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

[Daryoush Mehrtash]

So does this mean that the reason for complexity of generics is the Java
inheritance?

BTW, in addition to the article I posted, This site:
http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html   has a FAQ
on Java generics that is 500+ pages long!     In Haskell you have
parametrized types but I don't think you need 500+ page faq to explain it.

daryoush

On Thu, Oct 16, 2008 at 12:27 PM, Jonathan Cast
<jonathanccast at fastmail.fm>wrote:

> On Thu, 2008-10-16 at 12:27 -0700, Robert Greayer wrote:
> > --- On Thu, 10/16/08, Jonathan Cast <jonathanccast at fastmail.fm> 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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