Cubist's first bug report
mark at livecode.com
Mon Jun 6 13:06:19 EDT 2016
I just thought I'd sum up the situation with this bug and provide a
little more explanation.
On 2016-06-05 16:12, Quentin Long wrote:
> This handler *should* end up generating a 16-item string of integers
> which sum to exactly 100. What it *actually does* end up generating,
> is a 16-item string of integers whose sum may or may not fall
> somewhere within the range 80-120. Not sure what the hell is going on
> here, but I am not at all happy about it. Perhaps other people might
> like to try this code on their systems, and see if it works as
> intended for them?
This bug has the same underlying cause as
http://quality.livecode.com/show_bug.cgi?id=7919 - although that bug was
originally just reported against array subscript chunks, rather than
chunks in general.
This particular issues has always been present and is a side-effect of
how the engine currently handles chunk expressions.
If you have a command such as:
add 1 to item random() of tVar
Then the engine will do the equivalent of the following:
get item random() of tVar
add 1 to it
put it into item random() of tVar
This means that a chunk expression which contains expressions which
cause side-effects (or use functions which do not return the same result
for identical inputs - such as 'random()' / 'any') will not necessarily
work how you expect. This is because those side-effect causing
expressions will get evaluated twice.
From 7 onwards, the original bug report case was fixed. For commands
add 1 to tVar[random()]
The engine does something more like:
put random() into tIndex1
add 1 to it
put it into tVar[tIndex1]
This is because we changed the way that array subscripting operations
work then they are used as a container (i.e. read/write) so that the
'path' to the element is only evaluated once. A side-effect of this was
that we were able to implement the ability for array elements to be able
to passed by-ref (to @ parameters) which they previously could not.
We still need to change the way more general chunk expressions work to
do a similar thing. However, as it is quite a large piece of work to do
(and the behavior has always been the current way) it hasn't yet floated
to the top of the list. Once it is done though (as a bonus) it should be
possible to make arbitrary chunk expressions be passed by-ref like array
elements can now.
Beyond using 'any' in a container chunk expression (which should work
appropriately as it is part of the chunk expression itself), caution
should always be taken when composing commands where the sub-expressions
have overlapping side-effects:
command whatShouldThisDo? sInput
add char sIndex of line addOneToIndex() of sInput to tSum
add 1 to sIndex
Here, what range is used to compute the substring depends *entirely* on
order of evaluation which is not entirely obvious. One possible ordering
is strict left to right:
However, the more natural ordering from the point of view of the
operations being performed is actually:
This is because it follows the pattern of the underlying operations
which are actually needed:
1. Evaluate source container
2. Evaluate line range
3. Evaluate char of line
4. Evaluate number to add to
This ordering will actually end up with more 'generally' efficient code
also - predominantly because it ensures that the values which have been
evaluated only need to live for the minimum amount of time.
(As a side note, the way to see why this is a more 'natural' ordering
from the point of view of code execution is to rewrite the operation in
question in procedural form:
add(sInput.LineOf(addOneToIndex()).CharOf(sIndex), tSum -> tSum)
Here you can see that the second ordering *is* left to right, but only
after transforming from chunk syntax to function syntax.)
In an environment where side-effects could be completely known then
there wouldn't really be a problem here - you could choose any
well-defined ordering and the compiler could rearrange evaluations in
cases where there are no side-effect problems to ensure efficiency.
(Also, where side-effects do make things less efficient, the compiler
could warn you about this).
Unfortunately for us though, the dynamicity of the message path means
that it is impossible for the compiler to efficiently (thus making it
worthwhile!) check side-effects for cases where you have any non-private
handler call as a sub-expression - thus, the ordering of evaluation is
probably best made the one which generally provides the best performance
and to generally avoid sub-expressions with side-effects altogether
(which you can always do by storing expressions with side-effects into
temporary variables and *then* parsing them to a command).
Mark Waddingham ~ mark at livecode.com ~ http://www.livecode.com/
LiveCode: Everyone can create apps
More information about the Use-livecode