[PD] definitions of data types

Frank Barknecht fbar at footils.org
Mon Feb 6 14:11:23 CET 2006


Hallo,
IOhannes m zmoelnig hat gesagt: // IOhannes m zmoelnig wrote:

> Hans-Christoph Steiner wrote:
> > Here are some attempts at definitions:
> > 
> > list:  a series of 2 or more elements whose first element is either the 
> > selector "list", or a float, which causes the "list" selector to be
> 
> i think this is a bad start.
> implying "list" or "float" is an exception and should be handled as
> such: at the very end of the documentation! if people would use [float
> 1( instead of [1(, then i think that there would be less confusion.
> so we should get people to _think_ [float 1(, even though they _write_ [1(.

I agree that a "bottom up" approach to get clear definitions would be
better. That is, first start with the possible elements/atoms:
floats/numbers, symbols, words (i.e. "symbols" without "symbol"
selector), bang and pointer.

After these are defined, one could talk about how messages with more
than one atom are handled: those with a proper "list" selector, those
with an implied "list" selector and those with a different selector
(out of band-/meta-messages). 

Maybe this whole discussion should not so much focus on "data type"
anyway, but instead on "messages", because all of these things in
practice are only important when it comes to how Pd objects deal with
messages while communicating with each other.

Here's my quick try at definitions using that approach.


==============
Messages in Pd
==============

  Objects in Pd send each other various data in the form of messages.
  Messages can transport various kinds of information: single numbers
  and words, groups of numbers and words etc. Understanding the various
  ways to built and work with messages is a key to understanding Pd. 


Nomenclature (not scientific)
-----------------------------

  number: something you count and compute with like 1, 0.1, 1.21e+20
  
  word: the thing you can look up in a dictionary and that is not a
  number as above. Normally must not contain whitespace (unless
  properly escaped).

Atomic messages
---------------

bang: 

  The most basic message is "bang". It consists of the single word
  "bang" for example written in a message box. There also is the
  object [bang] (short [b]) which transforms every incoming message to
  a "bang"-message.
  
  Anatomy of a bang-message:
    
    "bang"

float:

  A message that starts with the word "float" followed by exactly one
  number. The first word "float" is also called the selector.
  Examples "float 1", "float -3.4", "float 1.21e+20". A message that
  only contains a number internally gets the word "float" prepended.

  Anatomy of a float-message: 
  
    Selector: float (can be implied)
    
    Data:     one number

symbol: 

  A message that starts with the word "symbol" followed by (exactly)
  one word. The first word "symbol" is also called the selector. If
  there are more words or numbers following the data word, they get
  truncated to create a proper symbol-message.

  Anatomy of a symbol-message: 
  
    Selector: symbol (can not be implied!)
    
    Data:     one word
    
pointer:

  (Uhm, how to define a pointer?)

List-messages
-------------

The official text by Miller (list-help.pd): 

  Messages in Pd are somewhat artificially divided into two classes.
  First are data-holding messages (bang, float, symbol, list) which are
  the primary way of communicating between objects. Second is
  "everything else" (you could call them out-of-band messages or
  metamessages) that describe changes in configuration, read and write
  files, quit Pd, etc. These are provided so that complex objects don't
  need to have 100 separate inlets for every possible functionality.
  It's not clear whether this was a good design choice, but it's
  entrenched.

  The distinction becomes visible, and ugly, when the leading item in
  a data-holding message is a symbol (i.e. word, fbar). In this case,
  to disambiguate it from the other sort, the printed form of the
  message has a selector, "list" or "symbol" prepended to it.
  Underneath, there is always a selector in fromt of data messages,
  but it is implied if the first data item is a number.

My take: 

  A list-message is a multi-atom message starting with the word "list"
  as selector. List-messages can be generated using the [list] object
  family, which all convert their input to a list-message and - except
  [list trim] - they all output proper list-messages. [list trim]
  splits off the "list"-selector and just passes the data part of a
  list-message.

  List-messages with just one element are handled specially, see
  below.

  Anatomy of a list-message: 
  
    Selector: list (can be implied, if data starts with a number)

    Data:     normally at least two atoms (words, numbers or
              pointers). One-atom list-messages get converted to
              atomic messages, see below.
    
One-element lists
.................

  If a list-message's data contains zero or one elements, the message
  gets  converted to an atomic message according to the following
  rules:

  No data:     conversion to bang-message.
  One word:    conversion to symbol-message
  One number:  conversion to float message.
  One pointer: conversion to pointer

Implied list-selector
.....................

  A multi-atom message that starts with a number is a list-message,
  too. (Cf. implied selector in float-messages)

Meta-messages
-------------

  Messages that are neither pointers, float-, symbol-, list- nor
  bang-messages. 
  
  Meta-messages can be generated out of atomic and list-messages using
  [list trim]. Many objects like [openpanel], [tabread] etc. accept
  and generate meta-message for administrative tasks. 

  Anatomy of a meta-message:

    Selector: a word that is not "float", "symbol", "list" nor "bang"

    Data:     anything

  Examples: 

    "reset"
    "read -resize soundfile.wav"
    "set pd-%s"
    "/synth/param 0 127"

Hey, that was a short definition of meta-messages. 

Ciao
-- 
 Frank Barknecht                 _ ______footils.org_ __goto10.org__




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