Homework #1

An Introduction to the Squeak World

Your Name: _____________________________________

Date Due: 14 January 2002

Please write directly on this handout and turn it back in. Go through every step below. As you do each thing, place a check mark next to it to indicate that you actually did it. Fill-in answers where a blank line is provided.

Getting Started

Download the SQUEAK Software and put it on your computer. The software is available at www.squeak.org.

    Squeak 3.0Alpha8MT -- This is the Virtual machine, the application program.
    Squeak3.0.image    -- This binary file contains the initial collection of objects
    Squeak3.0.changes  -- This text file contain more source code
    SqueakV3.sources   -- This text file contains distributed Smalltalk source code

To start up the system, run the VM on the "image" file. In the Macintosh, this is done by double clicking on the "image" file. Later you will create your own "image" and "changes" file, but you will always use the "VM" and the "sources" files.

• Download the SQUEAK System and get into it.

Using the Mouse

Smalltalk, from which Squeak is derived and with which it is still somewhat compatible, was designed assuming a three-button mouse. If your mouse has fewer buttons, you must press extra keys with the mouse button or buttons to simulate the mouse buttons that you are missing.

For platform independence, the mouse buttons are usually referred to by colors in the Squeak software and documentation. Consider buying a 3-button mouse; they are quite inexpensive, and make the Squeak world much more friendly. (They are pretty useful in many other applications too!) Software that comes with the mouse, or that is available on the Internet, will let you set up the buttons so that they do the right things. The following chart shows what keys must be held down when mouse-clicking to simulate Squeak's buttons.

Symbolic	MacOS	Windows	3-button	Use
Red	Mouse button	Left-button	Left-button	Selecting, moving the insertion cursor
Yellow	Option-button	Right-button	Middle-button	Application-specific menus
Blue	-button	Alt-Left-button	Right-button	Window and graphics manipulation

My mouse also has a scrolling wheel. I have this set up so that "wheel up" maps to -upArrow (on my Macintosh) and "wheel down" maps to -downArrow. This lets me use the wheel to control scrolling in my Squeak windows.

One of the advantages of referring to the buttons by colors is that you may choose to map them to different physical buttons from those shown above. For example, if you are left-handed, you might choose to reverse the red and blue buttons. On my mouse, the middle button is actually the scrolling wheel. Because the yellow button is used very frequently, I prefer to put the yellow button on the right, and to put the blue button on the wheel in the middle.

Placing some colored labels on the mouse buttons will help your fingers to follow these directions.

The World Menu

Red-click outside of any window; you will see the world menu. Notice that most Squeak menus are not modal; you can leave them on the screen for as long as you wish by selecting keep this menu up. Do this. Also, notice that menus appear when you click the mouse, but do not disappear when you release it; they stay visible until you make a selection, or until you click outside of the menu. You can even move the menu around by grabbing its title bar.

Bring up the open... submenu and select workspace. You should get a large window labeled Workspace.

Terminating and Restarting a Squeak Session

A Squeak session is normally terminated by writing out all of your objects to a disk file called a snapshot or image file. When you first start Squeak, the Squeak Virtual Machine is loaded with an initial set of objects, which includes a vast amount of pre-existing code and programming tools (all of which are objects). You will modify these objects during your Squeak session.

When you terminate Squeak, you will save a snapshot of your memory containing all of your objects. The next time you use Squeak, memory will be reloaded from this file and the state of the system will be exactly as you left it. The snapshot file will be named xxxxx.image, where xxxxx is something you will choose, like YourName.image. There will be a corresponding "changes" file, with a name such as YourName.changes.

• From the world menu, select new morph ... >> Demo >> BlobMorph. A "Blob" will attach itself to the mouse cursor. (The jargon is: the blob is "in hand"). Put the Blob down (by red-clicking) somewhere on the screen.
• Make your first snapshot by selecting save as... from the world menu. Give a name to your image file when requested, such as YourName.image. (Saving may take a second or two.) You may save an image any time you wish, by selecting the save option from the system menu. If the Squeak system crashes, and you restart it, the state will revert to the last snapshot you made. You can use this feature to go back to an earlier state if you get your Squeak machine into a really big mess.
• Another file called YourName.changes will also be created. All the source code that you compile or execute is saved there. For the most part the changes can be ignored, but if a crash occurs you can use it to recover changes made after the last snapshot. See the box (at the end of this worksheet) for instructions.
• Select quit on the world menu to exit the Squeak environment.
• Restart Squeak from your saved image. The blob should still be ... well, behaving like a blob! Yes, running processes are saved and restored too.

