That's what I was refering to as having to check the runtime type. I
don't feel like you gained much in your sample, you could just replace
"String | Integer" with Object and add a "default: throw new
AssertionError();". Obviously you gained type safety, but it would be
much more useful if unions were structural:
public static void closeSilently(InputStream|OutputStream|Reader|
Writer|Channel preCloseable) {
try {
preCloseable.close();
} catch(IOException e) {}
}
With kind regards
Ben
On 19 Aug., 15:57, James Iry <[email protected]> wrote:
> I don't follow. Union types don't seem particularly structural to me.
> They're the disjunction of other types, and if the other types are
> nominative then so is the union of them. A type like String | Integer
> contians all expressions that are Strings or Integers. The idea would be
> that you can write the followig. Here I'm borrowing a touch of Scala syntax
> to allow switch on runtime type and use | to indicate disjunction of types.
>
> void foo(String | Integer argument) {
> switch(argument) {
> case s : String => "It was a String " + s
> break;
> case i : Integer => "It was an Integer " + i
> }
>
> }
>
> The types of actual arguments would be checked statically. The type system
> would allow foo("hello") and foo(42) but would not allow foo(false). For
> backward compatibility, the argumenttype could be erased to the least upper
> bound (in this case Object) in the bytecode.
>
> Similarly, you could write
>
> String | Integer foo(Boolean flag) {
> if (flag) return 42;
> else return "hello";
>
> }
> On Wed, Aug 19, 2009 at 4:21 AM, Ben Schulz <[email protected]> wrote:
>
> > I'm not much of a PLTist, but I'm guessing the reason uions don't make
> > it into general purpose, object oriented languages is that they're
> > inherently structural. Just a guess though. (The alternative is having
> > to check the runtime type and downcast, which pretty much defeats the
> > purpose.)
>
> > With kind regards
> > Ben
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