A failed attempt to port Wadler's original Generic Java solution to expression problem to modern Java

wadler-expression-problem-generic-java-code-in-modern-java.java

// Wadler's the expression problem email https://homepages.inf.ed.ac.uk/wadler/papers/expression/expression.txt
// introduced the term expression problem and suggested a solution in Generic Java. The code within the email
// actually does not work. Below is Wadler's original words.

// 2.  A caveat with regard to inner interfaces

// In GJ as it is currently implemented, type parameters do not scope
// over static members, and further, a type parameter may be indexed only
// by non-static classes or interfaces defined in the bound.  And in Java
// as it is currently defined, all inner interfaces are taken as static,
// whether declared so are not.  This makes the mechanism for indexing
// type variables by inner classes useless for interfaces, greatly
// reducing its utility.  In particular, it invalidates the solution just
// presented, which depends on Visitor being an interface.

// Fortunately, it looks possible to relax either the GJ or the Java
// constraint.  So far as I can see, the only difference in making an
// interface non-static is that it can now include non-static inner
// classes; so the change would not render invalid any existing Java
// programs.  But this point requires further study.  Also, I should note
// that since the changes have not been implemented yet, I have not
// actually run the proposed solution.  (I did translate from GJ to Java
// by hand, and run that.)

// I tried to port the original code into modern java (with a different example).
// But I found I was unable to refer to the sibling trait within the same class.
// I tried to use `This.Visitor` in interface `PlaneShape`, which failed with error
// Cannot make a static reference to the non-static type This Java (536871434)
// This seemed to be essentially Wadler's caveat.
// I don't know if we can work around this 20 years later.

// class Module1F<This extends Module1F<This>> with
// final class Module1 extends Module1F<Module1> {}
// can be used to make This type variable in Module1F refer to exactly the
// same class (instead of possibly subclasses).
// Quoting The Expression Problem by Philip Wadler
// This use of `This' is the standard trick to provide accurate static typing in the prescence of subtypes (sometimes called MyType or ThisType).
// See also Is there a way to refer to the current type with a type variable?
// https://stackoverflow.com/questions/7354740/is-there-a-way-to-refeclass
class Module1F<This extends Module1F<This>> {
    // A Visitor trait that is bounded by the trait Module1F.
    // We may think this as a Visitor specialized to the PlaneShape defined below.
    // Quoting The Expression Problem by Philip Wadler
    // The key trick here is the use of This.Exp and This.Visitor, via the
    // mechanism described in `Do parametric types beat virtual types?'.
    // Recall that mechanism allows a type variable to be indexed by any
    // inner class defined in the variable's bound.
    interface Visitor<R> {
        public R forTriangle(Double a, Double b, Double c);
    }

    // The data type we want to extend.
    interface PlaneShape {
        // Instead of passing a generic Visitor to this function, we pass a
        // This.Visitor.
        // This sibling interface Visitor may use methods specific to some data
        // variants,
        // e.g. forTriangle method specific to Triangles here!

        // There is actually an error in the code below. The error is:
        // Cannot make a static reference to the non-static type This Java (536871434),
        // which as far as I know means that This.Visitor<R> is not a static type,
        // and accessing This.Visitor<R> from a static context is not allowed.
        public <R> R visit(This.Visitor<R> v);
    }

    // A data type variant of the PlaneShape interface.
    class Triangle implements PlaneShape {
        protected final Double a_, b_, c_;

        public Triangle(Double a, Double b, Double c) {
            a_ = a;
            b_ = b;
            c_ = c;
        }

        // The public entry point for the visitor, used to run a specific operator for
        // this data variant.
        public <R> R visit(Visitor<R> v) {
            return v.forTriangle(a_, b_, c_);
        }
    }

    // Implement an operator based on visitor pattern.
    class Perimeter implements Visitor<Double> {
        public Double forTriangle(Double a, Double b, Double c) {
            return a + b + c;
        }
    }
}

final class Module1 extends Module1F<Module1> {
}

class Module2F<This extends Module2F<This>> extends Module1F<This> {
    interface Visitor<R> extends Module1F.Visitor<R> {
        public R forSquare(Double x);
    }

    class Square implements PlaneShape {
        protected final Double x_;

        public Square(Double x) {
            x_ = x;
        }

        public <R> R visit(Visitor<R> v) {
            return v.forSquare(x_);
        }
    }

    class Perimeter extends Module1F<This>.Perimeter implements Visitor<Double> {
        public Double forSquare(Double x) {
            return 4 * x;
        }
    }

    class Area implements Visitor<Double> {
        public Double forTriangle(Double a, Double b, Double c) {
            Double s = (a + b + c) / 2.0;
            Double area = Math.sqrt(s * (s - a) * (s - b) * (s - c));
            return area;
        }

        public Double forSquare(Double x) {
            return x * x;
        }
    }
}

final class Module2 extends Module2F<Module2> {
}

final class Main {
    static public void main(String[] args) {
        Module2 m1 = new Module2();
        Module2.PlaneShape e1 = m1.new Triangle(3.0, 4.0, 5.0);
        System.out.println("Perimeter: " + e1.visit(m2.new Perimeter()));

        Module2 m2 = new Module2();
        Module2.PlaneShape e2 = m2.new Square(3.0);
        System.out.println("Perimeter: " + e2.visit(m2.new Perimeter()));
        System.out.println("Area: " + e2.visit(m2.new Area()));
    }
}