[PD] [PD-dev] recursion in Gem - a tutorial

Mathieu Bouchard matju at artengine.ca
Mon Jul 23 23:50:41 CEST 2007

On Thu, 19 Jul 2007, Claude Heiland-Allen wrote:

> Again, I tried to create something similar in Pd, but ran into 
> re-entrancy bugs whenever I tried to use recursion.  I do not write 
> externals lightly, but in this case I think it was necessary.  If 
> someone knows how to solve this in pure Pd, I'd be delighted.

To follow up on our discussion on the FreeNode #dataflow channel... think 
about what it takes for any object to figure out "hey, I'm being used 
recursively!". Basically, if I use [+] recursively, there's no way that 
just by the order of use of the inlets it can figure out whether anything 
is recursive or not. What it can use is the outlets, but it can only 
figure out recursion when the left inlet gets stuffed, so if that is 
preceded immediately by a right inlet message, there's no way for that 
object to know whether the right inlet message is intended to accompany 
the recursive call or just configure the object for the benefit of the 
next non-recursive call. Both possibilities are legitimate uses that have 
to be supported.

Your only possibility, then, is to make something explicit, with methods 
"push" and "pop" that can handle backups of states of enough subobjects, 
or else do something really heavy for creating instances recursively by 
dynamic patching. (we're at the edge of what makes sense in a 
pd-style of dataflow... I suppose any solution will look weird)

(In DesireData it could look at pd_stack, but that wouldn't be fast and 
it's not enough because it still needs a state-preserving system)

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| Mathieu Bouchard - tél:+1.514.383.3801, Montréal QC Canada

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