如何在Java中模拟Haskell的“Either a b”

Question

How can I write a typesafe Java method that returns either something of class a or something of class b? For example:

public ... either(boolean b) {
  if (b) {
    return new Integer(1);
  } else {
    return new String("hi");
  }
}

What is the cleanest way?

( The only thing that comes to my mind is using exceptions which is obviously bad, as it is abusing a error-handling mechanism for a general language feature ...

public String either(boolean b) throws IntException {
  if (b) {
    return new String("test");
  } else {
    throw new IntException(new Integer(1));
  }
}

)

Answer 1

My general formula for simulating algebraic data types is:

• The type is an abstract base class, and the constructors are subclasses of that
• The data for each constructor are defined in each subclass. (This allows constructors with different numbers of data to work correctly. It also removes the need to maintain invariants like only one variable is non-null or stuff like that).
• The constructors of the subclasses serve to construct the value for each constructor.
• To deconstruct it, one uses instanceof to check the constructor, and downcast to the appropriate type to get the data.

So for Either ab , it would be something like this:

abstract class Either<A, B> { }
class Left<A, B> extends Either<A, B> {
    public A left_value;
    public Left(A a) { left_value = a; }
}
class Right<A, B> extends Either<A, B> {
    public B right_value;
    public Right(B b) { right_value = b; }
}

// to construct it
Either<A, B> foo = new Left<A, B>(some_A_value);
Either<A, B> bar = new Right<A, B>(some_B_value);

// to deconstruct it
if (foo instanceof Left) {
    Left<A, B> foo_left = (Left<A, B>)foo;
    // do stuff with foo_left.a
} else if (foo instanceof Right) {
    Right<A, B> foo_right = (Right<A, B>)foo;
    // do stuff with foo_right.b
}

Answer 2

Here is a statically checked type-safe solution; this means you cannot create runtime errors. Please read the previous sentence in the way it is meant. Yes, you can provoke exceptions in some way or the other...

It's pretty verbose, but hey, it's Java!

public class Either<A,B> {
    interface Function<T> {
        public void apply(T x);
    }

    private A left = null;
    private B right = null;
    private Either(A a,B b) {
        left = a;
        right = b;
    }

    public static <A,B> Either<A,B> left(A a) {
        return new Either<A,B>(a,null);
    }
    public static <A,B> Either<A,B> right(B b) {
        return new Either<A,B>(null,b);
    }

    /* Here's the important part: */
    public void fold(Function<A> ifLeft, Function<B> ifRight) {
        if(right == null)
            ifLeft.apply(left);
        else
            ifRight.apply(right);
    }

    public static void main(String[] args) {
        Either<String,Integer> e1 = Either.left("foo");
        e1.fold(
                new Function<String>() {
                    public void apply(String x) {
                        System.out.println(x);
                    }
                },
                new Function<Integer>() {
                    public void apply(Integer x) {
                        System.out.println("Integer: " + x);
                    }
                });
    }
}

You might want to look at Functional Java and Tony Morris' blog .

Here is the link to the implementation of Either in Functional Java. The fold in my example is called either there. They have a more sophisticated version of fold , that is able to return a value (which seems appropriate for functional programming style).

Answer 3

您可以通过编写一个通用类与哈斯克尔密切的对应Either对两种类型，参数L和R与两个构造（一个以在L ，并且在一个了结R和两个方法L getLeft()和R getRight() ，使得他们要么返回施工时传递的值，或抛出异常。

Answer 4

The suggestions already provided, although feasible, are not complete as they rely on some null references and effectively make "Either" masquerade as a tuple of values. A disjoint sum is obviously one type or the other.

I'd suggest having a look at the implementation of FunctionalJava 's Either as an example.

Answer 5

The big thing is not to try to write in one language whilst writing in another. Generally in Java you want to put the behaviour in the object, rather than having a "script" running outside with encapsulation destroyed by get methods. There is no context for making that kind of suggestion here.

One safe way of dealing with this particular little fragment is to write it as a callback. Similar to a very simple visitor.

public interface Either {
    void string(String value);
    void integer(int value);
}

public void either(Either handler, boolean b) throws IntException {
    if (b) {
        handler.string("test");
    } else {
        handler.integer(new Integer(1));
    }
}

You may well want to implement with pure functions and return a value to the calling context.

public interface Either<R> {
    R string(String value);
    R integer(int value);
}

public <R> R either(Either<? extends R> handler, boolean b) throws IntException {
    return b ?
        handler.string("test") :
        handler.integer(new Integer(1));
}

(Use Void (capital 'V') if you want to get back to being uninterested in the return value.)

