Object-oriented programming is a programming paradigm that uses "Object_" and their interactions to design applications and computer programs.... , dynamically typed
Type system
In computer science, a type system may be defined as "a tractable syntactic method for proving the absence of certain program behaviors by classifying phrases according to the kinds of values they compute.".... , reflective
Reflection (computer science)
In computer science, reflection is the process by which a computer program can observe and modify its own structure and behaviour. The programming paradigm driven by reflection is called reflective programming.... programming
Computer programming
Computer programming is the process of writing, testing, debugging/troubleshooting, and maintaining the source code of computer programs. This source code is written in a programming language.... language
Programming language
A programming language is a machine-readable artificial language designed to express computations that can be performed by a machine, particularly a computer.... . Smalltalk was created as the language to underpin the "new world" of computing exemplified by "human–computer symbiosis." It was designed and created in part for education
Education
File:Inukshuk Monterrey 1.jpgEducation can be seen as a product or a process and considered in a broad sense or a technical sense. According to philosophy of education George F.... al use, more so for constructionist learning
Constructionist learning
Constructionist learning is inspired by constructivism theories of learning that propose that learning is an active process wherein learners are actively constructing mental models and theories of the world around them.... , at Xerox PARC by Alan Kay
Alan Kay
Alan Curtis Kay is an United States computer scientist, known for his early pioneering work on object-oriented programming and Window graphical user interface design.... , Dan Ingalls, Adele Goldberg
Adele Goldberg (computer scientist)
Adele Goldberg is a computer scientist who wrote or co-wrote books on the programming language Smalltalk-80. In the 1970s she worked for Xerox's PARC laboratory on the Xerox Alto.... , Ted Kaehler, Scott Wallace, and others during the 1970s.
The language was first generally released as Smalltalk-80 and has been widely used since.
Object-oriented programming is a programming paradigm that uses "Object_" and their interactions to design applications and computer programs.... , dynamically typed
Type system
In computer science, a type system may be defined as "a tractable syntactic method for proving the absence of certain program behaviors by classifying phrases according to the kinds of values they compute.".... , reflective
Reflection (computer science)
In computer science, reflection is the process by which a computer program can observe and modify its own structure and behaviour. The programming paradigm driven by reflection is called reflective programming.... programming
Computer programming
Computer programming is the process of writing, testing, debugging/troubleshooting, and maintaining the source code of computer programs. This source code is written in a programming language.... language
Programming language
A programming language is a machine-readable artificial language designed to express computations that can be performed by a machine, particularly a computer.... . Smalltalk was created as the language to underpin the "new world" of computing exemplified by "human–computer symbiosis." It was designed and created in part for education
Education
File:Inukshuk Monterrey 1.jpgEducation can be seen as a product or a process and considered in a broad sense or a technical sense. According to philosophy of education George F.... al use, more so for constructionist learning
Constructionist learning
Constructionist learning is inspired by constructivism theories of learning that propose that learning is an active process wherein learners are actively constructing mental models and theories of the world around them.... , at Xerox PARC by Alan Kay
Alan Kay
Alan Curtis Kay is an United States computer scientist, known for his early pioneering work on object-oriented programming and Window graphical user interface design.... , Dan Ingalls, Adele Goldberg
Adele Goldberg (computer scientist)
Adele Goldberg is a computer scientist who wrote or co-wrote books on the programming language Smalltalk-80. In the 1970s she worked for Xerox's PARC laboratory on the Xerox Alto.... , Ted Kaehler, Scott Wallace, and others during the 1970s.
The language was first generally released as Smalltalk-80 and has been widely used since. Smalltalk-like languages are in continuing active development, and have gathered loyal communities of users around them. was ratified in 1998 and represents the standard version of Smalltalk.
History
There are a large number of Smalltalk variants, as there are with many computer languages. The unqualified word Smalltalk is often used to indicate the Smalltalk-80 language, the first version to be made publicly available and created in 1980.
Smalltalk was the product of research by a group of researchers led by Alan Kay
Alan Kay
Alan Curtis Kay is an United States computer scientist, known for his early pioneering work on object-oriented programming and Window graphical user interface design.... at Xerox Palo Alto Research Center (PARC); Alan Kay designed most of the early Smalltalk versions, which Dan Ingalls implemented. The first version, known as Smalltalk-71, was created in a few mornings on a bet that a programming language based on the idea of message passing
Message passing
Message passing in computer science, is a form of communication used in parallel computing, object-oriented programming, and interprocess communication.... inspired by Simula
Simula
Simula is a name for two programming languages, Simula I and Simula 67, developed in the 1960s at the Norwegian Computing Center in Oslo, by Ole-Johan Dahl and Kristen Nygaard.... could be implemented in "a page of code." A later variant actually used for research work is now known as Smalltalk-72 and influenced the development of the Actor model
Actor model
In computer science, the Actor model is a mathematical model of concurrent computation that treats "actors" as the universal primitives of concurrent digital computation: in response to a message that it receives, an actor can make local decisions, create more actors, send more messages, and determine how to respond to the next message receiv... . Its syntax and execution model
Actor model
In computer science, the Actor model is a mathematical model of concurrent computation that treats "actors" as the universal primitives of concurrent digital computation: in response to a message that it receives, an actor can make local decisions, create more actors, send more messages, and determine how to respond to the next message receiv... were very different from modern Smalltalk variants.