Collapse and / or close unneeded windows

Each window has a title bar with a "close" icon (an x) in the upper left and a "collapse" icon in the upper right.

• In the initial state, you will see several windows on the screen. Collapse the windows called Getting Started... and Welcome To... Then expand them back to their original size.
• Close the Workspace window that you created earlier.
• Blue-click on the blob. You will see a collection of colored dots. Click in the pink handle. The blob should go away. (You may have to try several times as the Blob squirms around and tries to get away from the mouse.)

Workspaces

• Open another workspace. This time, instead of using the world menu, mouse over to the Tools flap at the right hand edge of the screen. The flap will pull out, like a drawer. Inside are a collection of tools. The off-white rectangle third from the top is a workspace. If you let the mouse linger over the tools, balloons will identify them. Drag a workspace out of the flap.
• Practice resizing and moving windows. To move the workspace window, grab it in the title bar at the top. To make the window larger or smaller, move the mouse to the lower right corner. A yellow dot will appear; this is a dragable resize handle. Note that a scroll bar appears when the cursor is in the workspace. Ignore the scroll bar for now.
• The window menu. Every window has menu of items that apply specifically to that window. Bring up the window menu for your workspace by red-clicking the small menu icon at the left of the title bar, just to the right of the close icon. Select change title ... to change the label in the title tab of the workspace. This brings up a prompter—this is the same kind of prompter used when saving a snapshot. (It's actually an object of class FillInTheBlank.) Experiment with full screen, and with changing the stacking order. Select window color; the system is now waiting for you to select a color for the window. Position the mouse anywhere on the screen and click it to select a color.
• Close the workspace window by pressing the "close" icon in the upper left corner.
• Window Selection. At any time only one window is active. The active window is in front and has its label highlighted. The mouse cursor must be in the window in which you wish to type. Create a second workspace and experiment with changing the active window with the red button. Experiment with overlapping views.
• Text Editing in a Workspace. Type a sentence or two into a workspace. Use the return, tab and backspace keys. Drag with the red button to select some characters. (Press, drag, and release.) Type characters to replace the highlighted characters. Try inserting and deleting characters after repositioning the cursor. Click the mouse button twice without moving it. This is called double-clicking, regardless of how long a time elapses between the clicks. What does double-click do when the cursor is in the middle of a word, at the beginning of a line, at the beginning of the workspace or after quotes, brackets or parentheses? (The bracket/quote feature will come in handy in matching nested brackets and parentheses when you are writing code.) Bring up the yellow button menu and practice using copy, cut and paste. (There is a "copy buffer" for text that has been cut. "Copy" saves into this buffer without cutting. You can also copy and paste between Squeak and other applications.
• Command keys can be used as shortcuts keys for copy, cut, and paste, as well as for many other editing operations—see the Squeak Language Reference Sheet for a full list.
  • In this worksheet, we will write command-c to mean that you type c while holding down the shortcut key. But the actual key that you press to get a shortcut depends on your operating system. On the Macintosh, hold down the key. On an IBM-compatible PC, hold down alt.
  • Some command key shortcuts are written with a capital letter. For example, command-T inserts the text ifTrue:. To type these capitalized shortcuts you can hold down shift and the shortcut key while you type t, or, more conveniently, you can hold down ctrl while you type t.
  • Here are some of the more commonly used editing shortcuts.

  General Editing Commands

  Key	Description	Notes
  z	Undo
  x	Cut
  c	Copy
  v	Paste
  a	Select all
  D	Duplicate. Paste the current selection over the prior selection, if it is non-overlapping.	1
  e	Exchange. Exchange the contents of current selection with the contents of the prior selection	1
  y	Swap. If there is no selection, swap the characters on either side of the insertion cursor, and advance the cursor. If the selection has 2 characters, swap them, and advance the cursor.
  w	Delete preceding word

  1. These commands are a bit unusual: they concern and affect not only the current selection, but also the immediately preceding selection.
     
• Scrolling. Using copy and paste, type in more text than will fit in the window. The scroll bar works as you expect; whether scroll bars are on the right or the left, and whether they flop out when they are needed or are permanently on view, can be controlled by preferences. (World menu>> help>>preferences). Notice that there is a tiny menu icon at the top of the scroll bar. Press the red mouse button here to get the yellow button menu that applies to the scrolling text pane. This is a nice shortcut, especially when using a stylus or a one-button mouse.
• Message Sending. Type 2+3 in the workspace, select it and then select the print it menu option, or type command-p. (Print it and do it work on the selected text, but if there is no selection, they work on the whole of the current line.) Both commands evaluate the selected expression. The print it command displays the result, but the do it option doesn't display the returned value. The short-cut key for do it is command-d. These are very common actions, so practice the shortcut keys.

