviceB b = new DeviceB();

a.setActive(true);

assertThat(a.isActive()).isTrue();
assertThat(b.isActive()).isFalse();

public class PatternMatching {
    private Pattern[] patterns;

    public PatternMatching(Pattern... patterns) { this.patterns = patterns; }

    public Object matchFor(Object value) {
        for (Pattern pattern : patterns)
            if (pattern.matches(value))
                return pattern.apply(value);

        throw new IllegalArgumentException("cannot match " + value);
    }
}

public interface Pattern {
    boolean matches(Object value);
    Object apply(Object value);
}

public class ClassPattern<T> implements Pattern {

    private Class<T> clazz;

    private Function<T, Object> function;

    public ClassPattern(Class<T> clazz, Function<T, Object> function) {
        this.clazz = clazz;
        this.function = function;
    }

    public boolean matches(Object value) {
        return clazz.isInstance(value);
    }

    public Object apply(Object value) {
        return function.apply((T) value);
    }

    public static <T> Pattern inCaseOf(Class<T> clazz, Function<T, Object> function) {
        return new ClassPattern<T>(clazz, function);
    }
}

PatternMatching pm = new PatternMatching(
    inCaseOf(Integer.class, x -> "Integer: " + x),
    inCaseOf(Double.class, x -> "Double: " + x)
);

System.out.println(pm.matchFor(42));
System.out.println(pm.matchFor(1.42));

Integer: 42
Double: 1.42

public class StringPattern implements Pattern {
    private final String pattern;
    private final Function<String, Object> function;

    public StringPattern(String pattern, Function<String, Object> function) {
        this.pattern = pattern;
        this.function = function;
    }

    @Override
    public boolean matches(Object value) { return pattern.equals(value); }

    @Override
    public Object apply(Object value) { return function.apply((String) value); }

    public static Pattern inCaseOf(String pattern, Function<String, Object> function) {
        return new StringPattern(pattern, function);
    }
}

public class IntegerPattern implements Pattern {
    private final Integer pattern;
    private final Function<Integer, Object> function;

    public IntegerPattern(int pattern, Function<Integer, Object> function) {
        this.pattern = pattern;
        this.function = function;
    }

    @Override
    public boolean matches(Object value) { return pattern.equals(value); }

    @Override
    public Object apply(Object value) { return function.apply((Integer) value); }

    public static Pattern inCaseOf(int pattern, Function<Integer, Object> function) {
        return new IntegerPattern(pattern, function);
    }
}

public class OtherwisePattern implements Pattern {
    private final Function<Object, Object> function;

    public OtherwisePattern(Function<Object, Object> function) {
        this.function = function;
    }

    @Override
    public boolean matches(Object value) { return true; }

    @Override
    public Object apply(Object value) { return function.apply(value); }

    public static Pattern otherwise(Function<Object, Object> function) {
        return new OtherwisePattern(function);
    }
}

PatternMatching pm = new PatternMatching(
    inCaseOf(Integer.class, x -> "Integer: " + x),
    inCaseOf(      "world", x -> "Hello " + x),
    inCaseOf(           42, _ -> "forty-two"),
    otherwise(              x -> "got this object: " + x.toString())
);

public static int fact(int n) {
    return ((Integer) new PatternMatching(
            inCaseOf(0, _ -> 1),
            otherwise(  _ -> n * fact(n - 1))
    ).matchFor(n));
}

public static int fact_old(final int n) {
    return ((Integer) new PatternMatching(
            inCaseOf(0, new Function<Integer, Object>() {
                @Override
                public Object apply(Integer _) {
                    return 1;
                }
            }),
            otherwise(new Function<Object, Object>() {
                @Override
                public Object apply(Object _) {
                    return n * fact(n - 1);
                }
            })
    ).matchFor(n));
}

public interface Function<T, R> {
  R apply(T input);
}

public abstract class Option<T> {

  // opérations de base

  public abstract T get();

  public abstract boolean isPresent();

