## Re: [XP] Test-first an infinite loop

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• ... I don t think you are confused at all. I think you are spot on. Bill is asking in the situation where an infinite loop will be the implementation how, or
Message 1 of 31 , Apr 5, 2010
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On Mon, Apr 5, 2010 at 8:59 AM, mthlr <mackinnon.m@...> wrote:

>
>
> I'm not sure I understand where the issue is.
>
> The infinite loop is an implementation choice.
> TDD says we need to write our test first and then do the simplest thing to
> make the tests go green.
> It seems like we're picking the solution and then trying to figure out the
> test to check for it (which isn't really tdd, is it?)
>
> Or am I misreading this discussion?
>
> On another hand, there are likely implementations that we can't reach (as
> I'd imagine Godel's incompleteness theorem (from mathematics) can apply
> here)
>
> Just a touch confused...
>
I don't think you are confused at all. I think you are spot on. Bill is
asking in the situation where an infinite loop "will" be the implementation
how, or is it even possible, does one get there TDD.

By "will be" I mean the obvious and simplest implementation appears to be an
infinite loop. So we sit down and start writing tests to see where it will
go. Has anybody ever done this and gotten to the infinite loop?

--
Curtis Cooley
curtis.cooley@...
home:http://curtiscooley.com
blog:http://ponderingobjectorienteddesign.blogspot.com
===============
Leadership is a potent combination of strategy and character. But if you
must be without one, be without the strategy.
-- H. Norman Schwarzkopf

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• Well, an infinite loop is definitely a special case construct - not something that you want to see in normal usage. Sounds like what we are looking for is a
Message 31 of 31 , Apr 9, 2010
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Well, an infinite loop is definitely a special case construct - not something that you want to see in normal usage.

Sounds like what we are looking for is a good use case - maybe a daemon that keeps running some processing method even when given no time by the current thread.

So I'd make the processing method simply increment a counter.

The first test just sets the counter to 0 and spawns a thread that runs our main method. The test then checks that the counter value eventually becomes at least 1 - and hence that our processing method got called. Note that spawning the thread is probably part of the test, not the method under test.

No loop required - just a call to our processing method.

The next test spawns the thread again, but this time checks the counter eventually becomes at least, say, 10.

I'd posit that the simplest way to pass the test is to surround the worker method call with a while(1) construct. If you wanted to go via ten calls to the worker method first, knock yourself out.

For completeness, I'd probably write a test that checks the counter with a decent time interval between the checks, and show that the counter value was continuing to increment. If you haven't reached while(1) already, then this test will surely get you there...

Yes, it isn't a fast test, and yes, it involves spawning threads. Not pretty. But I'd argue that you are working with a special use case here - even though on the face of it, the construct is a really simple one. It also gives a nice clean design (separating the "never stop" from the "processing" stuff) that lets you do nice things like checking error conditions and termination conditions if those ever become interesting.

On 8 Apr 2010, at 23:09, Tom wrote:

> Eggzackly - hence my hesitation.
>
> >
> > On Thu, Apr 8, 2010 at 11:06 AM, Tom <rossentj@...> wrote:
> > >
> > >
> > >
> > > I hesitate to suggest such ugly complexity - but ... maybe have the test start the target in a separate thread which it can kill when it's satisfied ... in a "finally" clause, of course....
> > >
> >
> > At which point it may no longer be the simplest thing that could
> > possibly work ;-)
> >
> > How would you change the test so that it was?
> >
>
>

--
Bill Michell
billmichell@...

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