OpenEuphoria

Re: Major Bug in Interpreter [Attn: Robert Craig]

• Posted by Robert Craig <rds at RapidEuphoria.com> Feb 13, 2005
• 471 views

Mario Steele wrote:
> I just wanted report this bug before 2.5 went Final, cause this is indeed
> a major bug.  I've tested this under 2.5 and 2.4, and 2.4 works correctly,
> but 2.5 does not.  You can see it happening when you invoke the trace
> like I have it written.
> 
> }}}
<eucode>
> with trace
> sequence stack
> 
> constant NewValue = {0,0,1,0}
> stack = {}
> 
> trace(1)
> function recursive_call(integer i)
> 	integer ib
> 	stack &= 0
> 	ib = length(stack)
> 	stack[ib] = NewValue
> 	for x = 1 to 4 do
> 		if stack[ib][i] = 0 then
> 			stack[ib][i] = recursive_call(i+1)
> 		end if
> 	end for
> 	return ib
> end function
> 
> integer i
> i = recursive_call(1)
> ? stack
> machine_proc(26,0)
> </eucode>
{{{


It's not a bug.
It's a change in the way things work in a rare,
ambiguous, and undocumented area.

I should mention this in the 2.5 alpha release notes
or somewhere.

In order to efficiently support the new $ feature,
I had to change the order of operations when subscripts
are performed on the left-hand-side of an assignment. e.g.

stack[ib][i] = recursive_call(i+1)

In 2.4, all expressions are evaluated first, then
the lhs subscripting is performed and the rhs value is assigned.

In 2.5, I (generally) need to perform each lhs subscript as I go along,
from left to right, before evaluating the subscript expression
that comes after it, since I may need to know the proper value of $
in the subsequent subscript expression.

In 2.4, you obviously counted on recursive_call() being completed
before the subscripted assignment was done, and you counted
on stack being changed by the side-effects of that call, before
the assignment to stack was done.
None of this was documented in the Reference Manual. 
You just tried it, and it worked.

In 2.5, the lhs var being subscripted will keep its original
value (I just increment its ref count), and will then be 
modified by the assignment. The effect of any side-effects 
on that var (i.e. stack) will be lost
as the assignment is completed. If I don't preserve the original
value, I can easily have crashes due to your having 
"pulled the rug out from under me" while I'm in the middle of
trying to subscript a now non-existent value. And, no, I don't
want to insert checks all over the place for this bizarre situation.

You only have to worry about this change if you are doing 
a subscripted assignment with multiple subscripts on the lhs,
and in the same statement you are calling a function 
that has the side-effect of modifying the lhs var (even as it
is being assigned-to).

You can make things work the way you want, and also
make your code more readable, by performing the function call 
in a separate statement before trying to modify the lhs variable.

Regards,
   Rob Craig
   Rapid Deployment Software
   http://www.RapidEuphoria.com