After significant revisions which froze some aspects of execution semantics to gain performance (by adopting a Simula
Simula
Simula is a name for two programming languages, Simula I and Simula 67, developed in the 1960s at the Norwegian Computing Center in Oslo, by Ole-Johan Dahl and Kristen Nygaard.... -like class inheritance model of execution), Smalltalk-76 was created. This system had a development environment
Development environment
Development environment may refer to Integrated development environment.In hosted software development, Development environment refers to an server tier designated to a specific stage in a release process.... featuring most of the tools now familiar including a class library code browser/editor. Smalltalk-80 added metaclass
Metaclass
In object-oriented programming Computer programming, a metaclass is a Class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances.... es, to help maintain the "everything is an object" (except variables) paradigm by associating properties and behavior with individual classes, and even primitives such as integer and boolean values (for example, to support different ways of creating instances).
Smalltalk-80 was the first language variant made available outside of PARC, first as Smalltalk-80 Version 1, given to a small number of companies (Hewlett-Packard
Hewlett-Packard
The Hewlett-Packard Company , commonly referred to as HP, is a technology corporation headquartered in Palo Alto, California, United States.... , Apple Computer
Apple Computer
Apple Inc., formerly Apple Computer Inc., is an United States multinational corporation which designs and manufactures consumer electronics and software products.... , Tektronix
Tektronix
Tektronix, Inc. is a United States company best known for its test and measurement equipment such as oscilloscopes, logic analyzers, and video and mobile test protocol equipment.... , and DEC
Digital Equipment Corporation
Digital Equipment Corporation was a pioneering United States company in the computer industry. It is often referred to within the computing industry as DEC .... ) and universities (UC Berkeley) for "peer review" and implementation on their platforms. Later (in 1983) a general availability implementation, known as Smalltalk-80 Version 2, was released as an image (platform-independent file with object definitions) and a virtual machine
Virtual machine
In computer science, a virtual machine is a software implementation of a machine that executes programs like a real machine.Definitions... specification. ANSI Smalltalk has been the standard language reference since 1998.
Two of the currently popular Smalltalk implementation variants are descendants of those original Smalltalk-80 images. Squeak
Squeak
The Squeak programming language is a Smalltalk implementation, derived directly from Smalltalk-80 by a group at Apple Computer that included some of the original Smalltalk-80 developers.... is an open source
Open source
Open source is an approach to design, development, and distribution offering practical accessibility to a product's source . Some consider open source as one of various possible design approaches, while others consider it a critical Strategy element of their business operations.... implementation derived from Smalltalk-80 Version 1 by way of Apple Smalltalk. VisualWorks
VisualWorks
VisualWorks is one of the leading commercial implementations of the Smalltalk programming language and environment.The lineage of VisualWorks goes back to the first Smalltalk-80 implementation by Xerox PARC.... is derived from Smalltalk-80 version 2 by way of Smalltalk-80 2.5 and ObjectWorks (both products of ParcPlace Systems, a Xerox PARC spin-off company formed to bring Smalltalk to the market). As an interesting link between generations, in 2002 Vassili Bykov implemented Hobbes, a virtual machine running Smalltalk-80 inside VisualWorks. (Dan Ingalls later ported Hobbes to Squeak).
During the late 1980s to mid-1990s, Smalltalk environments — including support, training and add-ons — were sold by two competing organizations: ParcPlace Systems and Digitalk, both California based. ParcPlace Systems tended to focus on the Unix/Sun Microsystems market, while Digitalk emphasized Intel-based PCs that were running either Microsoft Windows or IBM's OS/2. Both companies, however, struggled to take Smalltalk mainstream due to Smalltalk's substantial memory footprint, limited run-time performance, and initially lack of supported connectivity to SQL
SQL
SQL is a database computer language designed for the retrieval and management of data in relational database management systems , database schema creation and modification, and database object access control management.... -based relational database servers. While the high price point of the ParcPlace Smalltalk limited its market penetration to mid-sized and large commercial organizations, the Digitalk products initially tried to reach a wider audience with a lower price point. IBM having initially supported the Digitalk product entered the market with a Smalltalk product in 1995 called VisualAge/Smalltalk. Easel introduced Enfin at this time on Windows and OS/2. Enfin became much more popular in Europe, as IBM introduced it into IT shops prior to their development of IBM Smalltalk (later VisualAge). Enfin was later acquired by Cincom Systems, and is now sold under the name ObjectStudio, and is part of the Cincom Smalltalk product suite.
In 1995, ParcPlace and Digitalk merged into ParcPlace-Digitalk and then rebranded in 1997 as ObjectShare, located in Irvine, CA. ObjectShare (NASDAQ
NASDAQ
The NASDAQ is an United States stock exchange. It is the largest Electronic trading screen-based Stock trading market in the United States.... : OBJS) was traded publicly until 1999, when it was delisted and dissolved. The merged company never managed to find an effective response to Java in terms of market positions and by 1997 its owners were looking to sell the business. In 1999, Seagull Software acquired the Java development lab of ObjectShare (original Smalltalk/V, Visual Smalltalk development team), and still owns VisualSmalltalk, although worldwide distribution rights for the Smalltalk product remained with ObjectShare. VisualWorks was sold to Cincom
Cincom
Cincom Systems is a privately held, multinational, computer technology corporation founded in 1968 by Thomas M. Nies, Tom Richley and Claude Bogardus.... and is now part of Cincom Smalltalk. Cincom has backed Smalltalk quite strongly, putting out multiple new releases of VisualWorks and ObjectStudio each year since 1999.
