hlovatt schrieb:
> Tennent's book is quite old and talks about Pascal, I don't recall any
> specific discussion of exceptions and exceptions were not part of
> standard Pascal. So my guess is that any form of non-local jumps would
> not be covered by Tennent directly. The princiople says that the
> meaning of the code should not change if enclosed in a block, e.g.
> while loop, but it means any type of block and therefore also a try
> catch block.
I found out it is from around 1977, another reason why it is so hard to
find... citeseer doesn't have papers that old ;) But I also think that
an exception would be counted as exit point. On the other hand
simulating an exception by control variables is not very nice.
> So my reading is that any form of non-block structured jump is
> excluded by the principle, i.e. no return, break, continue, or throw
> statements. All these have to be simulated with a status variable.
> Interestingly if an inner class was used instead of a block then a
> control variable could be used and exceptions avoided all together:
>
> enum Status { CONTINUE, BREAK; }
>
> class Loop implements Method1< Void, Object > {
> Status status = CONTINUE;
> void breakLoop() { status = BREAK; }
> }
>
> class List {
> private Object[] data;
> ...
> void each( Loop l ) {
> for ( int i = 0; i < data.length && l.status == CONTINUE; i++ )
> { l.call( data[ i ] ); }
> }
> }
>
> // My favourite syntax follows!
> list.each method( e ) { if ( someCondition ) { breakLoop() } };
but you are not showing that the call to breakLoop() has to be at a
position where no further commands follow. so if you have
while (true) {
println "1"
if (b) break;
println "2"
}
you would have to split it in a b==true and b!=true part:
while (doLoop) {
if (b) {
println "1"
doLoop=false
} else {
println "1"
println "2"
}
}
which might have less exit points, but is also less readable in my eyes.
All in all it seems I will be no fan of Tennent's idea. In my last post
I used something compareable to your breakLoop() command, but I had in
mind that it throws an exception, that is cached before the block is
left and then a iterator method from outside could handle that.... in
your case, instead of doing a simple l.call you would have to put a
try-catch around to be able to skip parts of the contents of your Loop
subclass.
bye blackdrag
--
Jochen "blackdrag" Theodorou
Groovy Tech Lead (http://groovy.codehaus.org)
http://blackdragsview.blogspot.com/
http://www.g2one.com/
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