6.8.1 A Simple Error

First, let’s take a look at a typical error. Type:

   7 plus: 1

This will print out:

   7 did not understand selector 'plus:'
   <blah blah>
   UndefinedObject>>#executeStatements

The first line is pretty simple; we sent a message to the 7 object which was not understood; not surprising since the plus: operation should have been +. Then there are a few lines of gobbledegook: just ignore them, they reflect the fact that the error passed throgh GNU Smalltalk’s exception handling system. The remaining line reflect the way the GNU Smalltalk invokes code which we type to our command prompt; it generates a block of code which is invoked via an internal method executeStatements defined in class Object and evaluated like nil executeStatements (nil is an instance of UndefinedObject). Thus, this output tells you that you directly typed a line which sent an invalid message to the 7 object.

All the error output but the first line is actually a stack backtrace. The most recent call is the one nearer the top of the screen. In the next example, we will cause an error which happens deeper within an object.