Cincom, Gemstone and Object Arts, plus other vendors continue to sell Smalltalk environments, IBM has 'end of life'd VisualAge Smalltalk having in the late 1990s decided to back Java and it is, as of 2006, supported by Instantiations
Instantiations
.Instantiations is a software company headquartered in Portland Oregon, USA. The company develops Java development tools for the Eclipse platform, including solutions for static code analysis, GUI building and testing , and performance analysis.... , Inc. which has renamed the product VASmalltalk and released a new version. The open Squeak
Squeak
The Squeak programming language is a Smalltalk implementation, derived directly from Smalltalk-80 by a group at Apple Computer that included some of the original Smalltalk-80 developers.... implementation has an active community of developers, including many of the original Smalltalk community, and has recently been used to provide the Etoys environment on the OLPC project, a toolkit for developing collaborative applications Croquet Project
Croquet Project
The Croquet Project is an international effort to promote the continued development of Croquet, a free software Computer software Platform and a Computer network operating system for developing and delivering deeply collaborative multi-user online Application software.... , and the Open Cobalt
Open Cobalt
Open Cobalt is a free and open source software virtual world browser and construction toolkit application software for accessing, creating, publishing, and hyperlinking Avatar -based multi-user virtual worlds that are accessible both on local area networks or across the Internet.... virtual world application. GNU Smalltalk
GNU Smalltalk
GNU Smalltalk is an implementation of the Smalltalk programming language by the GNU Project.The implementation, unlike other Smalltalk environments, uses text files for program input and interprets the contents as Smalltalk code.... is a free software
Free software
Free Software or software libre is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with minimal restrictions only to ensure that further recipients can also do these things and to prevent consumer-facing hardware... implementation of a derivative of Smalltalk-80 from the GNU
GNU
GNU is a computer operating system composed entirely of free software. Its name is a recursive acronym for GNU's Not Unix; it was chosen because its design is Unix-like, but differs from Unix by being free software and containing no Unix code.... project.
A significant development, that has spread across all current Smalltalk environments, is the increasing usage of the Seaside and AIDA/Web
AIDA/Web
AIDA/Web is an object-oriented, open source Smalltalk web application server using the model-view-controller architectural pattern. First developed in 1996 by Janko Mivsek, AIDA/Web was designed to integrate the quite distinct object and web philosophies.... web frameworks to simplify the building of complex web applications.
Influences
Smalltalk has influenced the wider world of computer programming in four main areas. It inspired the syntax and semantics of other computer programming languages. Secondly, it was a prototype for a model of computation
Model of computation
In computability theory and computational complexity theory, a model of computation is the definition of the set of allowable operations used in computation and their respective costs.... known as message passing
Message passing
Message passing in computer science, is a form of communication used in parallel computing, object-oriented programming, and interprocess communication.... . Thirdly, its WIMP
WIMP (computing)
In human?computer interaction, WIMP stands for "Window , Icon , Menu , pointing device", denoting a style of interaction using these elements. It was coined by Merzouga Wilberts in 1980.... GUI
Gui
Gui or guee is a generic term to refer to grillinged dishes in Korean cuisine. These most commonly have meat or fish as their primary ingredient, but may in some cases also comprise grilled vegetables or other vegetarian ingredients.... inspired the windowing environments of personal computers in the late twentieth and early twenty-first centuries, so much so that the windows of the first Macintosh
Macintosh
File:Imac alu.pngMacintosh, commonly shortened to Mac, is a brand name which covers several lines of personal computers designed, developed, and marketed by Apple Inc.... desktop look almost identical to the MVC windows of Smalltalk-80. Finally, the integrated development environment
Integrated development environment
An integrated development environment also known as integrated design environment or integrated debugging environment is a software application that provides comprehensive facilities to computer programmers for software development.... was the model for a generation of visual programming tools that look like Smalltalk's code browsers and debuggers.
Smalltalk laid the groundwork and proved all the principles that led to the development and commercial success of Java.
Python is a general-purpose high-level programming language. Its design philosophy emphasizes code readability. Python's core syntax and semantics are Minimalism , while the standard library is large and comprehensive.... and Ruby
Ruby (programming language)
Ruby is a dynamic programming language, reflection , general purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features.... have reimplemented some Smalltalk ideas with an environment similar to that of Awk or Perl
Perl
In computer programming, Perl is a high-level programming language, List of programming languages by category, Interpreter , dynamic programming language.... . The Smalltalk "metamodel" also serves as the inspiration for the object model design of Perl 6
Perl 6
Perl 6 is a planned major revision to the Perl programming language. It is a language specification which introduces elements of many modern and historical languages.... .
The syntax and runtime behaviour of the Objective-C
Objective-C
Objective-C is a Reflection , Object-oriented programming programming language which adds Smalltalk-style message passing to C .Today it is used primarily on Mac OS X, iPhone OS, and GNUstep, three environments based on the OpenStep standard, and is the primary language used for the NEXTSTEP, OpenStep#OPENSTEP, and Cocoa application framew... programming language is strongly influenced by Smalltalk. Objective-C is the preferred language for software development on Mac OS X
Mac OS X
Mac OS X is a line of computer operating systems developed, marketed, and sold by Apple Inc., and since 2002 has been included with all new Macintosh computer systems.... .
