Re: [eclipse-users] Debugging constraints

On 28/09/2007, at 7:55 AM, Kish Shen wrote:

> Hi Malcolm,
>
> I don't think there is any easy way to debug constraints. The  
> delayed goals that implements the propagation of the constraints do  
> not necessarily map easily to the constraints. You can trace the  
> execution of these delayed goals using the tracer, and sometimes  
> you may be able to tell what the `source' constraint for it was.

If I was writing the system again from scratch, I'd tag each delayed  
goal with the details of the place in the code where it was initially  
suspended. This could then be used in a debugger as a replacement for  
the calling stack (which is clearly not very helpful). That way you  
could see not only the fine-grained constraints, but also where they  
originated from, rather than having to guess.

Going further, you could even use this information to place  
breakpoints on certain constraints, by anotating the line where they  
were delayed (rather than the code of the constraint itself, which  
may be called in many places). The breakpoint could identify the  
particular goal created, and then interrupt when that goal is executed.

Basically, imagine that there were no delays and that goals executed  
in standard top-to-bottom order. What would the calling stack be at  
each point in time? Delayed goals allow us to shuffle the order of  
execution, but we should still be able to reconstruct that same  
calling stack, which would provide much more useful debugging  
information.

Malcolm

--
            "Cleanliness is not next to godliness nowadays,
                 for cleanliness is made an essential
               and godliness is regarded as an offence.
                                     - G.K.Chesterton, On Lying in Bed