Smalltalk

There are a large number of Smalltalk variants.[8] The unqualified word Smalltalk is often used to indicate the Smalltalk-80 language and compatible VM, the first version to be made publicly available and created in 1980. The first hardware-environments which ran the Smalltalk VMs were Xerox Alto computers.

Smalltalk was the product of research led by Alan Kay at Xerox Palo Alto Research Center (PARC); Alan Kay designed most of the early Smalltalk versions, Adele Goldberg wrote most of the documentation, and Dan Ingalls implemented most of the early versions. The first version, termed Smalltalk-71, was created by Kay in a few mornings on a bet that a programming language based on the idea of message passing inspired by Simula could be implemented in "a page of code".[4] A later variant used for research work is now termed Smalltalk-72 and influenced the development of the Actor model. Its syntax and execution model were very different from modern Smalltalk variants.

After significant revisions which froze some aspects of execution semantics to gain performance (by adopting a Simula-like class inheritance model of execution), Smalltalk-76 was created. This system had a development environment featuring most of the now familiar tools, including a class library code browser/editor. Smalltalk-80 added metaclasses, 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 to create instances).

Smalltalk-80 was the first language variant made available outside of PARC. In 1981 it was shared with Tektronix, Hewlett-Packard, Apple Computer, and DEC for review and debugging on their platforms.[9] [10] The August 1981 issue of Byte Magazine was devoted to Smalltalk-80 and brought its ideas to a large audience. Several books on Smalltalk-80 were also published. Smalltalk-80 became the basis for all future commercial versions of Smalltalk.[11] The final release of Smalltalk-80 Version 1 was in November 1981.[12] Xerox only distributed Version 1 to Apple, DEC, HP, and Tektronix, but these companies were allowed unrestricted redistribution via any system they built. This encouraged the wide spread of Smalltalk.[13] Later, in 1983, Xerox released Smalltalk-80 Version 2. This version was generally available to the public, although under a restrictive license. Versions 1 and 2 were fairly similar, although Version 2 did have some added features such as a spelling corrector. Each release consisted of a virtual image (platform-independent file with object definitions) and a virtual machine specification.[13]

ANSI Smalltalk has been the standard language reference since 1998.[14] Two currently popular Smalltalk implementation variants are descendants of those original Smalltalk-80 images. Squeak is an open source implementation derived from Smalltalk-80 Version 1 by way of Apple Smalltalk. VisualWorks 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 2001 Vassili Bykov implemented Hobbes, a virtual machine running Smalltalk-80 inside VisualWorks.[15] (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 focused on Intel-based PCs running Microsoft Windows or IBM's OS/2. Both firms struggled to take Smalltalk mainstream due to Smalltalk's substantial memory needs, limited run-time performance, and initial lack of supported connectivity to SQL-based relational database servers. While the high price of 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. IBM initially supported the Digitalk product, but then 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 far more popular in Europe, as IBM introduced it into IT shops before 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: OBJS) was traded publicly until 1999, when it was delisted and dissolved. The merged firm never managed to find an effective response to Java as to market positioning, and by 1997 its owners were looking to sell the business. In 1999, Seagull Software acquired the ObjectShare Java development lab (including the original Smalltalk/V and Visual Smalltalk development team), and still owns VisualSmalltalk, although worldwide distribution rights for the Smalltalk product remained with ObjectShare who then sold them to Cincom.[16] VisualWorks was sold to Cincom and is now part of Cincom Smalltalk. Cincom has backed Smalltalk strongly, releasing multiple new versions of VisualWorks and ObjectStudio each year since 1999.

Cincom, GemTalk, and Instantiations, continue to sell Smalltalk environments. IBM has 'end of life'd VisualAge Smalltalk having in the late 1990s decided to back Java instead and it is, as of 2005, supported by Instantiations, Inc.[17] who renamed the product VA Smalltalk (VAST Platform) and continue to release new versions yearly. The open Squeak 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, and the Open Cobalt virtual world application. GNU Smalltalk is a free software implementation of a derivative of Smalltalk-80 from the GNU project. Pharo Smalltalk is a fork of Squeak oriented toward research and use in commercial environments.

A significant development, that has spread across all Smalltalk environments as of 2016, is the increasing usage of two web frameworks, Seaside and AIDA/Web, to simplify the building of complex web applications. Seaside has seen considerable market interest with Cincom, Gemstone, and Instantiations incorporating and extending it.

Smalltalk was one of many object-oriented programming languages based on Simula.[18] Smalltalk is also one of the most influential programming languages. Virtually all of the object-oriented languages that came after—Flavors,[19] CLOS, Objective-C, Java, Python, Ruby,[20] and many others—were influenced by Smalltalk. Smalltalk was also one of the most popular languages for agile software development methods, rapid application development (RAD) or prototyping, and software design patterns.[21] The highly productive environment provided by Smalltalk platforms made them ideal for rapid, iterative development.