There is also a modular Smalltalk-like implementation designed for scripting called S#
Script.NET
Script.NET or S# is a metaprogramming language that provides scripting functionality in Microsoft .NET applications, allowing runtime execution of custom functionality, similar to VBA in Microsoft Office applications.... , or Script.NET. S# uses just-in-time compilation
Just-in-time compilation
In computing, just-in-time compilation , also known as dynamic translation, is a technique for improving the runtime performance of a computer program.... technology and supports an extended Smalltalk-like language written by David Simmons of Smallscript Corp.
Object-oriented programming
As in other object-oriented languages, the central concept in Smalltalk-80 (but not in Smalltalk-72) is that of an object. An object is always an instance of a class. Classes are "blueprints" that describe the properties and behavior of their instances. For example, a Window class would declare that windows have properties such as the label, the position and whether the window is visible or not. The class would also declare that instances support operations such as opening, closing, moving and hiding. Each particular Window object would have its own values of those properties, and each of them would be able to perform operations defined by its class.
A Smalltalk object can do exactly three things:
Hold state (references to other objects).
Receive a message from itself or another object.
In the course of processing a message, send messages to itself or another object.
The state an object holds is always private to that object. Other objects can query or change that state only by sending requests (messages) to the object to do so. Any message can be sent to any object: when a message is received, the receiver determines whether that message is appropriate. (Alan Kay has that despite the attention given to objects, messaging is the most important concept in Smalltalk.)
Smalltalk is a "pure" object-oriented programming language, meaning that, unlike Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java .... and C++
C++
C++ is a general-purpose programming language. It is regarded as a middle-level language, as it comprises a combination of both high-level programming language and low-level programming language language features.... , there is no difference between values which are objects and values which are primitive types. In Smalltalk, primitive values such as integers, booleans and characters are also objects, in the sense that they are instances of corresponding classes, and operations on them are invoked by sending messages. A programmer can change the classes that implement primitive values, so that new behavior can be defined for their instances--for example, to implement new control structures--or even so that their existing behavior will be changed. This fact is summarised in the commonly heard phrase "In Smalltalk everything is an object" (which would more accurately be expressed as "all values are objects", as variables aren't).
In object-oriented programming, a class is a programming language construct that is used as a blueprint to create Object s. This blueprint includes Attribute s and Method s that the created objects all share.... themselves are also objects. Each class is an instance of the metaclass
Metaclass
In object-oriented programming Computer programming, a metaclass is a Class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances.... of that class. Metaclasses in turn are also objects, and are all instances of a class called Metaclass. Code blocks are also objects.
In computer science, reflection is the process by which a computer program can observe and modify its own structure and behaviour. The programming paradigm driven by reflection is called reflective programming.... system, implemented in Smalltalk-80 itself. Smalltalk-80 provides both structural and computational reflection. Smalltalk is a structurally reflective system whose structure is defined by Smalltalk-80 objects. The classes and methods that define the system are themselves objects and fully part of the system that they help define. The Smalltalk compiler compiles textual source code into method objects, typically instances of CompiledMethod. These get added to classes by storing them in a class's method dictionary. The part of the class hierarchy that defines classes can add new classes to the system. The system is extended by running Smalltalk-80 code that creates or defines classes and methods. In this way a Smalltalk-80 system is a "living" system, carrying around the ability to extend itself at run time.
Since the classes are themselves objects, they can be asked questions such as "what methods do you implement?" or "what fields/slots/instance variables do you define?". So objects can easily be inspected, copied, (de)serialized
Serialization
In computer science, in the context of data storage and transmission, serialization is the process of converting an object into a sequence of bits so that it can be stored on a storage medium or transmitted across a computer network connection link.... and so on with generic code that applies to any object in the system.
Smalltalk-80 also provides computational reflection, the ability to observe the computational state of the system. In languages derived from the original Smalltalk-80 the current activation of a method is accessible as an object named via a keyword, thisContext. By sending messages to thisContext a method activation can ask questions like "who sent this message to me". These facilities make it possible to implement co-routines
Coroutine
In computer science, coroutines are program components that generalize subroutines to allow multiple entry points for suspending and resuming of execution at certain locations.... or Prolog
Prolog
Prolog is a logic programming language. It is a general purpose language often associated with artificial intelligence and computational linguistics.... -like back-tracking without modifying the virtual machine. One of the more interesting uses of this is in the Seaside web framework which relieves the programmer of dealing with the complexity of a Web Browser's back button by storing continuations for each edited page and switching between them as the user navigates a web site. Programming the web server using Seaside can then be done using a more conventional programming style.
When an object is sent a message that it does not implement, the virtual machine sends the object the doesNotUnderstand: message with a reification
Reification (computer science)
Reification is a process through which a computable/addressable object - a resource - is created in a system, as a proxy for a non computable/addressable object.... of the message as an argument. The message (another object, an instance of Message) contains the selector of the message and an Array of its arguments. In an interactive Smalltalk system the default implementation of doesNotUnderstand: is one that opens an error window reporting the error to the user. Through this and the reflective facilities the user can examine the context in which the error occurred, redefine the offending code, and continue, all within the system, using Smalltalk-80's reflective facilities.
