To interrupt the running of your code, find a place in your code where you want to gather information. Then, specify a breakpoint by adding the message halt or the expression self halt.
For example, evaluate the following code in a Workspace with the Execute command:
(OrderedCollection new) halt; add: 'Alix'; add: 'Sam'; add: 4; add: (Date today); at: 4 put: 'Cathy'; yourself.
A debugger opens, showing the error message '(ExHalt) A halt has occurred.' The top of the stack has the following messages:
[] in <optimized block>(ExceptionalEvent class)>>#initializeSystemExceptions Signal>>#evaluate: ExceptionalEvent>>#applyDefaultHandler: ExceptionalEvent>>#signalWithArguments: ExceptionalEvent>>#signal OrderedCollection(Object)>>#halt UndefinedObject>>#Doit
Select OrderedCollection(Object)>>#halt in the message stack and inspect self in the list of variables. Notice that the ordered collection exists but contains no entries.