[Pharo-project] Better debugging...

Camillo Bruni camillo.bruni at inria.fr
Mon Apr 25 23:29:12 CEST 2011

I think a nice way to have a decent debugger is to run the program on top of a changeable interpreter. Since a classical debugger is nothing else but an interpreter with slightly changed semantics.

The object flow VM does a great job at tracing back stuff.. however it might pose a massive overhead since it collects tons and tons of data.

In Pinocchio[1] we implemented very nice and simple debuggers by "just" changing the current interpreter to take a user-action into account on each message send. Since this is implemented on top of a metacircular interpreter this was quite simple to achieve. So the whole implementation of the known debugger functionality (step, over...) maybe took an afternoon. Since you have access to the full interpreter protocol it is very easy to intercept different sorts of actions. As described in the linked paper, it was very easy to implement the object-flow behavior in Pinocchio.

So for me there is only one real way to have a decent debugger. And that means that it is a first-class interpreter whose semantics I can change at will. Thus I can create a specific debugger for my applications. 

But I guess that making the first-class interpreters work right away in Pharo will take a while to do, hence I propose the following additions to the current interpreter to make it partly useable:

- interception of message sends 
	- per thread [ok]
	- of specific classes [missing]
	- of specific instances [missing]

- interception of instance variable access, read/write
	- per thread [missing] 
	- of specific classes [missing]
	- of specific instances [missing]
	- of specific instance variables [missing]

Furthermore I really want to be able to script all of that stuff, eg. provide a simple test block that gets invoked on each message send. Returning true => halt, Returning false => continue execution.

[1] http://scg.unibe.ch/archive/papers/Verw10aPinocchio.pdf

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