Computer Programming with GNU Smalltalk - Free

Unary, Binary and Keyword Messages
Objects, Messages and Classes: Part I 21
There are 3 types of messages. The difference between them is about the selectors, arguments and the
precedence. What is precedence? It will be explained in a few minutes.
Unary messages are messages without any argument. It is called unary because the resulting expression
involves only one object. An example for an expression with unary message would be:
'Hello World!' size
As you can see there are no arguments here and the overall expression involves only the string object
'Hello World!'.
Binary messages are messages with an argument. But the other characteristic of binary messages is that
their selectors have up to 2 characters which are not alphanumeric. An example for an expression with
binary message would be:
3 + 4
+ is a selector here and 4 is the argument for it. The expression involves two integer objects of 3 and 4.
+ is one character long and it is not an alphanumeric character. So it meets the requirements of being
called as a binary message.
Finally, keyword messages are messages with one or more arguments whose selectors are composed of
alphanumeric characters. They usually have a colon at the end of each selector word which indicates
that it is demanding an argument. An example for an expression with keyword message would be:
3 bitAt: 3 put: 1
Our selector here is bitAt:put: where our arguments are 3 and 1. Overall expression involves three
objects but it could have been one or two or another number greater than three.
We gave the selector and argument features of message types, but we didn't mention about precedence
of them. Precedence is a concept used for deciding which one of the messages will be executed first, if
there are many messages in one expression. If you cannot imagine an expression with many messages
don't worry, here it is:
3 + 5 bitAt: 3 put: 1 printNl
As you might see, here are three messages: +, bitAt:put: and printNl. Which one should be
evaluated first? It depends on precedence rules.
There are some basic rules for message precedence in Smalltalk, they are:
1. Unary messages are evaluated first,
2. Binary messages are evaluated second,