5.2: Three Types of Messages

Pharo distinguishes between three kinds of messages to reduce mandatory parentheses. A few simple rules based on the such different message determine the order in which the messages are sent.

There are three different types of messages:

• Unary messages are messages that are sent to an object without any other information. For example, in 3 factorial, factorial is a unary message.
• Binary messages are messages consisting of operators (often arithmetic). They are binary because they always involve only two objects: the receiver and the argument object. For example in 10 + 20, + is a binary message sent to the receiver 10 with argument 20.
• Keyword messages are messages consisting of one or more keywords, each ending with a colon (:) and taking an argument. For example in anArray at: 1 put: 10, the keyword at: takes the argument 1 and the keyword put: takes the argument 10.

Unary messages

Unary messages are messages that do not require any argument. They follow the syntactic template: receiver messageName. The selector is simply made up of a succession of characters not containing : (e.g., factorial, open, class).

89 sin
>>> 0.860069405812453

3 sqrt
>>> 1.732050807568877

Float pi
>>> 3.141592653589793

'blop' size
>>> 4

true not
>>> false

Object class
>>> Object class "The class of Object is Object class (BANG)"

Important

Unary messages are messages that do not require any argument. They follow the syntactic template: receiver selector.

Binary messages

Binary messages are messages that require exactly one argument and whose selector consists of a sequence of one or more characters from the set: +, -, *, /, &, =, >, |, <, ~, and @. Note that -- (double minus) is not allowed for parsing reasons.

100@100
>>> 100@100 "creates a Point object"

3+4
>>> 7

10 - 1
>>> 9

4 <= 3
>>> false

(4/3) * 3 == 4
>>> true "equality is just a binary message, and Fractions are exact"

(3/4) == (3/4)
>>> false "two equal Fractions are not the same object"

Important

Binary messages are messages that require exactly one argument and whose selector is composed of a sequence of characters from: +, -, *, /, &, =, >, |, <, ~, and @. -- is not possible. They follow the syntactic template: receiver selector argument.

Keyword messages

Keyword messages are messages that require one or more arguments and whose selector consists of one or more keywords each ending in :. Keyword messages follow the syntactic template: receiver selectorWordOne: argumentOne wordTwo: argumentTwo.

Each keyword takes an argument. Hence r:g:b: is a method with three arguments, playFileNamed: and at: are methods with one argument, and at:put: is a method with two arguments. To create an instance of the class Color one can use the method r:g:b: (as in Color r: 1 g: 0 b: 0), which creates the color red. Note that the colons are part of the selector.

Note

In Java or C++, the method invocation Color r: 1 g: 0 b: 0 would be written Color.rgb(1, 0, 0).

1 to: 10
>>> (1 to: 10) "creates an interval"

Color r: 1 g: 0 b: 0
>>> Color red "creates a new color"

12 between: 8 and: 15
>>> true

nums := Array newFrom: (1 to: 5).
nums at: 1 put: 6.
nums
>>> #(6 2 3 4 5)

Important

Keyword messages are messages that require one or more arguments. Their selector consists of one or more keywords each ending in a colon (:). They follow the syntactic template: receiver selectorWordOne: argumentOne wordTwo: argumentTwo.