Suppose I have the following class:
public class FixExpr {
Expr<FixExpr> in;
}
Now I want to introduce a generic argument, abstracting over the use of Expr:
public class Fix<F> {
F<Fix<F>> in;
}
But Eclipse doesn't like this:
The type F is not generic; it cannot be parametrized with arguments <Fix<F>>
Is this possible at all or have I overlooked something that causes this specific instance to break?
Some background information: in Haskell this is a common way to write generic functions; I'm trying to port this to Java. The type argument F in the example above has kind * -> * instead of the usual kind *. In Haskell it looks like this:
newtype Fix f = In { out :: f (Fix f) }
I think what you're trying to do is simply not supported by Java generics. The simpler case of
public class Foo<T> {
public T<String> bar() { return null; }
}
also does not compile using javac.
Since Java does not know at compile-time what T is, it can't guarantee that T<String> is at all meaningful. For example if you created a Foo<BufferedImage>, bar would have the signature
public BufferedImage<String> bar()
which is nonsensical. Since there is no mechanism to force you to only instantiate Foos with generic Ts, it refuses to compile.