  Key	Description
  d	Do "it" (where "it" is a Squeak expression)
  i	Inspect "it": evaluate "it" and open an inspector on the result. ("it" is a Squeak expression). Exception: in a method list pane, i opens an inheritance browser.
  p	Print "it". Evaluate "it" and insert the results immediately after "it." (where "it" is a Smalltalk expression)
  I	Open the Object Explorer on "it" (where "it" is an expression)

• Undo, Searching. Experiment with undo, to undo the last change. Experiment with searching the workspace for a string. Use the shortcut keys as well as the menu items:

  Key	Description
  f	Find. Set the search string from a string entered in a dialog. Then, advance the cursor to the next occurrence of the search string.
  g	Find again. Advance the cursor to the next occurrence of the search string.
  h	Set Search String from the selection.
  j	Replace the next occurrence of the search string with the last replacement made
  A	Advance argument. Advance the cursor to the next keyword argument, or to the end of string if no keyword arguments remain.
  J	Replace all occurrences of the search string with the last replacement made
  S	Replace all occurrences of the search string with the present change text

• Accept, Cancel and Confirmers. Figure out how the accept and cancel commands work. Here are the shortcut keys.

  Key	Description
  l	Cancel (also "revert"). Cancel all edits made since the pane was opened or since the last save
  s	Accept (also "save"). Save the changes made in the current pane.
  o	Spawn. Open a new window containing the present contents of this pane, and then reset this window to its last saved state (that is, cancel the present window).

  • Workspaces keep a back-up copy. Accept writes into this copy.
  • Make some changes. Cancel takes the workspace back to the last copy saved by accept. Do an accept and then close the workspace.
  • In another workspace, make some changes without accepting them and try closing this workspace. A confirmer should appear; try ignoring the question—what happens?

Graphics Demonstrations

• In a workspace, type Spline example, select it and do it. Use the red button to click off several points on the screen. Then click another button: you should see a curve. What does restore display do to the curve? Can the curve lie inside a window? What do scroll bars do to the curve? Why?
• Type in Spline example1, then select do it. This is a typo and you should see a window pop up. Push the Abandon button. (example1 is a valid message, just not for Splines.)
• Type in Spline exampl then do it. This is also a typo, but this time Squeak suggests some possible corrections. Select cancel or example. (Here exampl is not a valid message on any object, so the system looks for messages with similar spellings.)

Accessing Files

• From the world menu, select open...>>file list. Alternatively, drag a file list browser from the Tools flap. You should see a pink window with several "sub-windows" known as panes. This is a file list browser, and gives you a view of the current directory.
  • In the upper-left window pane you'll see the directory hierarchy, directories, so you know where you are; the current directory is at the bottom.
  • In the upper-right pane, you'll see all the files in this directory.
  • Select a file name in the upper-right pane. (Choose a small text file.) You should see its contents in the bottom pane.
  • Try using the yellow-button menu in the upper-right pane to change the sort order.
• The bottom pane is similar to a workspace. You may edit files this way. Actually, you are editing an in-memory copy. Use accept (or command-s) to write the in-memory copy back to disk. Use cancel to revert to the old version without updating.
• The middle-left pane, containing just a *, defines a filtering pattern. You can change the pattern by accepting new text into this pane. Only directories and file names matching this pattern will be listed in the upper-right pane.
• The file list browser also supports ftp. Servers appear as if they were volumes at the root directory level. Yellow-click in the upper-left pane to add a new ftp server.

File Creation and Removal

• In the upper-right pane, select add new file and give it a name. This creates a zero-length file.
• Make some changes to the file, write them out, and read the changes back in.
• Use delete to remove this and other unneeded files. Select the more... menu option and take a look at the submenu options.
• The file In command in the File List Browser is important: it is used for reading and compiling Squeak source code. Selecting the file In command will read, parse, and compile code from the selected file. (At this time, we have no external code files to file in.)

Forms and Points

Forms are rectangular blocks of pixels, like mini-bitmaps.

