Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucsd!ames!uhccux!munnari.oz.au!murtoa.cs.mu.oz.au!charlie!aragorn!rad
From: rad@aragorn.cm.deakin.oz.au (Robert Alan Dew)
Newsgroups: comp.lang.smalltalk
Subject: Re: Smalltalk Types
Message-ID: <7776@charlie.OZ>
Date: 1 Sep 89 02:57:19 GMT
References: <56930@aerospace.AERO.ORG>
Sender: root@charlie.OZ
Reply-To: rad@aragorn.UUCP (Robert Alan Dew)
Organization: Department of Computing & Mathematics - Deakin University
Lines: 63

In article <56930@aerospace.AERO.ORG> abbott@itro3.aero.org writes:

>if an object has a response defined for a message sent to it, then it
>is of the "right type."  

Basically all objects produce a response to any message.  I guess you
want to exclude the doesNotUnderstand: response.

>In trying to understand what that means, I came up with the
>following.  I'm sure this must have been worked out properly somewhere.
>Does anyone know where?

Two suggestions:
 - forget about 'type'.
 - understand the following and there relationships
	class
	class instance
	message
	instance variable
	instance method
	class variable
	class method

	inheritance of
	  instance variables
	  instance methods
	  class variables
	  class methods
	
	self
	super

Anyway, if you want to think about types then class is the analogy.

>where a message is a message name, a list of the types of the
>parameters, and the type of the result, i.e.,
>
>Msg ::= Name(param1: Type1; param2: Type2; ..., paramn: Typen): Type

A message has:
 - a selector.  Which is a Symbol and does consist of the 'message names'.
			eg. #at:put:
 - arguments.  An Array containing the parameters.

A message does not return an object, therefore no result type is required.
For a better understanding separate messages from methods.
The method returns an object, the method is not restricted to returning
only one class of object.

Anyway, part of a system I am building includes a class where:
 - a variety of messages can be sent to instances of that class.
 - an instance of the class does not understand any of the messages, except
   for the doesNotUnderstand: and a few initialization messages.
 - the class instance was intended not to understand messages.  The
   response when an instance did not understand would be due to the
   method for doesNotUnderstand:. which was redefined in the class.

Robert Dew                                rad@aragorn.cm.deakin.oz
Department of Computing and Mathematics
Deakin University
Geelong 
Victoria
Australia 3217