Smalltalk and Object Orientation: an Introduction - Free

• marries: is the message selector
• thatPerson is the argument
As with every message expression it returns a result, which is what is saved into the variable newStatus.
34.3.4 Message types
There are a number of different types of message. These are:
• Unary Message have no arguments.
anArray size. theta sin. 4 even.
the message selector (i.e. size) can be any simple identifier.
• Keyword Messages have one (or more) keywords, each with an argument.
index max: limit
anArray at: first put: ‘John’
In the above examples max: and at:put: are the keywords (termed selectors). This means
that the name of a message selector is spread amongst the arguments. These can be any simple
identifier with a trailing colon. The argument can be an expression representing any object.
• Binary Messages have one argument, and the selector is one or two non -alphabetic symbols. For
example:
3 > 4. 100 / 17. oldIndex + 1.
The second character in a binary message selector cannot be a minus sign.
34.3.5 Parsing rules
The parsing rules of Smalltalk can be summarized in the following points:
• Multiple expressions are separated by full stops.
• Unary expressions parse left to right.
• Binary expressions parse left to right.
• Unary expressions take precedence over binary expressions.
• Parenthesized expressions (using round brackets) take precedence over unary expressions.
To summarize then the precedence order is unary
brackets).
- binar y -keyword (plus take into account round
34.4 Blocks
A block is a sequence of statements whose execution is deferred. They are evaluated in the context in
which they were defined. Each block is an object of class Context. The statements are enclosed in
square brackets e.g. []. The block will be executed when it is sent the message “value”. The result of the
last message sent is returned as the value of the block. For example:
| temp |
temp := [Transcript show: ‘Hello John’.].
Transcript show: ‘Hello World’.
temp value.
Blocks are most commonly used with control structures such as:
anObject
ifTrue: [Transcript show: ‘Hello Out There’.].
They may also be used in iteration statements, for example:
[x > 0]
whileTrue: [.....].
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