• In a workspace, execute f := Form fromUser. Then select a rectangle from the screen. (There are two ways to type the assignment operator. You may either type colon-equal (like Pascal) or you may use the left arrow, which is really the ASCII underscore (_) character. In other words, type the _ key and see <- on the display. Select a rectangle on the screen. (This expression created a new form and assigns it to variable f.)
• Execute f Form fromUser (Same statement, just use the left-arrow this time.)
• Execute f displayAt: 100 @ 100. Use restore display from the world menu if necessary.
• Use cmd-h / alt-h (which is the set search string menu option) to select the entire line. Then use cmd-d / alt-d (which is the do it menu option) to execute these statements.
• Expressions such as 100@100 specify Points; the @ character is a binary infix operator. For example 20@500 means the point with x=20 and y=500.
• The upper-left corner of the screen is position 0@0. The y coordinate increases down the screen.
• Experiment with different display locations and variable names by re-sending the above messages.
• Use f Form extent: 30@40 to create a blank form. The extent: message creates a form that is 30 pixels wide (x) and 40 pixels high (y).
• Execute f displayAt: Sensor waitButton. Click the mouse button.

Using a System Browser

The System Browser is the main tool used to read and write code in Smalltalk. It is OK to have many System Browsers open at once. Normally, if you change code in one Browser, the changes will be visible in any other. (Preferences >> smartUpdating controls this).

• Open a System Browser from the tool flap (or the world menu). The upper 4 panes in the browser contain lists. These lists work just like the lists in the file contents browser. Practice scrolling and selecting items from the lists. Note how selections in one list affect the lists to its right.
• Select Kernel-Numbers from the upper-left pane. This pane is the class category pane; because there are so many classes in Squeak, it is convenient to organize them into categories. Once you have selected Kernel-Numbers, you should see the class Integer in the class pane, just to the right.
• Look at some methods in the lower pane.
• Examine the menus in each pane of the browser. See if you can figure out what some of the menu options do. (Selecting accept in the lower pane will re-compile the method if you made any changes. Select cancel if wish to avoid a compile and revert to the previous text.)
• If you inadvertently change a method, use the versions menu item in the message list (top right) pane to get back the old one.
• Locate the even method in class Integer. How does this method determine its result?

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• Create a new Class category for the classes that you will define during this class. Call it something like MyStuff.

Executing Code: Sending Messages to Objects

• Type and execute with the following 5 lines:

         circleForm  Form extent: 25 @ 25.
         aCircle  Circle new.
         aCircle radius: 12.
         aCircle center: 12 @ 12.
         aCircle displayOn: circleForm.
  

• Describe what you think this code does. (Hint: you won't see any effects directly.)

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• Type in and evaluate some expressions to verify this code does what you say it does.

Comments

Do you have any comments about this script? For future reference, were any instructions unclear/frustrating (which ones)?

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What if you Crash Squeak? It is quite possible to crash Squeak—as an experimental system, Squeak lets you change anything, including things that are vital to make Squeak work! For example, try Smalltalk := nil. The good news is that you need never loose work, even if you crash and go back to the last saved version of your image,which might be hours old. This is because all of the code that you executed is saved in the changes file. All of it! That includes one liners that you evaluate in a workspace, as well as code that you add to a class while programming. So here are the instructions on how to get your code back. There is no need to read this until you need it. But when you do need it, you'll find it here waiting for you. In the worst case, you can use a text editor on the changes file, but since it is many megabytes in size, this can be slow. Squeak gives you better ways. How to get your code back Restart Squeak, and select help>>useful expressions from the world menu. This will give you a workspace full of useful expressions. The first three, Smalltalk recover: 10000. ChangeList browseRecentLog. ChangeList browseRecent: 2000. are most useful for recovery. If you execute ChangeList browseRecentLog, you will be given an opportunity to decide how far back in history you wish to browse. Normally, it's sufficient to browse changes as far back as the last Snapshot. (You can get much the same effect by editing ChangeList browseRecent: 2000 so that the number 2000 becomes something else, using trial and error.) One you have a recent changes browser, showing, say, changes back as far as your last snapshot, you will have a list of everything that you have done to Squeak during that time. You can delete items from this list using the yellow-button menu. When you are satisfied, you can file in what is left. It's a good idea to start a new change set, using the ordinary change set browser, before you do the file in, so that all of your recovered code will be in a new change set. You can them file out this change set. One useful thing to do in the Recent changes browser is to remove doIts. Usually, you won't want to file in (and thus re-execute) doIts. But there is an exception. Creating a class shows up as a doIt. Before you can file in the methods for a class, the class must exist. So, if you have created any new classes, first file in the class creation doIts, then remove doIts and file in the methods. When I am done, I like to file out my new ChangeSet, quit Squeak without saving the image, restart, and make sure that my new ChangeSet files back in cleanly.