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

[Jonathan Cast]

On Thu, 2008-10-16 at 09:48 -0700, Robert Greayer wrote:
> > On Thu, 2008-10-16 at 15:02 +1300, Richard O'Keefe
> > wrote:
> > > On 16 Oct 2008, at 12:09 pm, Jonathan Cast wrote:
> > > >> I am not sure how say in a Java language a
> > constructor can "conjure  
> > > >> up
> > > >> a value of an unknown type".
> > > >
> > > > Well, that's the point.  It can't, in
> > Haskell or in Java.  If you
> > > > understand that --- that you can't call the
> > default constructor of a
> > > > class that is not statically known at compile
> > time
> > > 
> > > If you understand that about Java, then you don't
> > understand Java.
> > 
> > God, I hope never to understand Java.  *shudder*
> > 
> > > Java reflection means that compile-time types are
> > backed up by
> > > runtime objects belonging to Type in general, to Class
> > if they
> > > are class types.  It also means that you can discover
> > the
> > > default constructor by using aClass.getConstructor(),
> > and you
> > > can invoke it by using .newInstance().
> > 
> > Wait, what?  Why can't Java use this to keep template
> > parameters around
> > at run time?  Or is the article (as per which
> > Set<Integer> and
> > Set<Double> are identical at run time) full of it?
> > 
> 
> The article (whichever it was) wasn't full of it... Set<Integer> and
> Set<Double> are identical at runtime.  You cannot, given a Class
> object at runtime that happens to be the Class object corresponding to
> Set<?>, conjure up an instance of Set... but simply for the reason
> that Set has no constructors (it is an interface).

I think we've found our difference between Haskell and Java...

So Set<Double> is an interface, thus you can't copy construct it.  Makes
sense --- but does it belong in an article about gotchas with
*generics*?

> You can, however, given a class object that happens to be the class
> object corresponding to (say) HashSet,

Can I have HashSet<Integer>?  Could I construct HashSet<?>, if I did?

>  conjure up an instance of a HashSet, and assign it to a variable of
> the (static) type Set<Integer> or Set<Double>... i.e.
> 
> Set<Integer> foo = (Set<Integer>) hashSetClass.newInstance();
> Set<Double> bar = (Set<Double>) hashSetClass.newInstance();
> 
> which will generate warnings about unsafe casts, but nevertheless can
> compile, and won't cause any exceptions at runtime.

jcc