Annotations on generic types parameters

[John Demme]

Hi All-

Is is possible to annotate the UInt32 in List(UInt32)? In the following example the annotation applies to the whole list, but I'm interested in just annotating the contained type:

struct Ex1 {
     array1 @0 :List(UInt32) $hide();
}


My understanding is that UInt32 is just a generic type parameter and that parameters can be annotated. I also tried something like this: "annotation baz(parameter) :Int32;" but got "error: Not a valid annotation target.".

... I suspect I'm not interpreting "parameter" correctly. Is it method parameter? If so, is there any other way to do what I'm looking for? If not, I just change the semantics of my annotations but I wanted to check first.

Thanks,

John

[Kenton Varda]

Hi John,

Unfortunately, annotations apply only to declarations, not to types. So, you can't annotate the argument to a type function.

The "parameter" annotation target means parameters to RPC methods, like:

    interface Foo {

      bar @0 (baz :UInt32 $annotation);

    }

One thing you could try is wrapping your list elements in a struct:

    struct Ex1 {

      array1 @0 :List(Element);

      struct Element {

        value @0 :UInt32 $hide();

      }

    }

Unfortunately in this particular case, this will double the memory usage of your list, because elements of a struct list will be aligned to 8 bytes, whereas UInt32s are 4 bytes.

-Kenton

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[John Demme]

Hi Kenton-

Yeah, that's what I thought. I'll just modify my annotation semantics.

Thanks!

~John