notation used in Niall Ross' Smalltalk examples

1) When writing Smalltalk textual examples, the usual symbol to express that a class has a method is >>

   MyClass>>myMethod
      someCode

means MyClass has the method named myMethod whose implementation is shown by someCode. This symbol is purely for writing code in a document; it has no meaning to any Smalltalk compiler or IDE.

2) The return symbol in Smalltalk is ^

   MyClass>>myMethod
      ^someCode

means the method myMethod returns the result of someCode.

3) Code evaluation is left to right subject to method precedence. The precedence of a Smalltalk method is:

• unary methods (e.g. 'printString' in the expression '12 printString' is a unary method)
• binary (e.g. '+' in the expression '12 + 3' is a binary method)
• keyword (e.g. 'computeGreatestCommonDivisor:' in the expression '12 computeGreatestCommonDivisor: 9' is a keyword method)

   12 > 3 ifTrue: [Transcript show: 12 + 9 sqrt].

the binary method > is sent to the SmallInteger 12 with parameter another SmallInteger (3) and only then is the keyword method ifTrue: sent to the result (which is the Boolean true), with parameter a BlockClosure. When the block is evaluated, the unary method sqrt is sent first to 9 before the binary method + is sent to 12 with parameter the result (3). Lastly, the keyword method show: is sent to the Transcript with the result of the addition (15) as parameter.

These precedence rules allow Smalltalk to be written with a relatively small number of brackets and let text epressions read reasonably naturally. However a side-effect is that arithmetic precedence is not the usual one. The result of

   1 + 2 * 5

is 15 (1 added to 2 then all multiplied by 5) not 11; to get the latter, you must write

   1 + (2 * 5)

As everyone, not just Java and C programmers, is likely to be more familiar with the idea of multiplication and division having higher precedence than addition and subtraction, this is a cost you pay for the clarity of the syntax in other areas; you just have to remember it. (One of the code-checking utility rules detects all occurences of the pattern A + B * C and similar in code you write, presenting them for your review, so helps you remember.)

Back to new code syntax

Back to polymorphism instead of a case statement

(Written by Niall Ross as part of Smalltalk for Java Programmers.)