CSE 505 -- Smalltalk Intro

Background

The Learning Research Group at Xerox Palo Alto Research Center (PARC) developed Smalltalk as a language for the Dynabook, during the 70s and early 80s. The Dynabook was a yet-to-be-developed "laptop of the future." Alan Kay, Dan Ingalls, Adele Goldberg were key players in the development of Smalltalk. They selected simulation as the metaphor for programming the Dynabook. Smalltalk was influenced by Lisp, Simula, Ivan Sutherland's Sketchpad.

Characteristics of Smalltalk-72:

interpreted
extensible syntax: fred move up x inches
more object-oriented than Simula -- even integers are objects
'class' is an object (but a rather special one). No subclassing

Characteristics of Smalltalk-80 (now also known just as Smalltalk):

everything is an object, even classes, blocks (closures), activation records, etc.
classes can have subclasses
not statically typed
rich programming environment
definitely no longer a programming language for kids!

Squeak is an interesting and free version. Smalltalk is also a commercial product (ParcPlace/Digitalk, IBM Smalltalk). From the point of view of studying programming languages, it is an excellent example of a pure object-oriented language.

Smalltalk influenced the development of other object-oriented languages, such as C++, Objective C, CLOS, Java, and our own Cecil language, as well as others.

Language Overview

We'll now give a quick overview of the language, via a few small examples. Basic concepts:

objects
instances
classes
messages and methods

Syntax:

unary messages

    examples:   "new", "copy"
		Date today    Time now hours    
		Array new   someCollection copy

keyword messages

    examples:   new:,  at:,  at: put:
	        Array new: 10  
	        someArray at: 1 put: 54
	        anArray at: 1

binary messages -

    examples:   + - * /
                5 * 9,  3 + 2 * 5

Precedence:

unary>binary>keyword, left-to-right within same kind,
parentheses override

Question: What is the value of 3 + 2 * 5 ?

  25, NOT 13
  Use parens to get what you want: 3 + (2 * 5)

Note that we will very frequently be composing messages -- for example

Time now hours + 1

first sends the message now to the class Time, which returns the current time (an instance of Time). We then send this object the message hours, which returns an integer. Then we send the message + with the argument 1 to this integer, returning another integer (which will be the current hour plus 1).

Example 1: Stack

First we define a new class.

Object subclass: #Stack
   instanceVariableNames: 'anArray top'
   classVariableNames: ''
   poolDictionaries: ''

Now define some methods:


push: item
   top := top+1.
   anArray at: top put: item


pop | item | 
   item := anArray at: top.
   top := top-1.
   ^ item

setsize: n
   anArray := Array new: n.
   top := 0.

Some code to test the stack:


S := Stack new.
S setsize: 10.
S inspect.
S push: 'hi there'.
S push: 3.14159.
S pop

Adding error checking and growing:

push: item
      | save |
   top := top+1.
   top > anArray size ifTrue: 
      "anArray is about to overflow.  make a new array twice as big, and
       copy the old values into it"
      [save := anArray.
       anArray := Array new: 2*save size.
       1 to: save size do:
          [:k | anArray at: k put: (save at: k)]].
   anArray at: top put: item


pop | item | 
   self isEmpty ifTrue: [self error: 'trying to pop an empty stack'].
   item := anArray at: top.
   top := top-1.
   ^ item

isEmpty
   ^ top=0