Java中如何使用访客模式模拟Haskell的模式匹配

Question

I am new to Java and I am trying to use visitor pattern to simulate pattern matching. For this Haskell function

getOrDefault :: Maybe a -> a -> a
getOrDefault (Just x) = const x
getOrDefault Nothing = id

which returns a value of type a (given a value of type Maybe a and a default value), the following Java codes work.

interface Visitor<T, S> {
    S visitJust(Just<T> j);
    S visitNothing(Nothing<T> n);
}

abstract class Maybe<T> {
    public abstract <S> S accept(Visitor<T, S> v);

    public T getOrDefault(T defaultValue) {
        return this.accept(new Visitor<T, T>() {
            @Override
            public T visitJust(Just<T> j) { return j.value; }

            @Override
            public T visitNothing(Nothing<T> n) { return defaultValue; }
        });
    }
}

class Just<T> extends Maybe<T> {
    final T value;

    public Just(T value) {
        this.value = value;
    }

    public <S> S accept(Visitor<T, S> v) {
        return v.visitJust(this);
    }

}

class Nothing<T> extends Maybe<T> {
    public <S> S accept(Visitor<T, S> v) {
        return v.visitNothing(this);
    }
}

When I try to simulate this function that obtains the "length" of a value of type Maybe [a]

getLength :: Maybe [a] -> Int
getLength (Just x) = length x
getLength Nothing = -1

, the Java-version getLength I write is

public int getLength() {
    return this.accept(new Visitor<List<T>, Integer>() {
        @Override
        public Integer visitJust(Just<List<T>> j) { ... }

        @Override
        public Integer visitNothing(Nothing<List<T>> n) { ... }
    });
}

But it doesn't work, since T and List<T> are incompatible. How to fix this?

Answer 1

Declare getLength() as a static method with its own type variable:

  public static <T> int getLength(Maybe<List<T>> maybe) {
    return maybe.accept(new Visitor<List<T>, Integer>() {
      @Override
      public Integer visitJust(Just<List<T>> j) { ... }

      @Override
      public Integer visitNothing(Nothing<List<T>> n) { ... }
    });
  }