Smalltalk emerged from a larger program of Advanced Research Projects Agency (ARPA) funded research that in many ways defined the modern world of computing. In addition to Smalltalk, working prototypes of things such as hypertext, GUIs, multimedia, the mouse, telepresence, and the Internet were developed by ARPA researchers in the 1960s.[22][23] Alan Kay (one of the inventors of Smalltalk) also described a tablet computer he called the Dynabook which resembles modern tablet computers like the iPad.[4]

Smalltalk environments were often the first to develop what are now common object-oriented software design patterns. One of the most popular is the model–view–controller (MVC) pattern for user interface design. The MVC pattern enables developers to have multiple consistent views of the same underlying data. It's ideal for software development environments, where there are various views (e.g., entity-relation, dataflow, object model, etc.) of the same underlying specification. Also, for simulations or games where the underlying model may be viewed from various angles and levels of abstraction.[24]

In addition to the MVC pattern, the Smalltalk language and environment were highly influential in the history of the graphical user interface (GUI) and the what you see is what you get (WYSIWYG) user interface, font editors, and desktop metaphors for UI design. The powerful built-in debugging and object inspection tools that came with Smalltalk environments set the standard for all the integrated development environments, starting with Lisp Machine environments, that came after.[25]

Smalltalk-80: The Language and its Implementation, a.k.a. the "Blue book", an original book on the language

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 GUI's window class might declare that windows have properties such as the label, the position and whether the window is visible or not. The class might 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:

1. Hold state (references to other objects).
2. Receive a message from itself or another object.
3. 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 commented that despite the attention given to objects, messaging is the most important concept in Smalltalk: "The big idea is 'messaging'—that is what the kernel of Smalltalk/Squeak is all about (and it's something that was never quite completed in our Xerox PARC phase)."[26]

Unlike most other languages, Smalltalk objects can be modified while the system is running. Live coding and applying fixes 'on-the-fly' is a dominant programming methodology for Smalltalk and is one of the main reasons for its efficiency.

Smalltalk is a "pure" object-oriented programming language, meaning that, unlike C++ and Java, 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 or extend (through subclassing) 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 summarized in the commonly heard phrase "In Smalltalk everything is an object", which may be more accurately expressed as "all values are objects", as variables are not.

Since all values are objects, classes are also objects. Each class is an instance of the metaclass of that class. Metaclasses in turn are also objects, and are all instances of a class called Metaclass. Code blocks—Smalltalk's way of expressing anonymous functions—are also objects.[27]

Reflection is a term that computer scientists apply to software programs that have the ability to inspect their own structure, for example their parse tree or data types of input and output parameters. Reflection is a feature of dynamic, interactive languages such as Smalltalk and Lisp. Interactive programs with reflection (either interpreted or compiled) maintain the state of all in-memory objects, including the code object itself, which are generated during parsing/compilation and are programmatically accessible and modifiable.

Reflection is also a feature of having a meta-model as Smalltalk does. The meta-model is the model that describes the language, and developers can use the meta-model to do things like walk through, examine, and modify the parse tree of an object, or find all the instances of a certain kind of structure (e.g., all instances of the Method class in the meta-model).

Smalltalk-80 is a totally reflective system. Smalltalk-80 provides both structural and computational reflection. Smalltalk is a structurally reflective system which structure is defined by Smalltalk-80 objects. The classes and methods that define the system are also 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 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 and so on with generic code that applies to any object in the system.[28]

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 pseudo-variable (one of the six reserved words), 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 or Prolog-like back-tracking without modifying the virtual machine. The exception system is implemented using this facility. 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.[29]

An example of how Smalltalk can use reflection is the mechanism for handling errors. When an object is sent a message that it does not implement, the virtual machine sends the object the doesNotUnderstand: message with a reification 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 (a Notifier) 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.[30][31]

By creating a class that understands (implements) only doesNotUnderstand:, one can create an instance that can intercept any message sent to it via its doesNotUnderstand: method. Such instances are called transparent proxies.[32] Such proxies can then be used to implement a number of facilities such as distributed Smalltalk where messages are exchanged between multiple Smalltalk systems, database interfaces where objects are transparently faulted out of a database, promises, etc. The design of distributed Smalltalk influenced such systems as CORBA.

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 properly termed pseudo-variables, identifiers that follow the rules for variable identifiers but denote bindings that a programmer cannot change. The true, false, and nil pseudo-variables are singleton instances. self and super refer to the receiver of a message within a method activated in response to that message, but sends to super are looked up in the superclass of the method's defining class rather than the class of the receiver, which allows methods in subclasses to invoke methods of the same name in superclasses. thisContext refers to the current activation record. The only built-in language constructs are message sends, assignment, method return and literal syntax for some objects. From its origins as a language for children of all ages, 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 and is not hardwired into the language syntax.)