Yet another important use of doesNotUnderstand: is intercession. One can create a class that does not define any methods other than doesNotUnderstand: and does not inherit from any other class. The instances of this class effectively understand no messages. So every time a message is sent to these instances they actually get sent doesNotUnderstand:, hence they intercede in the message sending process. Such objects are called proxies. By implementing doesNotUnderstand: appropriately, one can create distributed systems where proxies forward messages across a network to other Smalltalk systems (a facility common in systems like CORBA
Common Object Request Broker Architecture
The Common Object Requesting Broker Architecture is a Standardization defined by the Object Management Group that enables software componentry written in multiple programming language and running on multiple computers to work together.... , COM+
Component Object Model
Component Object Model is an interface standard for software componentry introduced by Microsoft in 1993. It is used to enable interprocess communication and dynamic object creation in a large range of programming languages.... and RMI
Java remote method invocation
The Java Remote Method Invocation Application Programming Interface, or Java RMI, a Java application programming interface, performs the object-oriented equivalent of remote procedure calls.... but first pioneered in Smalltalk-80 in the 1980s), and persistent systems where changes in state are written to a database
Database
A database is a structured collection of records or data that is stored in a computer system. The structure is achieved by organizing the data according to a database model.... and the like. An example of this latter is Logic Arts' VOSS (Virtual Object Storage System) available for VA Smalltalk under dual open source
Open source
Open source is an approach to design, development, and distribution offering practical accessibility to a product's source . Some consider open source as one of various possible design approaches, while others consider it a critical Strategy element of their business operations.... and commercial licensing.
Syntax
Smalltalk-80 syntax is rather minimalist, based on only a handful of declarations and reserved words. In fact, only six keywords are reserved in Smalltalk: true, false, nil, self, super and thisContext. (These are not actually keywords, but pseudo-variables. The true, false, and nil pseudo-variables are singleton
Singleton pattern
In software engineering, the singleton pattern is a design pattern that is used to restrict Instantiation of a class to one object-oriented programming.... instances. Smalltalk does not really define any keywords.) The only built-in language constructs are message sends, assignment, method return and literal syntax for some objects. From its origins as a teaching language, standard Smalltalk syntax uses punctuation in a manner more like English than mainstream coding languages. The remainder of the language, including control structures for conditional evaluation and iteration, is implemented on top of the built-in constructs by the standard Smalltalk class library. (For performance reasons implementations may recognize and treat as special some of those messages; however, this is only an optimization, not hardwired into the language syntax).
Literals
The following examples illustrate the most common objects which can be written as literal values in Smalltalk-80 methods.
Numbers. The following list illustrates some of the possibilities.
42
-42
123.45
1.2345e2
2r10010010
16rA000
The last two entries are a binary and a hexadecimal number, respectively. The number before the 'r' is the radix
Radix
In numeral system, the base or radix is usually the number of unique Numerical digit, including zero, that a Positional notation numeral system uses to represent numbers.... or base. The base does not have to be a power of two; for example 36rSMALLTALK is a valid number (for the curious, equal to 80738163270632 decimal).
Characters are written by preceding them with a dollar sign:
$A
Strings are sequences of characters enclosed in single quotes:
'Hello, world!'
To include a quote in a string, escape it using a second quote:
'I said, Hello, world! to them.'
Double quotes do not need escaping, since single quotes delimit a string:
'I said, "Hello, world!" to them.'
Two equal strings (strings are equal if they contain all the same characters) can be different objects residing in different places in memory. In addition to strings, Smalltalk has a class of character sequence objects called Symbol. Symbols are guaranteed to be unique--there can be no two equal symbols which are different objects. Because of that, symbols are very cheap to compare and are often used for language artifacts such as message selectors (see below).
Symbols are written as # followed by characters. For example:
#foo
Arrays:
#(1 2 3 4)
defines an array of four integers.
And last but not least, blocks (anonymous function literals)
[... Some smalltalk code...]
Blocks are explained in detail further in the text.
Many Smalltalk dialects implement additional syntaxes for other objects, but the ones above are the bread and butter supported by all.
Variable declarations
The two kinds of variable commonly used in Smalltalk are instance variables and temporary variables. Other variables and related terminology depend on the particular implementation. For example, VisualWorks
VisualWorks
VisualWorks is one of the leading commercial implementations of the Smalltalk programming language and environment.The lineage of VisualWorks goes back to the first Smalltalk-80 implementation by Xerox PARC.... has class shared variables and namespace shared variables, while Squeak
Squeak
The Squeak programming language is a Smalltalk implementation, derived directly from Smalltalk-80 by a group at Apple Computer that included some of the original Smalltalk-80 developers.... and many other implementations have class variables, pool variables and global variables.
Temporary variable declarations in Smalltalk are variables declared inside a method (see below). They are declared at the top of the method as names separated by spaces and enclosed by vertical bars. For example:
| index |
declares a temporary variable named index. Multiple variables may be declared within one set of bars:
| index vowels |
declares two variables: index and vowels.
Assignment
A variable is assigned a value via the ':=' syntax. So:
vowels := 'aeiou'
Assigns the string 'aeiou' to the previously declared vowels variable. The string is an object (a sequence of characters between single quotes is the syntax for literal strings), created by the compiler at compile time.
In the original Parc Place image, the glyph of the underscore character (_) appeared as a left-facing arrow. Smalltalk originally accepted this left-arrow as the only assignment operator. Some modern code still contains what appear to be underscores acting as assignments, harking back to this original usage. Most modern Smalltalk implementations accept either the underscore or the colon-equals syntax.
