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

[Jonathan Cast]

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