The adage that "Smalltalk syntax fits on a postcard" may have originated in Alan Kay's original conception of the language, as related by him in practically every of tens or hundreds of public lectures, op. cit., or perhaps it could refer to a code snippet by Ralph Johnson, demonstrating all the basic standard syntactic elements of methods:[33][34]

exampleWithNumber: x
    | y |
    true & false not & (nil isNil) ifFalse: [self halt].
    y := self size + super size.
    #($a #a 'a' 1 1.0)
        do: [ :each |
            Transcript show: (each class name);
                       show: ' '].
    ^x < y

42
-42
123.45
1.2345e2
2r10010010
16rA000

$A

'Hello, world!'

'I said, ''Hello, world!'' to them.'

'I said, "Hello, world!" to them.'

#'foo'

#foo

#(1 2 3 4)

#[1 2 3 4]

[... Some smalltalk code...]

| index |

| index vowels |

vowels := 'aeiou'

42 factorial

aRatherBigNumber := 42 factorial

2 raisedTo: 4

'hello world' indexOf: $o startingAt: 6

new Rectangle(100, 200);

Rectangle width: 100 height: 200

3 + 4

3 > 4

3 factorial + 4 factorial between: 10 and: 100

(3 factorial + 4) factorial between: 10 and: 100

3 + 4 * 5

3 + (4 * 5)

3 factorial factorial log

 | window |
  window := Window new.
  window label: 'Hello'.
  window open

  Window new
    label: 'Hello';
    open

[ :params | <message-expressions> ]

[:x | x + 1]

[:x | x + 1] value: 3

expr ifTrue: [statements to evaluate if expr] ifFalse: [statements to evaluate if not expr]

'''ifTrue:''' trueAlternativeBlock '''ifFalse:''' falseAlternativeBlock<br />
     ^trueAlternativeBlock value

'''ifTrue:''' trueAlternativeBlock '''ifFalse:''' falseAlternativeBlock<br />
    ^falseAlternativeBlock value

positiveAmounts := allAmounts select: [:anAmount | anAmount isPositive]

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 two subclasses of Boolean both implement ifTrue:, where the implementation in subclass True always evaluates the block and the implementation in subclass False never evaluates the block.

The following code demonstrates this:

result := a > b
    ifTrue:[ 'greater' ]
    ifFalse:[ 'less or equal' ]

Blocks are also used to implement user-defined control structures, enumerators, visitors, exception handling, 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 collisions |
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].

The exception handling mechanism uses blocks as handlers (similar to CLOS-style exception handling):

[
  some operation
] on:Error do:[:ex |
  handler-code
  ex return
]

The exception handler's "ex" argument provides access to the state of the suspended operation (stack frame, line-number, receiver and arguments etc.) and is also used to control how the computation is to proceed (by sending one of "ex proceed", "ex reject", "ex restart" or "ex return").

This is a stock class definition:[36]

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 a message to the Object class to create a subclass called MessagePublisher. In other words: classes are first-class objects in Smalltalk which can receive messages just like any other object and can be created dynamically at execution time.

publish
    Transcript show: 'Hello World!'

quadMultiply: i1 and: i2
    "This method multiplies the given numbers by each other and the result by 4."
    | mul |
    mul := i1 * i2.
    ^mul * 4

MessagePublisher new

publisher := MessagePublisher new

MessagePublisher new publish

The Hello world program is used by virtually all texts to new programming languages as the first program learned to show the most basic syntax and environment of the language. For Smalltalk, the program is extremely simple to write. 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.

Most popular programming systems separate static program code (in the form of class definitions, functions or procedures) from dynamic, or run time, program state (such as objects or other forms of program data). They load program code when a program starts, and any prior program state must be recreated explicitly from configuration files or other data sources. Any settings the program (and programmer) does not explicitly save must be set up again for each restart. A traditional program also loses much useful document information each time a program saves a file, quits, and reloads. This loses details such as undo history or cursor position. Image based systems don't force losing all that just because a computer is turned off, or an OS updates.

Many Smalltalk systems, however, do not differentiate between program data (objects) and code (classes). In fact, classes are objects. Thus, most Smalltalk systems store the entire program state (including both Class and non-Class objects) in an image file. The image can then be loaded by the Smalltalk virtual machine to restore a Smalltalk-like system to a prior state.[37] This was inspired by FLEX, a language created by Alan Kay and described in his M.Sc. thesis.[38]

Smalltalk images are similar to (restartable) core dumps and can provide the same functionality as core dumps, such as delayed or remote debugging with full access to the program state at the time of error.[39]

Other languages that model application code as a form of data, such as Lisp, often use image-based persistence as well (see EMACS, for example). This method of persistence is powerful for rapid development because all the development information (e.g. parse trees of the program) is saved which facilitates debugging.