Answer 6

I've implemented it in a Scala-like fashion in the following way. It's a little verbose (it is Java, after all :)) but it's type safe.

public interface Choice {    
  public enum Type {
     LEFT, RIGHT
  }

  public Type getType();

  interface Get<T> {
     T value();
  }
}

public abstract class Either<A, B> implements Choice {

  private static class Base<A, B> extends Either<A, B> {
    @Override
    public Left leftValue() {
      throw new UnsupportedOperationException();
    }

    @Override
    public Right rightValue() {
      throw new UnsupportedOperationException();
    }

    @Override
    public Type getType() {
      throw new UnsupportedOperationException();
    }
  }

  public abstract Left leftValue();

  public abstract Right rightValue();

  public static <A, B> Either<A, B> left(A value) {
    return new Base<A, B>().new Left(value);
  }

  public static <A, B> Either<A, B> right(B value) {
    return new Base<A, B>().new Right(value);
  }

  public class Left extends Either<A, B> implements Get<A> {

    private A value;

    public Left(A value) {
      this.value = value;
    }

    @Override
    public Type getType() {
      return Type.LEFT;
    }

    @Override
    public Left leftValue() {
      return Left.this;
    }

    @Override
    public Right rightValue() {
      return null;
    }

    @Override
    public A value() {
      return value;    
    }
  }

  public class Right extends Either<A, B> implements Get<B> {

    private B value;

    public Right(B value) {
      this.value = value;
    }

    @Override
    public Left leftValue() {
      return null;
    }

    @Override
    public Right rightValue() {
      return this;
    }

    @Override
    public Type getType() {
      return Type.RIGHT;
    }

    @Override
    public B value() {
      return value;
    }
  }
}

Then you can pass Either<A,B> instances around on your code. The Type enum is mainly used on switch statements.

Creating Either values is simple as:

Either<A, B> underTest;

A value = new A();

underTest = Either.left(value);

assertEquals(Choice.Type.LEFT, underTest.getType());
assertSame(underTest, underTest.leftValue());
assertNull(underTest.rightValue());
assertSame(value, underTest.leftValue().value());

Or, in the typical situation where it is used instead of exceptions,

public <Error, Result> Either<Error,Result> doSomething() {
    // pseudo code
    if (ok) {
        Result value = ...
        return Either.right(value);
    } else {
        Error errorMsg = ...
        return Either.left(errorMsg);
    }
}

// somewhere in the code...

Either<Err, Res> result = doSomething();
switch(result.getType()) {
   case Choice.Type.LEFT:
      // Handle error
      Err errorValue = result.leftValue().value();
      break;
   case Choice.Type.RIGHT:
      // Process result
      Res resultValue = result.rightValue().value();
      break;
}

Hope it helps.

Answer 7

From http://blog.tmorris.net/posts/maybe-in-java/ I learned that you can make the outer class's constructor private so only nested classes can subclass it. This trick is just as type safe as the best above, but much less verbose, works for any ADT you want like Scala's case class.

public abstract class Either<A, B> {
    private Either() { } // makes this a safe ADT
    public abstract boolean isRight();
    public final static class Left<L, R> extends Either<L, R>  {
        public final L left_value;
        public Left(L l) { left_value = l; }
        public boolean isRight() { return false; }
    }
    public final static class Right<L, R> extends Either<L, R>  {
        public final R right_value;
        public Right(R r) { right_value = r; }
        public boolean isRight() { return true; }
    }
}

(started from top answer's code and style)

Note that:

• The finals on the subclass are optional. Without them you can subtype Left and Right, but still not Either directly. Thus without the final s Either has limited width but unbounded depth.

• With ADTs like this, I see no reason to jump on the whole anti- instanceof bandwagon. A boolean works for Maybe or Either, but in general instanceof is your best and only option.

Answer 8

Thanks to Derive4J algebraic data types are now very easy in Java. All you have to do is create the following class:

import java.util.function.Function;

@Data
public abstract class Either<A, B> {

  Either(){}

  /**
   * The catamorphism for either. Folds over this either breaking into left or right.
   *
   * @param left  The function to call if this is left.
   * @param right The function to call if this is right.
   * @return The reduced value.
   */
  public abstract <X> X either(Function<A, X> left, Function<B, X> right);
}

And Derive4J will take care of creating constructors for the left and rights cases, as well as a pattern matching syntax alla Haskell, mapper methods for each sides, and more.

Answer 9

There is a stand-alone implementation of Either for Java 8 in a small library, "ambivalence": http://github.com/poetix/ambivalence

It is closest to the Scala standard implementation - for example, it provides left and right projections for map and hashMap operations.

There is no direct access to the left or right values; rather, you join the two types by providing lambdas to map them into a single result type:

Either<String, Integer> either1 = Either.ofLeft("foo");
Either<String, Integer> either2 = Either.ofRight(23);
String result1 = either1.join(String::toUpperCase, Object::toString);
String result2 = either2.join(String::toUpperCase, Object::toString);

You can get it from Maven central:

<dependency>
    <groupId>com.codepoetics</groupId>
    <artifactId>ambivalence</artifactId>
    <version>0.2</version>
</dependency>

Answer 10

You don't need to settle with the instanceof checks or redundant fields. Surprisingly enough, Java's type system provides enough features to simulate the sum types cleanly.

Background

First of all, do you know that any data type can be encoded with just functions? It's called Church encoding . Eg, using the Haskell signature, the Either type could be defined as follows:

type Either left right =
  forall output. (left -> output) -> (right -> output) -> output

You can interpret it as "given a function on the left value and a function on the right value, produce the result of either of them".

