Smalltalk and Object Orientation: an Introduction - Free

This dialog box indicates that New is not understood by the class Set (note the terminology). This is
an example of how Smalltalk (and VisualWorks in particular) attempts to help you if it finds a message
it does not understand. In this case it did not understand “New”. It identified this as a problem and is
now allowing you (the user) to
• Abort and correct the error yourself,
• Proceed as though nothing had gone wrong (however it would probably fail in another way
immediately) or to
• Correct it, the error, with the system’s help.
The correction option is illustrated below. Click on the “correct it” option. You will then see the
following selection box:
Select “OK”.
This is an example of how Smalltalk attempts to help you if it finds a m essage it does not
understand. In this case it did not understand “New”, but it found that Sets do understand “new”. As
“new” is very similar to “New”, Smalltalk presented it as a possible alternative.
Once you have selected “OK” on the selection dialog, you should then see the following dialog.
For the moment select “proceed”. The problem here is not actually with item, but is a feature of
another problem which we will come back to later. Once you have selected “proceed” you will then see:
This dialog was generated because, Peter, does not have single quotes around it (single quotes are
used to indicate a string). The system therefore assumes that it is a variable which has yet to be defined.
At this point press “cancel”. You should find that Peter is h ighlighted in the Workspace. This
illustrates how VisualWorks attempts to point out where the problem is. Now place single quotes
around Peter, e.g.
temp add: 'Peter'.
Next re-select the whole lot and “do it” again. You will again see a dialog warning yo u that item
isn’t defined. For the moment select proceed.
You will now get an Exception raised. This is because the Transcript object does not understand the
message “show”. The dialog you see should look like this:
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