Understanding Smalltalk method syntax

TL;DR

This page introduces the syntax of methods and statements in Smalltalk.

You might first want to visit Understanding Pharo built-in data types, Understanding Smalltalk message syntax and Smalltalk method syntax on a postcard.

Methods

Methods are like code snippets, except that:

1. They start with a declaration of the message and its parameters

2. They must declare any temporary variables used

3. They may explicitly return a value at the end of a block of statements with ^.

The example below shows the #lineCount method of the String ArrayedCollection subclass: #String instanceVariableNames: '' classVariableNames: 'AsciiOrder CSLineEnders CSNonSeparators CSSeparators CaseInsensitiveOrder CaseSensitiveOrder LowercasingTable Tokenish TypeTable UppercasingTable' package: 'Collections-Strings-Base' class. The first line declares the unary message lineCount without any parameters. This is followed by a comment (in double quotes), and the declaration of the lineCount variable (between or-bars).

The lineCount variable is then initialized, updated, and finally returned using the ^ return symbol.

String>>#lineCount lineCount "Answer the number of lines represented by the receiver, where every line delimiter CR, LF or CRLF pair adds one line." | lineCount | lineCount := 0. self lineIndicesDo: [:start :endWithoutDelimiters :end | lineCount := lineCount + 1]. ^lineCount

Expressions

Smalltalk expressions consist of:

1. A variable (e.g., lineCount)

2. A literal (e.g., #lineCount; see Understanding Pharo built-in data types)

3. A message send (e.g., lineCount+1; see Understanding Smalltalk message syntax)

Statements

Statements in Smalltalk consist of expressions separated by periods.

An assignment statement assigns the value of an expression to a variable using the built-in := operator. Note that this is not a message send.

A method can return the value of an expression with ^. If no return value is explicitly specified, a method returns self (the value of the receiver) when it ends.

Variables

All variables used in a method must be declared. Variables may be:

1. formal parameters to the method (arguments)

2. instance variables of the object (see A gentle introduction to classes and methods in Smalltalk)

3. temporaries declared at the beginning of the method (e.g., | lineCount |)

4. pseudo-variables (i.e., self, super, true, false, nil, or thisContext; see Understanding Pharo built-in data types)