Definition

Expanding on this idea, in Java we can define an interface called Matcher , which includes both functions and then define the Sum type in terms of how to pattern-match on it. Here's the complete code:

/**
 * A sum class which is defined by how to pattern-match on it.
 */
public interface Sum2<case1, case2> {

  <output> output match(Matcher<case1, case2, output> matcher);

  /**
   * A pattern-matcher for 2 cases.
   */
  interface Matcher<case1, case2, output> {
    output match1(case1 value);
    output match2(case2 value);
  }

  final class Case1<case1, case2> implements Sum2<case1, case2> {
    public final case1 value;
    public Case1(case1 value) {
      this.value = value;
    }
    public <output> output match(Matcher<case1, case2, output> matcher) {
      return matcher.match1(value);
    }
  }

  final class Case2<case1, case2> implements Sum2<case1, case2> {
    public final case2 value;
    public Case2(case2 value) {
      this.value = value;
    }
    public <output> output match(Matcher<case1, case2, output> matcher) {
      return matcher.match2(value);
    }
  }

}

Usage

And then you can use it like this:

import junit.framework.TestCase;

public class Test extends TestCase {

  public void testSum2() {
    assertEquals("Case1(3)", longOrDoubleToString(new Sum2.Case1<>(3L)));
    assertEquals("Case2(7.1)", longOrDoubleToString(new Sum2.Case2<>(7.1D)));
  }

  private String longOrDoubleToString(Sum2<Long, Double> longOrDouble) {
    return longOrDouble.match(new Sum2.Matcher<Long, Double, String>() {
      public String match1(Long value) {
        return "Case1(" + value.toString() + ")";
      }
      public String match2(Double value) {
        return "Case2(" + value.toString() + ")";
      }
    });
  }

}

With this approach you can even find a direct resemblance of pattern-matching in such languages as Haskell and Scala.

Library

This code is distributed as part of my library of composite types (Sums and Products, aka Unions and Tuples) of multiple arities. It's on GitHub:

https://github.com/nikita-volkov/composites.java

Answer 11

Since you've tagged Scala, I'll give a Scala answer. Just use the existing Either class. Here's an example usage:

def whatIsIt(flag: Boolean): Either[Int,String] = 
  if(flag) Left(123) else Right("hello")

//and then later on...

val x = whatIsIt(true)
x match {
  case Left(i) => println("It was an int: " + i)
  case Right(s) => println("It was a string: " + s)
}

This is completely type-safe; you won't have problems with erasure or anything like that... And if you simply can't use Scala, at least use this as an example of how you can implement your own Either class.

Answer 12

Based on the answer by Riccardo, following code snippet worked for me:

public class Either<L, R> {
        private L left_value;
        private R right_value;
        private boolean right;

        public L getLeft() {
            if(!right) {
                return left_value;
            } else {
                throw new IllegalArgumentException("Left is not initialized");
            }
        }

        public R getRight() {
            if(right) {
                return right_value;
            } else {
                throw new IllegalArgumentException("Right is not initialized");
            }
        }

        public boolean isRight() {
            return right;
        }

        public Either(L left_v, Void right_v) {
           this.left_value = left_v;
           this.right = false;
        }

        public Either(Void left_v, R right_v) {
          this.right_value = right_v;
          right = true;
        }

    }

Usage:

Either<String, Integer> onlyString = new Either<String, Integer>("string", null);
Either<String, Integer> onlyInt = new Either<String, Integer>(null, new Integer(1));

if(!onlyString.isRight()) {
  String s = onlyString.getLeft();
}

Answer 13

The closest I can think of is a wrapper around both values that lets you check which value is set and retrieve it:

class Either<TLeft, TRight> {
    boolean isLeft;

    TLeft left;
    TRight right;

    Either(boolean isLeft, TLeft left1, TRight right) {
        isLeft = isLeft;
        left = left;
        this.right = right;
    }

    public boolean isLeft() {
        return isLeft;
    }

    public TLeft getLeft() {
        if (isLeft()) {
            return left;
        } else {
            throw new RuntimeException();
        }
    }

    public TRight getRight() {
        if (!isLeft()) {
            return right;
        } else {
            throw new RuntimeException();
        }
    }

    public static <L, R> Either<L, R> newLeft(L left, Class<R> rightType) {
        return new Either<L, R>(true, left, null);
    }

    public static <L, R> Either<L, R> newRight(Class<L> leftType, R right) {
        return new Either<L, R>(false, null, right);
    }
}

class Main {
    public static void main(String[] args) {
        Either<String,Integer> foo;
        foo = getString();
        foo = getInteger();
    }

    private static Either<String, Integer> getInteger() {
        return Either.newRight(String.class, 123);
    }

    private static Either<String, Integer> getString() {
        return Either.newLeft("abc", Integer.class);
    }   
}

Answer 14

Change your design so that you don't need this rather absurd feature. Anything you'd do with the return value would require some sort of if/else construct. It would just be very, very ugly.

From a quick Googling, it seems to me that the only thing Haskell's Either is commonly used for is error reporting anyway, so it looks like exceptions are actually to correct replacement.