However, it also has serious drawbacks as a true persistence mechanism. For one thing, developers may often want to hide implementation details and not make them available in a run time environment. For reasons of legality and maintenance, allowing anyone to modify a program at run time inevitably introduces complexity and potential errors that would not be possible with a compiled system that exposes no source code in the run time environment. Also, while the persistence mechanism is easy to use, it lacks the true persistence abilities needed for most multi-user systems. The most obvious is the ability to do transactions with multiple users accessing the same database in parallel.

Everything in Smalltalk-80, unless customised to avoid the possibility, is available for modification from within a running program. This means that, for example, the IDE can be changed in a running system without restarting it. In some implementations, the syntax of the language or the garbage collection implementation can also be changed on the fly. Even the statement true become: false is valid in Smalltalk, although executing it is not recommended except for demonstration purposes (see virtual machine, image-based persistence, and backups).

Smalltalk programs are usually compiled to bytecode, which is then interpreted by a virtual machine or dynamically translated into machine-native code.

• Objective-C
• GLASS (software bundle)
• Distributed Data Management Architecture

• Goldberg, Adele (December 1983). Smalltalk-80: The Interactive Programming Environment. Addison-Wesley. ISBN 0-201-11372-4.
• Goldberg, Adele; Kay, Alan, eds. (March 1976). Smalltalk-72 Instruction Manual (PDF). Palo Alto, California: Xerox Palo Alto Research Center. Retrieved 2011-11-11.
• Goldberg, Adele; Robson, David (May 1983). Smalltalk-80: The Language and its Implementation. Addison-Wesley. ISBN 0-201-11371-6.
• Goldberg, Adele; Robson, David (11 January 1989). Smalltalk 80: The Language. Addison-Wesley. ISBN 0-201-13688-0.
• Kay, Alan C. (March 1993). "The Early History of Smalltalk" (PDF). ACM SIGPLAN Notices. ACM. 28 (3): 69–95. doi:10.1145/155360.155364.
• Krasner, Glen, ed. (August 1983). Smalltalk-80: Bits of History, Words of Advice. Addison-Wesley. ISBN 0-201-11669-3.
• Nierstrasz, Oscar; Ducasse, Stéphane; Pollet, Damien; Black, Andrew P. (2009-10-07). Squeak by Example. Kehrsatz, Switzerland: Square Bracket Associates. ISBN 978-3-9523341-0-2.
• Nierstrasz, Oscar; Ducasse, Stéphane; Pollet, Damien; Black, Andrew P. (February 23, 2010). Pharo by Example. Kehrsatz, Switzerland: Square Bracket Publishing. ISBN 978-3-9523341-4-0. Archived from the original on October 21, 2009.
• Winston, Patrick Henry (September 3, 1997). On to Smalltalk. MIT, USA: Addison Wesley. ISBN 978-0-201-49827-1.
• Brauer, Johannes (2015). Programming Smalltalk – Object-Orientation from the Beginning. Springer. ISBN 978-3-658-06823-3.
• "Special Issue on Smalltalk". Byte. McGraw-Hill. 6 (8). August 1981. Retrieved 2013-10-18.
  • Goldberg, Adele (August 1981). "Introducing the Smalltalk-80 System". Byte. McGraw-Hill. 6 (8). Retrieved 2013-10-18.
  • Ingalls, Dan (August 1981). "Design Principles Behind Smalltalk". Byte. McGraw-Hill. 6 (8). Retrieved 2011-11-11.
  • Tesler, Larry (August 1981). "The Smalltalk Environment". Byte. McGraw-Hill. 6 (8). Retrieved 2016-05-02.

Wikibooks has a book on the topic of: Smalltalk Programming

• Free Online Smalltalk Books
• Cuis Smalltalk
• Pharo Smalltalk
• Squeak Smalltalk
• Cincom Smalltalk ObjectStudio
• Cincom Smalltalk VisualWorks
• Dolphin Smalltalk
• GNU Smalltalk
• Smalltalk/X
• StrongTalk
• Amber Smalltalk
• Redline Smalltalk
• Scarlet Smalltalk
• VA Smalltalk
• GemStone
• GLASS (GemStone, Linux, Apache, Seaside, and Smalltalk)
• Smalltalk MT
• Smalltalk-78 online emulator
• OpenSmalltalk cross-platform virtual machine for Squeak, Pharo, Cuis, and Newspeak
• Smalltalk-80 Bluebook implementations in C++: by dbanay and rochus-keller on GitHub