[Zeek-Dev] support for event handlers using a subset of parameters

[Jon Siwek]

On Tue, Feb 5, 2019 at 7:40 PM Robin Sommer <robin at corelight.com> wrote:

> The following would be even worst in terms of confusion:
>
>     global my_event: event(a: string, b: string);
>     event my_event(b: string)
>
> Now I need to know if the language goes by order of parameters or by
> parameter name.

The issue may be exaggerated because we're using contrived parameter
names.  If we make it a bit more real (still "shortened" for sake of
example):

    http_request: event(method: string, version: string);
    event http_request(version: string) { ... }

The "parameter order vs. name" issue doesn't even cross my mind here
because the intent is clear -- I understand the meaning of the
handler's parameter because the original author of the event chose a
meaningful and useful name for it (which will be the common case).
That's all the reader cares about -- understanding the meaning of any
given parameter.

> I do see the appeal of making things just work when event handlers
> change, but is there really no different way to support that?

If the goal is to avoid breaking user-code, I don't think we're in a
position to do anything else.  For example, we have:

    global my_event: event(a: string);

And we want to add a parameter:

    global my_event: event(a: string, b: string);

The user has already written their handler as:

    event my_event(a: string) { ... }

We can't magically change that user's code.  We have to, somehow, let
that work as it is, without user intervention.

- Jon