  // opérations monadiques

  public static <T> Option<T> wrap(T element) { return new Some<T>(element); }

  public static <T> Option<T> fail() { return new None<T>(); }

  public abstract <U> Option<U> andThen(Function<T, Option<U>> function);

}

public class None<T> extends Option<T> {

  public T get() { throw new IllegalStateException(); }

  public boolean isPresent() { return false; }

  public <U> Option<U> andThen(Function<T, Option<U>> function) {
    return new None<U>();
  }

}

public class Some<T> extends Option<T> {

  private T element;

  private Some(T element) { this.element = element; }

  public T get() { return element; }

  public boolean isPresent() { return true; }

  public <U> Option<U> andThen(Function<T, Option<U>> function) {
    return function.apply(this.element);
  }

}

public static Function<
  Map<String, Map<String, Map<String, Map<String, Product>>>>,
  Option<Map<String, Map<String, Map<String, Product>>>>
>
accessCountry(String country) {
    return getFromKey(country);
}

public static Function<
  Map<String, Map<String, Map<String, Product>>>,
  Option<Map<String, Map<String, Product>>>
>
accessCity(String city) {
    return getFromKey(city);
}

public static Function<
  Map<String, Map<String, Map<String, Product>>>,
  Option<Map<String, Map<String, Product>>>
>
accessCity(String city) {
    return getFromKey(city);
}

public static Function<
  Map<String, Map<String, Product>>,
  Option<Map<String, Product>>
>
accessSupplier(String supplier) {
    return getFromKey(supplier);
}

public static Function<
  Map<String, Product>,
  Option<Product>
>
accessProduct(String productCode) {
    return getFromKey(productCode);
}

public static <K, V> Function<Map<K, V>, Option<V>> getFromKey(final K key) {
    return new Function<Map<K, V>, Option<V>>() {
        @Override
        public Option<V> apply(Map<K, V> map) {
            V value = map.get(key);
            if (value == null) {
                return Option.none();
            }
            return Option.wrap(value);
        }
    };
}

public Option<Product> getProductFrom(
  Map<String, Map<String, Map<String, Map<String, Product>>>> productsByCountry,
  String country, String city, String supplier, String code) {

  return wrap(productsByCountry)
    .andThen(accessCountry(country))
    .andThen(accessCity(city))
    .andThen(accessSupplier(supplier))
    .andThen(accessProduct(code));
}

public Option<Product> getProductFrom(
  Map<String, Map<String, Map<String, Map<String, Product>>>> productsByCountry,
  String country, String city, String supplier, String code) {

  return getFrom(productsByCountry, country).andThen(
         cities -> getFrom(cities, city).andThen(
         suppliers -> getFrom(suppliers, supplier).andThen(
         products -> getFrom(products, code))));
}

public <K, V> Option<V> getFrom(Map<K, V> map, K key) {
  V value = map.get(key);
  if (value == null) {
    return Option.none();
  }
  return Option.wrap(value);
}

Integer value = map.get("a");
if (value == null) {
    value = DEFAULT;
}
process(value);

Integer value = map.get("a");
process(value == null ? DEFAULT : value);

public static <K, V> Optional<V> getFrom(Map<K, V> map, K key) {
    Optional.fromNullable(maps.get(key))
}

process(getFrom(map, "a").or(DEFAULT));

public Product getProductFrom(
  Map<String, Map<String, Map<String, Map<String, Product>>>> productsByCountry,
  String country, String city, String supplier, String code) {

    Map<String, Map<String, Map<String, Product>>> productsByCity = productsByCountry.get(country);
    if (productsByCity != null) {
        Map<String, Map<String, Product>> productsBySupplier = productsByCity.get(city);
        if (productsBySupplier != null) {
            Map<String, Product> productsByCode = productsBySupplier.get(supplier);
            if (productsByCode != null) {
                return productByCode.get(code);
            }
        }
    }
    return null;
}

public Optional<Product> getProductFrom(
  Map<String, Map<String, Map<String, Map<String, Product>>>> productsByCountry,
  String country, String city, String supplier, String code) {