Messages
The message is the most fundamental language construct in Smalltalk. Even control structures are implemented as message sends. Smalltalk adopts by default a synchronous
Asynchronous method dispatch
Asynchronous method dispatch is a data communication method used when there is a need for the server side to handle a large number of long lasting client requests.... , single dynamic message dispatch
Dynamic dispatch
In computer science, dynamic dispatch is the process of mapping a Message passing to a specific sequence of code at runtime. This is done to support the cases where the appropriate method cannot be determined at compile-time .... strategy (as contrasted to a synchronous, multiple dispatch
Multiple dispatch
Multiple dispatch or multimethods is the feature of some object-oriented programming languages in which a function or method can be dynamically dispatched based on the run time type of more than one of its arguments.... strategy adopted by some other object-oriented languages).
The following example sends the message 'factorial' to number 42:
42 factorial
In this situation 42 is called the message receiver, while 'factorial' is the message selector. The receiver responds to the message by returning a value (presumably in this case a factorial of 42). Among other things, the result of the message can be assigned to a variable:
aRatherBigNumber := 42 factorial
"factorial" above is what is called a unary message because only one object, the receiver, is involved. Messages can carry additional objects as arguments, as follows:
2 raisedTo: 4
In this expression two objects are involved: 2 as the receiver and 4 as the message argument. The message result, or in Smalltalk parlance, the answer is supposed to be 16. Such messages are called keyword messages. A message can have more arguments, using the following syntax:
'hello world' indexOf: $o startingAt: 6
which answers the index of character 'o' in the receiver string, starting the search from index 6. The selector of this message is "indexOf:startingAt:", consisting of two pieces, or keywords.
Such interleaving of keywords and arguments is meant to improve readability of code, since arguments are explained by their preceding keywords. For example, an expression to create a rectangle using a C++ or Java-like syntax might be written as:
new Rectangle(100, 200);
It's unclear which argument is which. By contrast, in Smalltalk, this code would be written as:
Rectangle width: 100 height: 200
The receiver in this case is "Rectangle", a class, and the answer will be a new instance of the class with the specified width and height.
Finally, most of the special (non-alphabetic) characters can be used as what are called binary messages. These allow mathematical and logical operators to be written in their traditional form:
3 + 4
which sends the message "+" to the receiver 3 with 4 passed as the argument (the answer of which will be 7). Similarly,
3 > 4
is the message ">" sent to 3 with argument 4 (the answer of which will be false).
Notice, that the Smalltalk-80 language itself does not imply the meaning of those operators. The outcome of the above is only defined by how the receiver of the message (in this case a Number instance) responds to messages "+" and ">".
A side effect of this mechanism is operator overloading. A message ">" can also be understood by other objects, allowing the use of expressions of the form "a > b" to compare them.
Expressions
An expression can include multiple message sends. In this case expressions are parsed according to a simple order of precedence. Unary messages have the highest precedence, followed by binary messages, followed by keyword messages. For example:
3 factorial + 4 factorial between: 10 and: 100
is evaluated as follows:
3 receives the message "factorial" and answers 6
4 receives the message "factorial" and answers 24
6 receives the message "+" with 24 as the argument and answers 30
30 receives the message "between:and:" with 10 and 100 as arguments and answers true
The answer of the last message send is the result of the entire expression.
Parentheses can alter the order of evaluation when needed. For example,
(3 factorial + 4) factorial between: 10 and: 100
will change the meaning so that the expression first computes "3 factorial + 4" yielding 10. That 10 then receives the second "factorial" message, yielding 3628800. 3628800 then receives "between:and:", answering false.
Note that because the meaning of binary messages is not hardwired into Smalltalk-80 syntax, all of them are considered to have equal precedence and are evaluated simply from left to right. Because of this, the meaning of Smalltalk expressions using binary messages can be different from their "traditional" interpretation:
3 + 4 * 5
is evaluated as "(3 + 4) * 5", producing 35.
Unary messages can be chained by writing them one after another:
3 factorial factorial log
which sends "factorial" to 3, then "factorial" to the result (6), then "log" to the result (720), producing the result 2.85733.
A series of expressions can be written as in the following (hypothetical) example, each ending with a period. This example first creates a new instance of class Window, stores it in a variable, and then sends two messages to it.
If a series of messages are sent to the same receiver as in the example above, they can also be written as a cascade with individual messages separated by semicolons:
(Window new)
label: 'Hello';
open
This rewrite of the earlier example as a single expression avoids the need to store the new window in a temporary variable. According to the usual precedence rules, the unary message "new" is sent first, and then "label:" and "open" are sent to the answer of "new".
Code blocks
A block of code (an anonymous function) can be expressed as a literal value (which is an object, since all values are objects.) This is achieved with square brackets:
[ :params | ]
Where :params is the list of parameters the code can take. This means that the Smalltalk code:
In mathematical logic and computer science, lambda calculus, also written as ?-calculus, is a formal system designed to investigate function definition, function application and recursion.... ):
In computer science, a closure is a function that is evaluated in an environment containing one or more bound variables. When called, the function can access these variables.... . It can (at any time) access the variables of its enclosing lexical scopes. Blocks are first class objects. That is, references to blocks can be passed as arguments, returned as values, or stored as a state, just like any other objects. Blocks can be asked to execute their code by sending them a "value"-message (with one argument for each parameter in the block).
The literal representation of blocks was an innovation which allowed certain code to be significantly more readable; it allowed algorithms involving iteration to be coded in a clear and concise way. Code that would typically be written with loops in some languages can be written concisely in Smalltalk using blocks, sometimes in a single line.
positiveAmounts := allAmounts select: [:amt | amt isPositive]
In computer science, functional programming is a programming paradigm that treats computation as the evaluation of function s and avoids program state and immutable object data.... , wherein patterns of computation (here selection) are abstracted into higher-order function
Higher-order function
In mathematics and computer science, higher-order functions or functional are function s which do at least one of the following:*take one or more functions as an input... s. For example, the message select: on a Collection is equivalent to the higher-order function filter
Filter (higher-order function)
In functional programming, filter is a higher-order function that processes a data structure in some order to produce a new data structure containing exactly those elements of the original data structure for which a given predicate returns the boolean value true.... on an appropriate functor.
