Re: Re[2]: [Haskell-cafe] Re: What I wish someone had told me...

On Thu, 2008-10-16 at 11:41 -0700, Robert Greayer wrote:

--- On Thu, 10/16/08, Jonathan Cast wrote:

...

But I can't say new HashSet()?

No... but you can say 'new HashSet<T>()' where T is a type variable, and then put a value of type T into your set, which is probably generally what you want. HashSet is a set of unknown (at compile time) element type. It is not safe to put any element into such a set. Consider:

void wrong(List<?> foo, List<?> bar) { foo.add(bar.get(0)); // illegal... but if it weren't... }

So if I say void wrong(List<?> foo, List<?> bar) I get two /different/ type variables implicitly filled in? If I declare a generic class, and then have a method, is there a way, in that method's parameter list, to say `the type parameter that was supplied when my class was instantiated'?

... List<Integer> x = ...; List<String> y = ...; wrong(x, y); // then this would put an String into a list of ints...

Yikes. So, in this instance, the difference between Haskell and Java is: if you want to disallow that call to wrong, in Haskell you can...

---

Perhaps there was confusion over what you meant by 'conjure up a value of an unknown type'... you can't explicitly instantiate a parameterized class with a wildcard type variable (e.g. new HashSet).

Right. Which is a logical consequence of parametric polymorphism.

However, you can conjure up an instance of any class for which you have a Class object handy, provided it is non-abstract and has public constructors, and then assign it to a variable with a wildcard in its type.

(Which is a logical consequence of non-parametric polymorphism, concerning which: yikes!) jcc