    Optional<Map<String, Map<String, Map<String, Product>>>> productsByCity
        = getFrom(productsByCountry, country);
    if (productsByCity.isPresent()) {
        Optional<Map<String, Map<String, Product>>> productsBySupplier
            = getFrom(productsByCity.get(), city);
        if (productsBySupplier.isPresent()) {
            Optional<Map<String, Product>> productsByCode
                = getFrom(productsBySupplier.get(), supplier);
            if (productsByCode.isPresent()) {
                return getFrom(productByCode.get(), code);
            }
        }
    }
    return Optional.absent();
}

public class OptionalMonad {
    public static <T> Optional<T> unit(T value) {
        return Optional.of(value);
    }

    public static <T, U> Optional<U> bind(
            Optional<T> value,
            Function<T, Optional<U>> function) {
        if (value.isPresent()) return function.apply(value.get());
        else return Optional.absent();
    }
}

public static <K, V> Function<Map<K, V>, Optional<V>> getFromKey(final K key) {
    return new Function<Map<K, V>, Optional<V>>() {
        @Override
        public Optional<V> apply(Map<K, V> map) {
            return getFrom(map, key);
        }
    };
}

public Optional<Product> getProductFrom(
  Map<String, Map<String, Map<String, Map<String, Product>>>> productsByCountry,
  String country, String city, String supplier, String code) {

    Optional<Map<String, Map<String, Map<String, Product>>>> productsByCity
        = bind(unit(productsByCountry), Maps2.<String, Map<String, Map<String, Map<String, Product>>>>getFromKey(country));
    Optional<Map<String, Map<String, Product>>> productsBySupplier
        = bind(productsByCity, Maps2.<String, Map<String, Map<String, Product>>>getFromKey(city));
    Optional<Map<String, Product>> productsByCode
        = bind(productsBySupplier, Maps2.<String, Map<String, Product>>getFromKey(supplier));
    Optional<Product> product
        = bind(productsByCode, Maps2.<String, Product>getFromKey(code));

    return product;
}

public Optional<Product> getProductFrom(
  Map<String, Map<String, Map<String, Map<String, Product>>>> productsByCountry,
  String country, String city, String supplier, String code) {

  return bind(bind(bind(bind(unit(productsByCountry),
    Maps2.<String, Map<String, Map<String, Map<String, Product>>>>getFromKey(country)),
    Maps2.<String, Map<String, Map<String, Product>>>getFromKey(city)),
    Maps2.<String, Map<String, Product>>getFromKey(supplier)),
    Maps2.<String, Product>getFromKey(code));
}

public
Function<
  Map<String, Map<String, Map<String, Map<String, Product>>>>,
  Optional<Map<String, Map<String, Map<String, Product>>>>
>
getFromCountry(String country) {
  // type inference system knows what to do here
  return Maps2.getFromKey(country);
}

// declaration of the other methods here
// ...

public Optional<Product> getProductFrom(
  Map<String, Map<String, Map<String, Map<String, Product>>>> productsByCountry,
  String country, String city, String supplier, String code) {

  return bind(
    bind(
      bind(
        bind(
          unit(productsByCountry),
          getFromCountry(country)),
          getFromCity(city)),
          getFromSupplier(supplier)),
          getFromCode(code));
}

def getProductFrom(products: Map[String, Map[String, Map[String, Map[String, Product]]]],
                   country: String, city: String, supplier: String, code: String): Option[Product] = {
  for (
    cities <- products.get(country);
    suppliers <- cities.get(city);
    codes <- vendors.get(supplier);
    product <- codes.get(code)
  ) yield product
}

