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The Smalltalk Language
Last updated at 7:22 am UTC on 8 November 2006
This is chapter 2 of the Squeak Tutorial
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Doing some simple things in Smalltalk


Smalltalk is a highly dynamic language, you can often see the results directly. Let's open a Workspace window to start playing around: Open the world menu, then click on "open", then "Workspace". In a workspace, you can execute small programs, and mix that with comments and results.

Let's see how Smalltalk computes. Type this into the workspace:
2 + 2


Now run the code:

You actually executed a small program! This works not only in a workspace but nearly everywhere in Smalltalk where you can edit some text: see a small snippet of code, select it, "print it" (keyboard shortcut: cmd-p) and look at the result.

Now for something more interesting:
100 factorial

Type (or copy) this into a new line, select it and "Print it". You see the factorial of 100, i.e. the product 1 * 2 * 3 * ... * 99 * 100. As you can see, Smalltalk has no problems with handling large numbers.

Printing
27 / 6

gives (9/2), a fraction. Fractions are just another kind of numbers available in Smalltalk.

One of the Smalltalk maxims is "Everything is an object". You've seen some numbers, they are objects of course. But there are also window objects, text objects, rectangles, dates. So let's see some graphical objects. Type (or copy) this:
EllipseMorph new openInHand

and then select it, right click and choose "Do it". A yellow ellipse appears under your mouse pointer, you can move it across the screen. Click once to put it somewhere. You can later pick it up with the mouse and put it anywhere you like. The yellow ellipse is an example of a Morph, a graphical object that can react to mouse clicks, change its appearance, contain other morphs, and be scripted.

The Object Universe


Smalltalk lets you think about your program in terms of objects. Objects collaborate to solve your problem, they can "talk" to each other to make things happen. It turns out that it is very natural to model a problem and its solution in terms of communicating objects, because that is close to our natural way of thinking. The Smalltalk system is full of objects, and you have created yourself a few new ones in the previous examples. For example, 27 is an integer object, and executing 27 / 6 creates a new fraction object, 9/2.

Objects communicate by sending each other messages. When an object receives a message, it will decide what to do. It may change its state (like a window that closes itself) and / or return an answer which is another object. Let's look at our example, EllipseMorph new openInHand again, and interpret this in terms of objects and messages. EllipseMorph is a class object, that means it can create new objects and describes how they react to messages. EllipseMorph new sends the message new to the EllipseMorph class. The class creates a new EllipseMorph, and answers that. The new morph is still invisible. By sending our newborn EllipseMorph the message openInHand we make it appear under the mouse pointer. Now every time we move the mouse, the ellipse morph is sent a notification message and reacts by placing itself at the new mouse position. When we click, another message is sent to the ellipse morph that makes it stop following the mouse.

These reactions to mouse clicks and mouse moves are part of the ellipse morphs behaviour, all ellipse morphs react in the same way. The behaviour is specified by the EllipseMorph class object, if you wanted to find out how this is programmed, you would start by looking into the code of the EllipseMorph class.

Let us send some messages to a new ellipse morph. Execute ("Do it") the following line:
ball := EllipseMorph new openInHand

Now we have created a new ellipse object and given it the name "ball". From now on we can refer to it by that name. Let's ask our ball for its position. Print ("Print it" , cmd-p) this:
ball center

The answer is printed right next to the code, for me it is 876@233. We send the message center to the ball, and it responds with a point object (the x@y-notation is used for point objects). Now pick up the ball and move it somewhere else. Can you see how its center changes?

Let's give the ball a drop shadow:
ball addDropShadow

This message changes the appearance of the morph. In more general terms, the state of the object is changed in response to the message.

Move it to the left margin of the screen:
ball left: 50

This message is a bit more complicated. It tells the morph to put its left bound at the coordinate 50. left: 50 is a message with an argument, the integer 50. Again the state (the position) of the object is changed.

Change its color
ball color: Color random

These are two messages. Color random creates a new random color object which is then used as an argument when sending ball the color: message. In more detail: Color random means we send the message random to the class object Color. Color knows how to respond: it creates three random numbers and uses these as values for hue, saturation, and brightness of a new color object. Finally, it returns the new color object.

Congratulations! You have created new objects and changed them by sending them messages. Before we show you more interesting things we'll tell you about the syntax of Smalltalk, so you can read programs easily.

All the syntax in one short Smalltalk Program

Smalltalk is a very simple language, and nearly all of its syntax can be shown in a single example. Open the Transcript window (World menu / Open / Transcript). The Transcript window shows basic systems messages and it is a place where you can print your own messages.

Copy and paste this program into a Workspace: (World menu / open / Workspace) (How?)

"Prints the integers between 1 and 10, and a string stating whether each is even or odd"
1 to: 10 do: [:n |
	n even ifTrue: [
		Transcript show: n; show: ' is even'; cr]
	ifFalse: [
		Transcript show: n; show: ' is odd'; cr]
]


Now run the code:
You should see a printout of the numbers 1 through 10 and whether each is odd or even. This short example demonstrates nearly all of the language features of Smalltalk. Let us disect it from the inside out:

Objects and Messages


Here are some of the objects used in the example:

Here are some of the messages:

'Sending a message' means the same thing as 'Calling a method' in other languages. There are three types of messages in Smalltalk:

Notes about message syntax


Blocks

Smalltalk and Ruby are the only two languages that have the concept of a block. A block is an object that contains executable Smalltalk code. It is, at once, an object and code. It is very similar to lambda forms in Lisp. This means:
Thus flow control, which is a language construct in many languages, can be implemented as a message send in Smalltalk.

Here is the code to find the maximum of two integers, first in C, then in Smalltalk. First C:
if (a > b) max = a;
else max = b;

Now in Smalltalk:
a > b ifTrue: [ max := a ]
ifFalse: [ max := b ]

In C, the if ... else construct is built into the language; In Smalltalk, ifTrue:ifFalse: is just a message send, like any other action. This functionality is made possible by Blocks.

There are two types of blocks in Smalltalk:

Examining the simple program

You now have all the understanding you need to understand the simple example:
"Prints the integers between 1 and 10, and a string stating whether each is even or odd"
1 to: 10 do: [:n |
	n even ifTrue: [
		Transcript show: n; show: ' is even'; cr]
	ifFalse: [
		Transcript show: n; show: ' is odd'; cr]
]

Let us examine it line by line:

Summary


For a quick reference card on this and other Squeak coding, see the Terse guide to Squeak

Move this to the browsers and classes chapter

Each has a typical use in Smalltalk: