Smalltalk and Object Orientation: an Introduction - Free

Messages with one or more arguments, separated by part of the method selector and composed of
alphanumeric symbols and a trailing colon, are called Keyword Messages. For example:
Transcript show: 'Hello John'.
10 max: 20.
aDictionary at: 'UK' put: 'United Kingdom'.
In the above examples show:, 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. Keyword
expressions have the lowest precedence.
7.6.3 Message selectors
A message selector is the term used to describe the method interface provided by one object to other
objects. For example, if an object possesses a method with the following definition then it possesses a
message selector of “addAddress:for:”.
addAddress: anAddress for: aName
addressBook at: aName put: anAddress.
Notice that the method selector only consists of the method name and does not include any of the
parameters dispersed amongst that name.
7.6.4 Precedence
Smalltalk has slightly different rules regarding precedence than many other languages. For those of you
who are unclear about precedence, this refers to the order in which operators are evaluated in an
expression. Many languages, such as C, have quite complex rule s regarding precedence, which
determine the order in which an expression such as:
2 + 5 * 3 - 1 / 2.
would be evaluated. Smalltalk is rather more intuitive. Essentially it handles parsing in a left to right
manner. For example, in the above example, 5 is added to 2, the result (7) is multiplied by 3 (to give
21). 1 is subtracted from this (giving 20) which is divided by 2. The result of this expression is therefore
10. That is, there is no precedence difference between +, -, * or /. If you wish to alter t he way in which
the arithmetic calculations are performed, then you can use round brackets. For example:
2 + (5 * 3) - 1 / 2.
This expression will be evaluated to the value 8. This rule also includes unary messages. For
example:
arraySize := #('a' 'b' 'c' 'd') size even
the message size is sent to the array object, and even is sent to the resulting object. The result of this
is then assigned to the variable arraySize.
Keyword messages are a special case, in that although they too are parsed in a left t o right manner,
the system assumes that all the keywords within one expression are part of the same message selector.
Thus an expression such as:
aDog name: 'Fido' age: 2.
will be parsed as calling a message name:age:. If this is not the intention then t he programmer must
indicate to the system that what is required is that the message name: and then a separate message
age: should be sent to aDog. For example, either as separate messages to aDog or using the
cascading mechanism. A potential point of confu sion here is when the intention is to send the second
method to the result obtained from the first. For example:
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