In this chapter we will give you a high-level tour of Pharo to help you get comfortable with the environment. There will be plenty of opportunities to try things out, so it would be a good idea if you have a computer handy when you read this chapter.
We will use this icon: to mark places in the text where you should try something out in Pharo. In particular, you will fire up Pharo, learn about the different ways of interacting with the system, and discover some of the basic tools. You will also learn how to define a new method, create an object and send it messages.
Pharo is available as a free download from http://pharo-project.org. There are three parts that you will need to download, consisting of four files (see Figure 1.1).
Download and install Pharo on your computer.
We recommend that you use the image provided on the Pharo by Example web page.2
Most of the introductory material in this book will work with any version, so if you already have one installed, you may as well continue to use it. However, if you notice differences between the appearance or behaviour of your system and what is described here, do not be surprised.
As you work in Pharo, the image and changes files are modified, so you need to make sure that they are writable. Always keep these two files together. Never edit them directly with a text editor, as Pharo uses them to store the objects you work with and to log the changes you make to the source code. It is a good idea to keep a backup copy of the downloaded image and changes files so you can always start from a fresh image and reload your code.
The sources file and the VM can be read-only — they can be shared between different users. All of these files can be placed in the same directory, but it is also possible to put the Virtual Machine and sources file in separate directory where everyone has access to them. Do whatever works best for your style of working and your operating system.
Type Time now in the workspace. Now action-click in the workspace. Select print it .
We recommend that right-handed people configure their mouse to click with the left button, action-click with the right button, and meta-click with the clickable scroll wheel, if one is available. If you are using a Macintosh without a second mouse button, you can simulate one by holding down the key while clicking the mouse. However, if you are going to be using Pharo at all often, we recommend investing in a mouse with at least two buttons.
You can configure your mouse to work the way you want by using the preferences of your operating system and mouse driver. Pharo has some preferences for customising the mouse and the meta keys on your keyboard. In the preference browser ( System … ⊳ Preferences … ⊳ Preference Browser… ), the keyboard category contains an option swapControlAndAltKeys that switches the action-click and meta-click functions. There are also options to duplicate the various command keys.
Click again on the Pharo background.
You will see the World menu again. Most Pharo menus are not modal; you can leave them on the screen for as long as you wish by clicking the push pin icon in the top-right corner. Do this.
The world menu provides you a simple means to access many of the tools that Pharo offers.
Have a closer look at the World and Tools … menus. (Figure 1.3 (a))
You will see a list of several of the core tools in Pharo, including the browser and the workspace. We will encounter most of them in the coming chapters.
Open a workspace. Type in the following text:
BouncingAtomsMorph new openInWorld
Now action-click. A menu should appear. Select do it (d) . (See Figure 1.5.)
A window containing a large number of bouncing atoms should open in the top left of the Pharo image.
You have just evaluated your first Smalltalk expression! You just sent the message new to the BouncingAtomsMorph class, resulting in a new BouncingAtomsMorph instance, followed by the message openInWorld to this instance. The BouncingAtomsMorph class decided what to do with the new message, that is, it looked up its methods for handling new message and reacted appropriately. Similarly the BouncingAtomsMorph instance looked up its method for responding to openInWorld and took appropriate action.
If you talk to Smalltalkers for a while, you will quickly notice that they generally do not use expressions like “call an operation” or “invoke a method”, but instead they will say “send a message”. This reflects the idea that objects are responsible for their own actions. You never tell an object what to do — instead you politely ask it to do something by sending it a message. The object, not you, selects the appropriate method for responding to your message.
Now click on the bouncing atoms window and drag it anywhere you like. You now have the demo “in hand”. Put it down by clicking anywhere.
Select World ⊳ Save as … , enter the name “myPharo”, and click on the OK button. Now select World ⊳ Save and quit .
Now if you go to the location where the original image and changes files were, you will find two new files called “myPharo.image” and “myPharo.changes” that represent the working state of the Pharo image at the moment before you told Pharo to Save and quit . If you wish, you can move these two files anywhere that you like on your disk, but if you do so you may (depending on your operating system) need to also move, copy or link to the virtual machine and the sources file.
Start up Pharo from the newly created “myPharo.image” file.
Now you should find yourself in precisely the state you were when you quit Pharo. The BouncingAtomsMorph is there again and the atoms continue to bounce from where they were when you quit.
