4.3: Message Sends

There are three kinds of messages in Pharo. This distinction has been made to reduce the number of mandatory parentheses.

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

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

+ - / \ * ~ < > = @ % | & ? , 

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

Message precedence. 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 >>> to show the result of evaluating an expression.

Precedence aside, execution is strictly from left to right, so:

1 + 2 * 3
>>> 9

return 9 and not 7. Parentheses must be used to alter the order of evaluation:

1 + (2 * 3)
>>> 7

Periods and semi-colons. 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.

---