On 2017-08-11 12:51, Lagi Pittas via use-livecode wrote:
Hi Mark,
I Beg to Differ about Next Repeat.
"It's not wat you do it's the way that you do it" comes to mind
Someone who writes spaghetti code can do it very well in any language
but
its much more difficult without the structure brought in by the
Algol/Pascal/Modula.
I'm not sure I quite get your point here...
Pure structured control flow does not allow any exit from a loop (it's
definition is actually related to properties of the 'control-flow graph'
(CFG) created by what are called 'basic blocks' when you analyze code at
a more fundamental level than the syntax - structured control-flow only
produces 'reducible CFGs' which have nice properties).
However, there are relatively straightforward transformation you can
apply to mostly-structured-control-flow with 'exit' to turn it into pure
structured control flow:
repeat while tCondition
... before ...
exit repeat
... after ...
end while
Can be re-written as:
repeat while tCondition and not tExit
... before ...
put true into tExit
if not tExit then
... after ...
end if
end repeat
The latter is 'pure structured control flow' - so 'exit loop' does no
harm; the compiler can do an 'easy' transformation on the code to make
it work; and that transformation is something which you have to do by
hand if 'exit loop' is not available.
The key thing here is extending 'exit loop' to allow you to exit from
more than one loop at once - if a language does not have this ability
directly, then you have to write code to do it yourself (which is always
possible - by the above transformation). Needing to exit multiple loops
is not uncommon - which is why that extension might well be useful.
Similarly, 'next repeat' is often useful at least in the current loop -
I'm not sure how useful it is for nested loops - it might not be.
However, there is a transformation you can do for that as well:
repeat while tCondition
... before ...
next repeat
... after ...
end while
Becomes:
repeat while tCondition
... before ...
put true into tNext
if not tNext then
... after ...
end if
end while
So, having written that, clearly it is fine (it is actually the same as
'exit repeat'; except that the boolean flag you need to create doesn't
become part of the loop condition).
If you only allow GOTOS within a procedure to a NON numeric label then
you
can write structured code in any language.
Not strictly true - I think the rules (in terms of the actual underlying
definition of structured control flow) is that you must not jump into a
block of code which has a jump from elsewhere at the top. e.g.
void func()
{
while(condition)
{
...
foo:
if (baz)
goto bar:
}
while(othercondition)
{
...
bar:
if (foobar)
goto foo:
}
}
This produces a non-reducible CFG if memory serves. So as soon as you
allow general GOTO, you end up with the ability to create irksome CFGs
from the point of view of a compiler.
Of course if it is time critical and every millisecond saved is
important
then if spaghetti code is needed to get the speed then there is no
"best
way" . But as I have said I have never missed Goto in nearly 40 Years
of
Coding in High level languages (you can't not use them in Machine code
which Dijkstra was acknowledging in his "Goto Considered Harmful"
article).
Indeed - the ability for a compiler to analyse code is the thing which
makes it potentially run faster. With GOTO you can do the optimizations
yourself; however, in almost all cases if you can only produce
structured control flow, then a suitable competent compiler can do the
work for you - making your code easier to read and maintain, whilst
still not loosing performance.
Warmest Regards,
Mark.
P.S. I should point out that LCS is not a 'suitably competent compiler'
as yet - however, that does not mean we should introduce language
features which make it much harder for it to be given greater
competence.
--
Mark Waddingham ~ m...@livecode.com ~ http://www.livecode.com/
LiveCode: Everyone can create apps
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