Re: Theories

 Some other of David's publications about Theories are 
listed at http://groups.csail.mit.edu/pag/pubs/David-Saff.html

Charlie

> -----Original Message-----
> From: [email protected] 
> [mailto:[email protected]] On 
> Behalf Of Kelly Anderson
> Sent: Tuesday, February 12, 2008 4:33 PM
> To: [email protected]; [email protected]
> Subject: [nunit-developer] Theories
> 
> Hope the cross posting isn't too irritating, but this 
> potentially relates to 3.0 too...
> 
> In the recent "Data Driven Addins" thread on nunit-developer, 
> Charlie pointed to this very interesting paper on Theories.
> 
> http://shareandenjoy.saff.net/tdd-specifications.pdf
> 
> Recommended reading. I agree with Charlie that this may be 
> among the more important recent developments in TDD. IMHO, 
> the idea seems to have more depth and applicability than BDD, 
> for example. One of the most interesting things that it 
> explains in the paper is Assumptions.
> An Assumption is a way of excepting something from a theory. 
> For example... if you have a theory about dividing integers, 
> (at the bottom of page 7 in the original paper) it could be 
> expressed in NUnit-like terms as:
> 
> [Theory]
> public void multiplyAnyAmountByInteger(int amount, int multiplier) {
>    Assume.That(multiplier, Is.Not(0)); // The original paper 
> incorrectly put amount here instead of multiplier.
>    Assert.That(amount, Is.EqualTo(new
> Dollar(amount).times(multiplier).divideBy(multiplier).getAmount());
> }
> 
> Apologies if I've messed up the Constraint syntax usage... 
> it's still a bit weird to me.
> 
> What you want to have happen here is that if the multiplier 
> is 0, the function returns before calling the Assert. Of 
> course, you have to specify what values are passed into such a test.
> 
> I've been giving some thought to how we might implement the 
> Assume part of this... and it seems like it would be fairly 
> straight forward to implement Assume in a fashion similar to 
> Assert, but if the condition is satisfied, you would throw a 
> specific exception, say of type AssumptionException, that 
> NUnit could catch and continue processing with no negative 
> impact on the validity of the theory. That is, it's still 
> green if you pass in an amount of 0 because it never got to 
> the Assert.
> 
> You might be able to do it in some other way, but throwing 
> the exception seems like the most logical to me.
> 
> Question... How hard would it be given the current class 
> hierarchy and structure of Assert to implement Assume? Does 
> it seem like a good idea?
> 
> The paper suggests using something along the lines of 
> Agitator to make calls to the Theory. I think I would leave 
> that as an exercise to the reader... so that you could do 
> something like this:
> 
> public IEnumerable TestData
> {
>   get
>   {
>      // Fill an ArrayList with random data... allowing the 
> user to be the Agitator.
>   }
> }
> 
> [Theory]
> [PropertyData("TestData")]
> public void multiplyAnyAmountByInteger(int amount, int multiplier) {
>    Assume.That(multiplier, Is.Not(0));
>    Assert.That(amount, Is.EqualTo(new
> Dollar(amount).times(multiplier).divideBy(multiplier).getAmount());
> }
> 
> Does this seem like the right way to go forward? Does Assume 
> feel like it might be a 3.0 thing, or a 2.5 thing? Does 
> anyone think we should integrate Agitator type functionality?
> 
> Could we implement it so that this would be possible?
> 
> [Theory]
> [PropertyData("TestData")]
> [InlineData(0, 0)]
> [InlineData(1, 0)]
> [InlineData(0, 1)]
> public void multiplyAnyAmountByInteger(int amount, int multiplier) {
>   ...
> }
> 
> Meaning that it used all the values from the IEnumerable 
> PropertyData and the InlineData as well? That seems like it 
> would be very cool.
> 
> -Kelly
> 
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