[Swift-devel] could we set the job sequence without a file dependency?

[Ben Clifford]

On Tue, 24 Mar 2009, Ian Foster wrote:

> I raised a technical issue: people sometimes want to express sequencing that
> relates to external side effects (e.g., output). How do we do that? We can, as
> Ben suggests, introduce an artificial data dependency. Or, we can provide an
> operator that expresses the sequencing. Each have pros and cons:

I don't really thing (a lot of the time) that it is an artificial data 
dependency - in Zhao and Mike's cases both, they have data dependencies.

> - The artificial data dependency is less clear in its semantics than the
> sequential operator

I think they both make one thing happen before the other. I don't think 
either way is less clear.

> - The artificial data dependency requires that you modify procedures to
> sequence them

> - The artificial data dependency is a real pain to write if you have to do it
> a lot

disagree

> + The artificial data dependency may allow for finer control over what is
> sequenced

I think they're probably about equivalent.

> I couldn't care less whether Swift looks like PCN. But I do have that
> experience, and recall the considerable frustration of some users who wanted
> to ensure that activity X happened before activity Y (whether for side
> effects, or debugging) and had to go to great lengths to introduce artificial
> data dependencies to achieve that goal.

The sequencing-for-debugging thing is something I think that external 
dependencies is perhaps wrong for. Not sure if putting an operator into 
the language is the right way - we've talked about different 
execution modes too, which might be a better thing to do.
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