Smalltalk and Object Orientation: an Introduction - Free

anArray at: 1 max: 10.
This will try and send the message at:max: to anArray. Here it is necessary to use round
brackets to ensure that it is the object returned as a result of accessing position 1 in the array which
receives the max: message. For example:
7.6.5 Parsing rules
(anArray at: 1) max: 10.
Although the order in which expressions are evaluated is left to right, there are three levels of
precedence amongst the categories of message which affect this evaluation. The three categories each
have a different precedence as indicated by the following table:
Message category Precedence Example
unary 1 size, rem, odd
binary 2 *, /, - , +
keyword 3 max:, min:, at:put:
This means that a unary message will be evaluated before a binary or keyword message. In turn a
binary message will be evaluated before a keyword message. If this order is not what is desired then
round brackets can be used to alter the evaluation sequences as above. For example:
2 * 3 max: 3 * 4 odd.
This would generate an error, because, the odd message at the end of the expression has the highest
precedence. This means that the result of sending the message odd to the value 4 would be used as the
parameter for the message ‘*’ to be sent to the value 3. As this would result is multiplying 3 by false,
the system would generate a run time error. To ensure that we obtain the correct result, we might place
brackets around parts of the expression, thus:
((2 * 3) max: (3 * 4)) odd.
This can be a common source of errors. If you have a problem with any of the parsing rules a quick
and easy way to make sure the system parses an expression in the way that you want, is to put brackets
around parts of the expression to describe your requirements. Then select the format menu option in
one of the VisualWorks browsers. This option not only formats the code according to Smalltalk
standards, it also removes any unnecessary brackets.
7.7 Summary
In this chapter you have learnt about classes in Smalltalk, how they are defined, how instance variables
are specified and how methods are constructed. You have also encountered many of the basic Smalltalk
language structures.
7.8 Further reading
Two good books to have a look at now, if you have not already done so, are [Lewis 1995] and [Hopkins
and Horan 1995].
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