Quick-start guide to Squeak
Last updated at 4:55 am UTC on 16 October 2017
This is intended to record the learning experiences of an experienced programmer in using Squeak. It is hoped that this will be useful for those who already know how to use an IDE, and what object-oriented programming is, who want to start using Squeak fast. This was written while using Squeak3.6#5429.
Getting ready
Click on the "tools" tab on the right-hand side of the screen. To create a window of the named kind, click and drag the icon into the main window area. You want
- a "Transcript" (good for displaying output),
- a "Workspace" (for typing commands), and
- a "Browser" (used to examine all of the code in the system).
Hello, world
Type the following into the workspace window:
Transcript show: 'Hello World'
To execute, you can middle-click and select "do it", or on a PC, hit Alt-d (Cmd-d on a Mac). The exciting message "Hello, World" appears in the Transcript window.
Creating a class
It appears that all class names must be unique in Squeak. One cannot have a class in both category "a" and "b" called "Foo" - categories do not provide namespacing. All class names must be capitalised.
Categories are displayed in the leftmost pane of the browser, with names like "Kernel-Objects". To create a new category, middle-click on the category list, to bring up a small menu with the item "Add item"; select that and you will be prompted for the name of your category.
To create a class, click on a category, and notice that the text in the bottom pane has changed to a weird object declaration with first line
Object subclass: #NameOfSubclass
If you change "#NameOfSubclass" to "#MyNewCapitalisedName", then middle-click and choose "Accept" (or do Alt-s), you should find that your category has now acquired a class named "MyNewCapitalisedName". Let's assume that you chose the name "Foo".
Adding methods
Once again, one adds methods by altering a prepared template. To access this, click on a class to show its method categories, then click on a category, which will show you a template. Replace the first line with "method_name[: arg1_name second_part_of_method_name: arg2_name ...]"; that is to say, replace it with the method name, then if it takes an argument, separate the name of the argument from the method name with a colon, and subsequent arguments require that the method have multiple parts to its name.
The rest of the template should be fairly self-explanatory, but one thing is worth mentioning now: if you don't explicitly return a value from a method, it will return the object upon which it was called (this is useful for chaining otherwise value non-returning method-calls together). To explicitly return a value, use the "^" operator, e.g.:
^'Shoes'
will return 'Shoes' from the method.
If it annoys you that your method is in the "As yet uncategorized" category, then you can add a category by middle-clicking on the category pane (The third pane from the left), and hitting "add category"; once you have a category, you can change the category of a method by (middle) clicking upon it in the method pane (fourth pane) and choosing "Change category".
Inheritance and polymorphism
Smalltalk is a single-inheritance language, with the usual semantics that members and method implementations are inherited by subclasses. As the method invocation semantics are by-name, bound at invocation-time, on untyped variables, Smalltalk does not require a way of defining interfaces, as provided in other single-inheritance languages.
To extend another class, simply modify the class-creation template used before. The first line, by default reads "Object subclass #Foo"; to make Foo a subclass of Boolean, change "Object" to "Boolean", and accept the changes. In fact, one can do this to any of the elements in the class definition, such as the category.
