[Haskell-cafe] Re: What I wish someone had told me...

[Jonathan Cast]

On Thu, 2008-10-16 at 10:02 -0700, Robert Greayer wrote:
> 
> 
> --- On Thu, 10/16/08, Jonathan Cast <jonathanccast at fastmail.fm> wrote:
> 
> > Can I have HashSet<Integer>?  Could I construct
> > HashSet<?>, if I did?
> 
> Yes:
> 
> HashSet<?> blah = (HashSet<?>) hashSetClass.newInstance();
> 
> ... compiles, and won't throw an exception if hashSetClass truly is
> the class object for HashSet.

But I can't say new HashSet<?>()?

> Pretty useless, because you can't put anything *into* a HashSet<?>
> object...

Even if my client hands my a ? object?

> blah.add("foo"); // doesn't typecheck...
> 
> but you can do:
> HashSet<String> blah = (HashSet<String>) hashSetClass.newInstance();
> blah.add("foo");
> 
> works fine..

jcc