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This is Frank Barknecht's take on how more clearly define things in PdDefinitions:

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:

Any atomic element that is not a valid number is considered a word or symbol, though we avoid the term "symbol" in this context to not confuse it with Pd's "symbol-messages". This includes, but is not limited to the things you can look up in a dictionary. More specifically a word is a string sequence that does not contain whitespace (unless properly escaped), semicolons or commas and is not a functional representation of a number.

Important note about "exotic" words: It is possible to create words that at first glance look like a number with makefilename %d. Internally these are of type "A_SYMBOL" in Pd and they are not functional as real numbers (i.e. you cannot do math or count and compute with them), but you can use them as selectors. They are seldomly used, but you should be aware of their existence. Besides these digit-words some other exotic words include the symbols for non-printable characters like BACKSPACE or SPACE, which can be created by sending their ASCII-code numbers to makefilename %c

In the following all these are included when we talk about "words".

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 is a float-message as well, the selector "float" then is implied.

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-element 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 elements (words, numbers or pointers). One-element 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-element 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.



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