On 18/11/2013 14:48, Stian Soiland-Reyes wrote:
For instance, if there are iterations over the nested workflow, now you would have iterations over the immediate first processors instead (and their downstreams) - which might in some cases require you to reconfigure List Handling on every step of the workflow.Many nested workflows are used with iterations both inside and outside the nested workflows - in which case there are several List Handling iteration patterns which can't easily be expressed on a merged workflow, say outer Dot product and inner Cross product.
Another cast that I hit recently relates to the ways in which merges
work; if you want to form (apologies for pseudo-SML syntax):
'a list • 'b list • 'c list -> ('a • 'b • 'c) list
without using a beanshell, you end up using a merge inside a nested
workflow, where the depth of the inputs to the nested workflow are 0 but
the values being fed in are lists. Remove the nesting and you get a
*completely* different operation which has an outer list which has the
three input lists as sub-lists; the outer list is working like a
poor-mans tuple. (FWIW, in my particular case, the three values are
actually derived from a single input by applications of — decidedly
non-trivial — functions, so I can guarantee that doing a dot product
works.) Changing the workflow to deal with the fact that the merges now
fail would involve introducing an entirely new processor. (I also
suspect that we perhaps ought to flag up any workflow that has a merge
of a value of depth greater than 0 as being potentially problematic; the
“poor-mans tuple” is a categorical canary in the type mine.)
I also use nested workflows for looping control and the workflow
concerned would *not* work without it. But Stian's already mentioned that.
The long and short of it is this: nested workflows are a critical
feature of Taverna. You can't unnest in all but the most trivial of
cases; if you want to understand anything complicated, you'll have to
deal with directly nesting. :-)
Donal.
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