Control structures
Control structures do not have special syntax in Smalltalk. They are instead implemented as messages sent to objects. For example, conditional execution is implemented by sending the message ifTrue: to a Boolean object, passing as an argument the block of code to be executed if and only if the Boolean receiver is true.
The following code demonstrates this:
result := a > b
ifTrue:[ 'greater' ]
ifFalse:[ 'less' ]
Blocks are also used to implement user-defined control structures, enumerators, visitors, pluggable behavior and many other patterns.
For example:
| aString vowels |
aString := 'This is a string'.
vowels := aString select: [:aCharacter | aCharacter isVowel].
In the last line, the string is sent the message select: with an argument that is a code block literal. The code block literal will be used as a predicate function that should answer true if and only if an element of the String should be included in the Collection of characters that satisfy the test represented by the code block that is the argument to the "select:" message.
A String object responds to the "select:" message by iterating through its members (by sending itself the message "do:"), evaluating the selection block ("aBlock") once with each character it contains as the argument. When evaluated (by being sent the message "value: each"), the selection block (referenced by the parameter "aBlock", and defined by the block literal "[:aCharacter | aCharacter isVowel]"), answers a boolean, which is then sent "ifTrue:". If the boolean is the object true, the character is added to a string to be returned.
Because the "select:" method is defined in the abstract class Collection, it can also be used like this:
| rectangles aPoint|
rectangles := OrderedCollection
with: (Rectangle left: 0 right: 10 top: 100 bottom: 200)
with: (Rectangle left: 10 right: 10 top: 110 bottom: 210).
aPoint := Point x: 20 y: 20.
collisions := rectangles select: [:aRect | aRect containsPoint: aPoint].
Classes
This is a stock class definition:
Object subclass: #MessagePublisher
instanceVariableNames:
classVariableNames:
poolDictionaries:
category: 'Smalltalk Examples'
Often, most of this definition will be filled in by the environment. Notice that this is actually a message to the "Object"-class to create a subclass called "MessagePublisher". In other words: classes are first-class object
First-class object
In computing, a first-class object , in the context of a particular programming language, is an entity which can be used in programs without restriction .... s in Smalltalk which can receive messages just like any other object and can be created dynamically at execution time.
Methods
When an object receives a message, a method matching the message name is invoked. The following code defines a method publish, and so defines what will happen when this object receives the 'publish' message.
publish
Transcript show: 'Hello, World!'
The following method demonstrates receiving multiple arguments and returning a value:
quadMultiply: i1 and: i2
"This method multiplies the given numbers by each other and the result by 4."
| mul |
mul := i1 * i2.
^mul * 4
The method's name is #quadMultiply:and:. The documentation is represented by a string, making it accessible from the program. Return value is specified with the ^ operator.
Note that objects are responsible for determining dynamically at runtime which method to execute in response to a message--while in many languages this may be (sometimes, or even always) determined statically at compile time.
Instantiating classes
The following code:
MessagePublisher new
creates (and returns) a new instance of the MessagePublisher class. This is typically assigned to a variable:
publisher := MessagePublisher new
However, it is also possible to send a message to a temporary, anonymous object:
MessagePublisher new publish
Hello World example
In the following code, the message "show:" is sent to the object "Transcript" with the String literal 'Hello, world!' as its argument. Invocation of the "show:" method causes the characters of its argument (the String literal 'Hello, world!') to be displayed in the transcript ("terminal") window.
Transcript show: 'Hello, world!'.
Note that a Transcript window would need to be open in order to see the results of this example.
Image-based persistence
Most popular programming systems separate program code (in the form of class definitions, functions or procedures) from program state (such as objects or other forms of application data). They load the program code when an application is started, and any previous application state has to be recreated explicitly from configuration files or other data sources. Any settings the application programmer doesn't explicitly save, you have to set back up whenever you restart. A traditional application also throws away a lot of useful document information every time you save a file, quit and reload. You lose details such as undo history or cursor position. Image based systems don't force you to scrap all that just because you need to turn off your computer, or update the OS.
Many Smalltalk systems, however, do not differentiate between application data (objects) and code (classes). In fact, classes are objects themselves. Therefore most Smalltalk systems store the entire application state (including both Class and non-Class objects) in an image
System image
A system image in computing is the state of a computer system or software system system stored in some Secondary storage form. The form of computer storage is often a Computer file.... file. The image can then be loaded by the Smalltalk virtual machine
Virtual machine
In computer science, a virtual machine is a software implementation of a machine that executes programs like a real machine.Definitions... to restore a Smalltalk-like system to a previous state. This was inspired by , a language created by Alan Kay
Alan Kay
Alan Curtis Kay is an United States computer scientist, known for his early pioneering work on object-oriented programming and Window graphical user interface design.... and described in his M.Sc. thesis.
Other languages that model application code as a form of data, such as Lisp
Lisp programming language
Lisp is a family of computer programming languages with a long history and a distinctive, fully parenthesized syntax. Originally specified in 1958, Lisp is the second-oldest high-level programming language in widespread use today; only Fortran is older.... , often use image-based persistence as well.
