CSE 341 : 10 May 2001

Smalltalk: the language

Smalltalk is small and simple. Like Scheme, it demonstrates the principle that power comes not from piling feature upon feature, but from pruning away restrictions.

Everything in Smalltalk is an object. Objects have a class, some instance variables, and methods, all of which are ultimately objects. All actions are performed by sending a message to an object; if the object has a method for handling that message, the method is executed.

Bindings

Objects are assigned/bound to names by using the assignment statement:

    x := 'hi'.

Here, we are binding x to the String object representing 'hi'. Differences relative to ML:

• Name-to-object bindings are not immutable, and assignment side-effects the current environment:

      x := 28.
  

  changes x's binding to point to a new object---in this case an integer with value 28.
• In the Workspace (I'll discuss this later), you can create a binding just by executing an assignment. However, in some contexts (e.g. inside methods), you must declare a variable name before assigning to it.

Message sending syntax

    x negated.       "Unary message syntax"
    x + 5.           "Infix message syntax"
    x gcd: 21.       "Keyword message syntax"

    "Keyword message with multiple arguments"
    'Hello, world' replaceFrom: 1 to: 6 with: 'byebye' startingAt: 1.

In keyword syntax, each argument to a message follows a keyword. This only seems weird because you are used to memorizing the argument order from languages like C. Since each keyword describes the argument that follows, I find this nicer than "normal" function syntax.

Common syntax gotchas

• Periods separate statements. Semicolons, as we'll see shortly, separate multiple messages sent to the same receiver.
• Strings are written using single quotes, and comments are written using double quotes.
• All binary messages associate left to right; arithmetic operations do not have standard precedence rules.