When you start Pharo for the first time, the Pharo virtual machine loads the image file that you provide. This file contains a snapshot of a large number of objects, including a vast amount of pre-existing code and a large number of programming tools (all of which are objects). As you work with Pharo, you will send messages to these objects, you will create new objects, and some of these objects will die and their memory will be reclaimed (i.e., garbage-collected).
When you quit Pharo, you will normally save a snapshot that contains all of your objects. If you save normally, you will overwrite your old image file with the new snapshot. Alternatively, you may save the image under a new name, as we just did.
In addition to the .image file, there is also a .changes file. This file contains a log of all the changes to the source code that you have made using the standard tools. Most of the time you do not need to worry about this file at all. As we shall see, however, the .changes file can be very useful for recovering from errors, or replaying lost changes. More about this later!
The image that you have been working with is a descendant of the original Smalltalk-80 image created in the late 1970s. Some of these objects have been around for decades!
You might think that the image is the key mechanism for storing and managing software projects, but you would be wrong. As we shall see very soon, there are much better tools for managing code and sharing software developed by teams. Images are very useful, but you should learn to be very cavalier about creating and throwing away images, since tools like Monticello offer much better ways to manage versions and share code amongst developers.
Using the mouse (and the appropriate modifier keys), meta-click on the BouncingAtomsMorph.5
You will see a collection of colored circles that are collectively called the BouncingAtomsMorph’s morphic halo. Each circle is called a handle. Click in the pink handle containing the cross; the BouncingAtomsMorph should go away.
Close all open windows. Open a transcript and a workspace. (The transcript can be opened from the World ⊳ Tools ... submenu.)
Position and resize the transcript and workspace windows so that the workspace just overlaps the transcript.
You can resize windows either by dragging one of the corners, or by meta-clicking the window to bring up the morphic halo, and dragging the yellow (bottom right) handle.
At any time only one window is active; it is in front and has its border highlighted.
The transcript is an object that is often used for logging system messages. It is a kind of “system console”.
Workspaces are useful for typing snippets of Smalltalk code that you would like to experiment with. You can also use workspaces simply for typing arbitrarily text that you would like to remember, such as to-do lists or instructions for anyone who will use your image. Workspaces are often used to hold documentation about a captured image, as is the case with the standard image that we downloaded earlier (see Figure 1.2).
Type the following text into the workspace:
Transcript show: ’hello world’; cr.
Try double-clicking in the workspace at various points in the text you have just typed. Notice how an entire word, entire string, or the whole text is selected, depending on whether you click within a word, at the end of the string, or at the end of the entire expression.
Select the text you have typed and action-click. Select do it (d) .
Notice how the text “hello world” appears in the transcript window (Figure 1.8). Do it again. (The (d) in the menu item do it (d) tells you that the keyboard shortcut to do it is CMD–d. More on this in the next section!)
If you want to evaluate an expression, you do not always have to action-click. Instead, you can use keyboard shortcuts. These are the parenthesized expressions in the menu. Depending on your platform, you may have to press one of the modifier keys (control, alt, command, or meta). (We will indicate these generically as CMD–key.)
Evaluate the expression in the workspace again, but using the keyboard shortcut: CMD–d.
In addition to do it , you will have noticed print it , inspect it and explore it . Let’s have a quick look at each of these.
Type the expression 3 + 4 into the workspace. Now do it with the keyboard shortcut.
Do not be surprised if you saw nothing happen! What you just did is send the message + with argument 4 to the number 3. Normally the result 7 will have been computed and returned to you, but since the workspace did not know what to do with this answer, it simply threw the answer away. If you want to see the result, you should print it instead. print it actually compiles the expression, executes it, sends the message printString to the result, and displays the resulting string.
Select 3+4 and print it (CMD–p).
This time we see the result we expect (Figure 1.9).
3 + 4 -→ 7
We use the notation -→ as a convention in this book to indicate that a particular Pharo expression yields a given result when you print it .
Delete the highlighted text “7” (Pharo should have selected it for you, so you can just press the delete key). Select 3+4 again and this time inspect it (CMD–i).
Now you should see a new window, called an inspector, with the heading SmallInteger: 7 (Figure 1.10). The inspector is an extremely useful tool that will allow you to browse and interact with any object in the system. The title tells us that 7 is an instance of the class SmallInteger. The left panel allows us to browse the instance variables of an object, the values of which are shown in the right panel. The bottom panel can be used to write expressions to send messages to the object.