// get a product or null
product = products?.get(country)?.get(city)?.get(supplier)?.get(code)

public static int factorial(int n) {
    int result = 1;
    for (int i = 1; i <= n; i++) {
        result *= i;
    }
    return result;
}

public static int factorial(int n) {
    if (n == 0) return 1;
    else return n * factorial(n - 1);
}

public interface Function<T, R> {
    R apply(T value);
}

Function<Integer, Integer> factorial = new Function<Integer, Integer>() {
    @Override public Integer apply(Integer n) {
        if (n == 0) return 1;
        else return n * apply(n - 1);
    }
};

Integer result = factorial.apply(5);

Integer result = new Function<Integer, Integer>() {
    @Override public Integer apply(Integer n) {
        if (n == 0) return 1;
        else return n * apply(n - 1);
    }
}.apply(5);

private static int fact(int n, int k) {
    if (n == 0) return k;
    else return fact(n - 1, n * k);
}

public static int factorial(int n) {
    return fact(n, 1);
}

-> factorial(5)  // first call
  -> factorial(4)
    -> factorial(3)
      -> factorial(2)
        -> factorial(1)
          -> factorial(0)  // here, we've reached the stop condition
          <- 1
        <- 1 = 1 * 1  // all remaining multiplications are executed
      <- 2 = 2 * 1
    <- 6 = 3 * 2
  <- 24 = 4 * 6
<- 120 = 5 * 24  // we have computed the result in the last return

-> fact(5, 1)  // first call
  -> fact(4, 5)  // result is built through the successive calls
    -> fact(3, 20)
      -> fact(2, 60)
        -> fact(1, 120)
          -> fact(0, 120)  // here, we've reached the stop condition
          <- 120  // the final result is obtained directly in the last recursive call
        <- 120
      <- 120
    <- 120
  <- 120
<- 120

public static int factorial(int n, int k) {
    while (!(n == 0)) {
        k = n * k;
        n = n - 1;
    }
    return k;
}

public static fib(int n) {
    if (n <= 1) return 1;
    else return fib(n-1) + fib(n-2);
}

public class FactorialFunction implements Function<Integer, Function<Integer, Integer>> {
    @Override
    Function<Integer, Integer> apply(final Integer n) {
        return new Function<Integer, Integer>() {
            @Override
            public Integer apply(Integer k) {
                if (n == 0) return k;
                else return FactorialFunction.this.apply(n - 1).apply(k * n);
            }
        };
    }
}

FactorialFunction fact = new FactorialFunction();
Integer result = fact.apply(5).apply(1); // factorial of 5

fact n k = if n == 0
  then k
  else fact (n-1) (n*k)

case class Pair[T1, T2](first: T1, second: T2)

// usage: val myPair = Pair(1, 2)

public class Pair<T1, T2> {
    private T1 first;
    private T2 second;

    public Pair(T1 first, T2 second) {
        this.first = first;
        this.second = second;
    }

    public static <T1, T2> Pair<T1, T2> Pair(T1 first, T2 second) {
        return new Pair(first, second);
    }
}

Pair<Integer, Pair<String, Pair<Boolean, Object>>> tree
  = Pair(1,
      Pair("hello",
        Pair(true, new Object())));

Brown Bag Lunch (BBL)

BBL host

Become a Bagger

My Sessions (as examples)

BBL Outside France

A really simple parser

Parse numbers

Yet another simple example to parse a sequence

Parse an arithmetic expression and evaluate it

Changeset

What a mixin is?

What are virtual extension methods?

A naive emulation of mixin in Java 8

“And Now for Something Completely Different”

Solution to the puzzle

Fix? Workaround?

Another approach

Conclusion

Main implementation

Pattern

A first Pattern example: pattern for types

Usual patterns

Factorial based on PatternMatching

Conclusion

Optional vs. null

Going further: Optional to the limit

Option(al) monad

Is there an happy end for this?

Conclusion

Factorial

The recursive solution in Java

Recursive solution in Guava

Tail recursion in Java

Motivation behind the tail recursion

Tail recursion in Guava

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