Smalltalk images are similar to (restartable) core dump
Core dump
In computing, a core dump consists of the recorded state of the working Computer storage of a computer program at a specific time, generally when the program has terminated abnormally .... s and can provide the same functionality as core dumps, such as delayed or remote debugging with full access to the application state at the time of error.
Level of access
Everything in Smalltalk-80 is available for modification from within a running program. This means that, for example, the IDE
Integrated development environment
An integrated development environment also known as integrated design environment or integrated debugging environment is a software application that provides comprehensive facilities to computer programmers for software development.... can be changed in a running system without restarting it. In some implementations, the syntax of the language or the garbage collection
Garbage collection (computer science)
In computer science, garbage collection is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage , or memory used by Object that will never be accessed or mutated again by the Application software.... implementation can also be changed on the fly
On the fly
Colloquial usageIn colloquial use, on the fly means something created when needed. The phrase is used: to explain that something wasn't planned ahead, or... . Even the statement true become: false is valid in Smalltalk, although executing it is not recommended.
Just-in-time compilation
Smalltalk programs are usually compiled to bytecode
Bytecode
Bytecode is a term which has been used to denote various forms of instruction sets designed for efficient execution by a software Interpreter as well as being suitable for further compilation into machine language.... , which is then interpreted by a virtual machine
Virtual machine
In computer science, a virtual machine is a software implementation of a machine that executes programs like a real machine.Definitions... or dynamically translated into machine-native code.
was planned as a commercial implementation of the Smalltalk language and development environment for Mac OS X.There has been no communication from the projects authors since early 2007 and no new versions have been made available since then....
VisualWorks is one of the leading commercial implementations of the Smalltalk programming language and environment.The lineage of VisualWorks goes back to the first Smalltalk-80 implementation by Xerox PARC.... by (, )
Dolphin Smalltalk, or "Dolphin" for short , is an implementation of the Smalltalk programming language by Object Arts, targeted at the Microsoft Windows platform....
F-Script is an object-oriented programming scripting language developed by Philippe Mougin. In a nutshell, F-Script is Smalltalk with support for Array programming....
GemStone is a proprietary application framework that was first available for Smalltalk as an object database.GemStone Systems was founded in 1982 as Servio Logic, and then became GemStone Systems, Inc in 1995....
GNU Smalltalk is an implementation of the Smalltalk programming language by the GNU Project.The implementation, unlike other Smalltalk environments, uses text files for program input and interprets the contents as Smalltalk code....
Little Smalltalk is a non-standard dialect of the Smalltalk programming language invented by Timothy Budd. It was originally described in the book: "A Little Smalltalk", Timothy Budd, Addison-Wesley, 1987, ISBN 0-201-10698-1....
Pocket Smalltalk is a Smalltalk environment that runs in Microsoft Microsoft Windows and cross compiler on the Palm Pilot platform.The resulting executables are usable on the Palm 3.5 platform and up.... runs on Palm Pilot
Potato is a Squeak interpreter written in Java, a direct derivative of JSqueak
Smalltalk MT is an implementation of the Smalltalk programming language created in 1994 to deal with some of the shortcomings of Smalltalk-80 style of implementations.... compiled Smalltalk for Windows
Smalltalk YX is an open source programming language. It's an implementation of the Smalltalk-80 standard.Syx is written in the C programming language and has the following purposes:...
The Squeak programming language is a Smalltalk implementation, derived directly from Smalltalk-80 by a group at Apple Computer that included some of the original Smalltalk-80 developers.... , an open source Smalltalk
StepTalk is the official GNUstep Scripting language framework that represents separate scriptable servers or applications as a single object environment.... (uses Smalltalk language atop an Objective-C
Objective-C
Objective-C is a Reflection , Object-oriented programming programming language which adds Smalltalk-style message passing to C .Today it is used primarily on Mac OS X, iPhone OS, and GNUstep, three environments based on the OpenStep standard, and is the primary language used for the NEXTSTEP, OpenStep#OPENSTEP, and Cocoa application framew... runtime)
Strongtalk is a Smalltalk environment with optional static typing support. Strongtalk can make some compile time checks, and offer "stronger" type-safety guarantees; this is the source of its name.... (for Windows, offers optional strong typing)
Susie: Scripting Using a Smalltalk Interpreter Engine
Visual Smalltalk Enterprise is a Smalltalk dialect that runs only on Microsoft Windows, and is the last in a long line of Smalltalk implementations first produced by Digitalk.... and family, including Smalltalk/V.
A byte is a basic unit of measurement of Computer storage in computer science. In many computer architectures it is a Byte addressing memory address space.... August 1981 Special Issue on Smalltalk
Alan Curtis Kay is an United States computer scientist, known for his early pioneering work on object-oriented programming and Window graphical user interface design.... ( and )
Smalltalk advocacy site.
New Little Smalltalk Version with an actively developed GUI.
is a Latin American group with a website in English to promote the Smalltalk technology
FAQ - http://smalltalk.awardspace.com
by Goldberg & Robson
: A non-profit organization which gathers both industrial and academics. Has various Smalltalk promotion activities including a yearly event since 1993.
by Dan Ingalls.
.
Smalltalk products and services.
Database Management for Smalltalk.
Ottawa Smalltalk Institute
Books
Free, printable PDF download or order a print-on-demand, softcover copy of the book about Squeak Smalltalk.
Permission obtained to make these books freely available. Over a dozen full texts scanned from print.