Type self squared in the bottom panel of the inspector on 7 and print it .
Close the inspector. Type the expression Object in a workspace and this time explore it (CMD–I, uppercased i).
This time you should see a window labelled Object containing the text
⊳ root: Object
root: Object. Click on the triangle to open it up (Figure 1.11).
The explorer is similar to the inspector, but it offers a tree view of a complex object. In this case the object we are looking at is the Object class. We can see directly all the information stored in this class, and we can easily navigate to all its parts.
The class browser6 is one of the key tools used for programming. As we shall see, there are several interesting browsers available for Pharo, but this is the basic one you will find in any image.
Open a browser by selecting World ⊳ Class browser .7
We can see a browser in Figure 1.12. The title bar indicates that we are browsing the class Object.
When the browser first opens, all panes are empty but the leftmost one. This first pane lists all known packages, which contain groups of related classes.
Click on the Kernel package.
This causes the second pane to show a list of all of the classes in the selected package.
Select the class Object.
Now the remaining two panes will be filled with text. The third pane displays the protocols of the currently selected class. These are convenient groupings of related methods. If no protocol is selected you should see all methods in the fourth pane.
Select the printing protocol.
You may have to scroll down to find it. Now you will see in the fourth pane only methods related to printing.
Select the printString method.
Now we see in the bottom pane the source code of the printString method, shared by all objects in the system (except those that override it).
There are several ways to find a class in Pharo. The first, as we have just seen above, is to know (or guess) what category it is in, and to navigate to it using the browser.
A second way is to send the browse message to the class, asking it to open a browser on itself. Suppose we want to browse the class Boolean.
Type Boolean browse into a workspace and do it .
A browser will open on the Boolean class (Figure 1.13). There is also a keyboard shortcut CMD–b (browse) that you can use in any tool where you find a class name; select the name and type CMD–b.
Use the keyboard shortcut to browse the class Boolean.
Notice that when the Boolean class is selected but no protocol or method is selected, instead of the source code of a method, we see a class definition (Figure 1.13). This is nothing more than an ordinary Smalltalk message that is sent to the parent class, asking it to create a subclass. Here we see that the class Object is being asked to create a subclass named Boolean with no instance variables, class variables or “pool dictionaries”, and to put the class Boolean in the Kernel-Objects category. If you click on the ? at the bottom of the class pane, you can see the class comment in a dedicated pane (see Figure 1.14).
Often, the fastest way to find a class is to search for it by name. For example, suppose that you are looking for some unknown class that represents dates and times.
Put the mouse in the package pane of the browser and type CMD–f, or select find class …(f) by action-clicking. Type “time” in the dialog box and accept it.
You will be presented with a list of classes whose names contain “time” (see Figure 1.15). Choose one, say, Time, and the browser will show it, along with a class comment that suggests other classes that might be useful. If you want to browse one of the others, select its name (in any text pane), and type CMD–b.
Note that if you type the complete (and correctly capitalized) name of a class in the find dialog, the browser will go directly to that class without showing you the list of options.
Sometimes you can guess the name of a method, or at least part of the name of a method, more easily than the name of a class. For example, if you are interested in the current time, you might expect that there would be a method called “now”, or containing “now” as a substring. But where might it be? The method finder can help you.
Select World ⊳ Tools ... ⊳ Method finder . Type “now” in the top left pane, and accept it (or just press the RETURN key).
The method finder will display a list of all the method names that contain the substring “now”. To scroll to now itself, move the cursor to the list and type “n”; this trick works in all scrolling windows. Select “now” and the right-hand pane shows you the classes that define a method with this name, as shown in Figure 1.16. Selecting any one of them will open a browser on it.
At other times you may have a good idea that a method exists, but will have no idea what it might be called. The method finder can still help! For example, suppose that you would like to find a method that turns a string into upper case, for example, it would translate ’eureka’ into ’EUREKA’.
Type ’eureka’ . ’EUREKA’ into the method finder and press the RETURN key, as shown in Figure 1.17.
The method finder will suggest a method that does what you want.8
An asterisk at the beginning of a line in the right pane of the method finder indicates that this method is the one that was actually used to obtain the requested result. So, the asterisk in front of String asUppercase lets us know that the method asUppercase defined on the class String was executed and returned the result we wanted. The methods that do not have an asterisk are just the other methods that have the same name as the ones that returned the expected result. So Character»asUppercase was not executed on our example, because ’eureka’ is not a Character object.
