Higher-kinded generics in Java

Question

Suppose I have the following class:

public class FixExpr {
  Expr in;
}

Now I want to introduce a generic argument, abstract

Answer 1

Still, there are ways to encode higer-kinded generics in Java. Please, have a look at higher-kinded-java project.

Using this as a library, you can modify your code like this:

public class Fix {
    Type.App> in;
}

You should probably add an @GenerateTypeConstructor annotation to your Expr class

@GenerateTypeConstructor
public class Expr {
    // ...
}

This annotation generates ExprTypeConstructor class. Now you can process your Fix of Expr like this:

class Main {
    void run() {
        runWithTyConstr(ExprTypeConstructor.get);
    }

     void runWithTyConstr(ExprTypeConstructor.Is tyConstrKnowledge) {
        Expr> one = Expr.lit(1);
        Expr> two = Expr.lit(2);

        // convertToTypeApp method is generated by annotation processor
        Type.App> oneAsTyApp = tyConstrKnowledge.convertToTypeApp(one);
        Type.App> twoAsTyApp = tyConstrKnowledge.convertToTypeApp(two);

        Fix oneFix = new Fix<>(oneAsTyApp);
        Fix twoFix = new Fix<>(twoAsTyApp);

        Expr> addition = Expr.add(oneFix, twoFix);
        process(addition, tyConstrKnowledge);
    }

     void process(
            Fix fixedPoint,
            ExprTypeConstructor.Is tyConstrKnowledge) {

        Type.App> inTyApp = fixedPoint.getIn();

        // convertToExpr method is generated by annotation processor
        Expr> in = tyConstrKnowledge.convertToExpr(inTyApp);

        for (Fix subExpr: in.getSubExpressions()) {
            process(subExpr, tyConstrKnowledge);
        }
    }

}