Re: OT: A bit of Sudoku

["J-P. Rosen" <rosen@adalog.fr>]

Le 05/06/2014 20:30, Adam Beneschan a écrit :
>> The only solution that I can see is to jump straight out of the 
>> tree.
>>> But that seems to lack elegance. The jump is made by raising an 
>>> exception which has been declared in, and is handled by, the 
>>> enclosing subprogram. The exception is 'silent' because the 
>>> handler contains a null statement.
>>> 
>>> I fear that perhaps I am missing something but have no idea 
>>> what.
On the contrary, I think exception are perfectly appropriate for that:
they allow to unwind the call stack directly up to the point where you
want to catch it by providing a handler.

I know that not everybody likes this idea, but to me exceptions are a
powerful programming structure, not limited to handling errors.

> Without seeing an actual program or any code at all, I can't really 
> tell, but ... when a caller calls itself recursively, isn't there 
> either a function result or an OUT parameter that allows the callee 
> to tell the caller whether it has succeeded?  In which case the 
> caller simply exits, and returns to *its* caller passing back the 
> correct answer and if necessary a flag indicating that it's 
> succeeded.  I have no idea whether I've identified the problem 
> correctly, but it's the best I can do without seeing any code. 
> Anyway, I think that's the general approach to handling backtracking 
> problems.
What you are doing here is just implementing by hand an exception
mechanism; that's the only solution when the language does not provide
exceptions, but fortunately we have them in Ada....

-- 
J-P. Rosen
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