3.3: Message Sends

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Page ID: 36344

Andrew P. Black, Stéphane Ducasse, Oscar Nierstrasz, Damien Pollet

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There are three kinds of messages in Squeak.

Unary messages take no argument. 1 factorial sends the message factorial to the object 1.
Binary messages take exactly one argument. 1 + 2 sends the message + with argument 2 to the object 1.
Keyword messages take an arbitrary number of arguments. 2 raisedTo: 6 modulo: 10 sends the message consisting of the message selector raisedTo:modulo: and the arguments 6 and 10 to the object 2.

Unary message selectors consist of alphanumeric characters, and start with a lower case letter.

Binary message selectors consist of one or more characters from the following set:

+--/\*∼<>=@%|& ?,

Keyword message selectors consist of a series of alphanumeric keywords, where each keyword starts with a lower-case letter and ends with a colon.

Unary messages have the highest precedence, then binary messages, and finally keyword messages, so:

2 raisedTo: 1 + 3 factorial → 128

(First we send factorial to 3, then we send + 6 to 1, and finally we send raisedTo: 7 to 2.) Recall that we use the notation expression \(\rightarrow\) result to show the result of evaluating an expression.

Precedence aside, evaluation is strictly from left to right, so

1+2*3 → 9

not 7. Parentheses must be used to alter the order of evaluation:

1+(2*3) → 7

Message sends may be composed with periods and semi-colons. A period-separated sequence of expressions causes each expression in the series to be evaluated as a statement, one after the other.

Transcript cr.
Transcript show: 'hello world'.
Transcript cr

This will send cr to the Transcript object, then send it show: 'hello world', and finally send it another cr.

When a series of messages is being sent to the same receiver, then this can be expressed more succinctly as a cascade. The receiver is specified just once, and the sequence of messages is separated by semi-colons:

Transcript cr;
    show: 'hello world';
    cr

This has precisely the same effect as the previous example.