2.2 Message send definitions

There are three kinds of messages in Cuis-Smalltalk:

• Unary messages take no argument.
  In 1 factorial the message #factorial is sent to the object 1.
• Binary messages take exactly one argument.
  In 1 + 2 the message #+ is sent to the object 1 with the argument 2.
• Keyword messages take an arbitrary number of arguments.
  In 2 raisedTo: 6 modulo: 10 the message consisting of the message selector #raisedTo:modulo: and the arguments 6 and 10 is sent to the object 2.

Unary message selectors consist of alphanumeric characters, and start with a lower case letter.

Binary message selectors consist of one or more characters from the following set:

+ - / \ * ~ < > = @ % | & ? ,

Keyword message selectors consist of a series of alphanumeric keywords, where each keyword starts with a lower-case letter and ends with a colon.

Unary messages have the highest precedence, then binary messages, and finally keyword messages, so:

2 raisedTo: 1 + 3 factorial
⇒ 128

First we send factorial to 3, then we send + 6 to 1, and finally we send raisedTo: 7 to 2.

Precedence aside, evaluation is strictly from left to right, so

1 + 2 * 3
⇒ 9

is not 7. Parentheses must be used to alter the order of evaluation:

1 + (2 * 3)
⇒ 7

Parentheses can also be used to make potentially confusing code more clear. For example, the strict left to right evaluation order can be confusing when applied to mathematical expressions. In the Spacewar! code snippet below, the parentheses make it more clear that the addition happens first:

newVelocity := (ai + ag) * t + velocity

To send multiple messages to the same receiver a cascade can be used to state the receiver once, followed by the cascade of messages separated by semicolons. Here is our earlier Example 1.2 code expressed with a cascade:

Transcript
   show: 'Hello World!';
   newLine;
   show: 'I am Cuising'

Another example of a cascade from the Spacewar! game:

aShip 
   velocity: 0 @ 0;
   morphPosition: randomCoordinate value @ randomCoordinate value

Observe the text here is formatted to ease code understanding. It is possible to write the cascade of messages in one line, but it reduces the readability of the code:

Transcript show: 'Hello World!'; newLine; show: 'I am Cuising'

The Transcript class is frequently helpful in presenting useful information when developing an application. An alternative to the Ctrl-d (Do it) shortcut is Ctrl-p (Print it), which executes the script and prints the result direcly in the Workspace.

In the Example 2.4, we have requested no special result. Selecting the text and typing Ctrl-p results in the default, which is to return the object to which a message is sent, in this case the Transcript.