On Thu, 2008-10-16 at 12:27 -0700, Robert Greayer wrote:
> --- On Thu, 10/16/08, Jonathan Cast <[EMAIL PROTECTED]> wrote:
> > 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'?
> >
>
> Yes -
> class Foo<T> {
> ...
> void right(List<T> foo, List<T> bar) {
> foo.add(bar.get(0));
> }
>
> Can also do it at the method level...
>
> void <T> alsoRight(List<T> foo, List<T> bar) { ... }
>
> > 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...
> >
>
> Not exactly... Java disallows 'wrong' from being written (without
> class casts and such), because it disallows calling a method which has
> a wildcard type in a contravariant position. IIRC, Scala handles all
> this more elegantly. If you stay away from '?', and stick to type
> variables with Java generics, though, how type checking with generics
> works in Java should be mostly unsurprising to a Haskeller.
Oh, good.
Daryoush Mehrtash:
It looks like the answer to your original question --- gotchas with Java
generics vs. Haskell polymorphism --- is that the `gotchas' you linked
to are consequent on using ? in your code, instead of true type
variables. So the truly problematic construct is ?, and the difference
is just that Haskell doesn't have an analogue to it.
jcc
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