You can also use the method finder for methods with arguments; for example, if you are looking for a method that will find the greatest common factor of two integers, you might try 25. 35. 5 as an example. You can also give the method finder multiple examples to narrow the search space; the help text in the bottom pane explains how.
The advent of Test Driven Development9 (TDD) has changed the way that we write code. The idea behind TDD is that we write a test that defines the desired behaviour of our code before we write the code itself. Only then do we write the code that satisfies the test.
Suppose that our assignment is to write a method that “says something loudly and with emphasis”. What exactly could that mean? What would be a good name for such a method? How can we make sure that programmers who may have to maintain our method in the future have an unambiguous description of what it should do? We can answer all of these questions by giving an example:
When we send the message shout to the string “Don’t panic” the result should be “DON’T PANIC!”.
To make this example into something that the system can use, we turn it into a test
self assert: (’Don’’t panic’ shout = ’DON’’T PANIC!’)
How do we create a new method in Pharo? First, we have to decide which class the method should belong to. In this case, the shout method that we are testing will go in class String, so the corresponding test will, by convention, go in a class called StringTest.
Open a browser on the class StringTest, and select an appropriate protocol for our method, in this case tests - converting , as shown in Figure 1.18. The highlighted text in the bottom pane is a template that reminds you what a Smalltalk method looks like. Delete this and enter the code from method 1.1.
Once you have typed the text into the browser, notice that the bottom pane is outlined in red. This is a reminder that the pane contains unsaved changes. So select accept (s) by action-clicking in the bottom pane, or just type CMD–s, to compile and save your method.
If this is the first time you have accepted any code in your image, you will likely be prompted to enter your name. Since many people have contributed code to the image, it is important to keep track of everyone who creates or modifies methods. Simply enter your first and last names, without any spaces, or separated by a dot.
Because there is as yet no method called shout, the browser will ask you to confirm that this is the name that you really want — and it will suggest some other names that you might have intended (Figure 1.20). This can be quite useful if you have merely made a typing mistake, but in this case, we really do mean shout, since that is the method we are about to create, so we have to confirm this by selecting the first option from the menu of choices, as shown in Figure 1.20.
Run your newly created test: open the SUnit TestRunner from the World menu.
The leftmost two panes are a bit like the top panes in the browser. The left pane contains a list of categories, but it’s restricted to those categories that contain test classes.
Select CollectionsTests-Text and the pane to the right will show all of the test classes in that category, which includes the class StringTest. The names of the classes are already selected, so click Run Selected to run all these tests.
You should see a message like that shown in Figure 1.21, which indicates that there was an error in running the tests. The list of tests that gave rise to errors is shown in the bottom right pane; as you can see, StringTest»#testShout is the culprit. (Note that StringTest»#testShout is the Smalltalk convention to identify the testShout method of the StringTest class.) If you click on that line of text, the erroneous test will run again, this time in such a way that you see the error happen: “MessageNotUnderstood: ByteString»shout”.
The window that opens with the error message is the Smalltalk debugger (see Figure 1.22). We will look at the debugger and how to use it in Chapter 6.
The error is, of course, exactly what we expected: running the test generates an error because we haven’t yet written a method that tells strings how to shout. Nevertheless, it’s good practice to make sure that the test fails because this confirms that we have set up the testing machinery correctly and that the new test is actually being run. Once you have seen the error, you can Abandon the running test, which will close the debugger window. Note that often with Smalltalk you can define the missing method using the Create button, edit the newly-created method in the debugger, and then Proceed with the test.
Now let’s define the method that will make the test succeed!
Select class String in the browser, select the converting protocol, type the text in
method 1.2 over the method creation template, and accept it. (Note: to get a ↑
↑, type ).
↑ self asUppercase, ’!’
The comma is the string concatenation operation, so the body of this method appends an exclamation mark to an upper-case version of whatever String object the shout message was sent to. The ↑ tells Pharo that the expression that follows is the answer to be returned from the method, in this case the new concatenated string.
Does this method work? Let’s run the tests and see.
Click on Run Selected again in the test runner, and this time you should see a green bar and text indicating that all of the tests ran with no failures and no errors.
When you get to a green bar10, it’s a good idea to save your work and take a break. So do that right now!
This chapter has introduced you to the Pharo environment and shown you how to use some of the major tools, such as the browser, the method finder, and the test runner. You have also seen a little of Pharo’s syntax, even though you may not understand it all yet.