[PD] Should message objects be able to pre-parse $0 into valid dollarzero?

Bill Gribble grib at billgribble.com
Wed Sep 10 20:51:35 CEST 2014

On 09/10/2014 02:21 PM, Miller Puckette wrote:
> This leads to an interesting point - '$0' might recently have become more
> important than it was before because of the multiple-libpd-instances features
> in 0.46 - any libpd patch wanting to support multiple instances will need
> some $0-ish disambiguation.

Scope hygiene and namespace management was one of the main issues that 
led me to start working on MFP [1].  While my code base is totally 
different from PD's, I think the approach I came up with for this works 
pretty well and is at least a proof-of-concept that hints that working 
this stuff out in PD will make for cleaner patches.

I don't think there's enough overlap in the implementation for a 
detailed description to be useful, but the highlights are: The app knows 
about lexical scopes.  Every patch has at least one scope and possibly 
more (nested scopes are possible, UI is a bit kludgy).  Any time a name 
is created (by naming an object in a patch, or by creating an [r] 
object) it goes into a specific scope.  Trying to resolve names, by 
creating an [s] for example, searches in the local scope of the [s] 
first, then works upward.  There are always-available names ("patch", 
"self", "app") which resolve to the patch, the object containing the 
name reference, and the top-level program respectively.  Names can 
include explicit tree-walking to target an object in a scope that's not 
a parent of the current scope (i.e. [s someotherpatch.scopename.name])

With this approach there's no need for $0 or anything like it. Names are 
local to the object in which they were created.  The whole thing is 
still young, but I am finding that patches have a lot less name-mangling 
and are a lot more readable to my programmer's eyes.

As a bonus, with clean scope semantics you can pretty easily create user 
patches that have dynamic (per-instance) numbers of inlets and outlets, 
with fairly arbitrary semantics about how the templating and cloning of 
patch-pieces works.  The trick is "hygienic scope cloning"... take all 
the objects in a scope, copy them into a new scope within the patch.  
This lets you use [s] and [r] within portions of a patch that need to be 
cut-n-pasted to add more inlets/outlets, and have everything Just Work.  
Last I looked, this was pretty hard to do in PD.

Bill Gribble

[1] https://github.com/bgribble/mfp

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