[darcs-users] darcs patch: rewrite partitionFL and partitionRL to reduce the numb...

[Ganesh Sittampalam]

On Sat, 1 Nov 2008, David Roundy wrote:

> On Thu, Oct 30, 2008 at 05:58:52AM +0000, Ganesh Sittampalam wrote:
>> On Wed, 29 Oct 2008, Jason Dagit wrote:
>>
>>> On Wed, Oct 29, 2008 at 1:35 PM, Ganesh Sittampalam <ganesh at earth.li>
>>> wrote:
>>>> Hi David,
>>>>
>>>> No rush to deal with this one, I'm just sending it in case you want to
>>>> and so that others can review it.
>>>>
>>>> This addresses a different quadratic blowup I found when running
>>>> whatsnew -sl on a directory with lots of files in it. It's a nice
>>>> advert for type witnesses being very helpful in creating confidence
>>>> in a refactoring.
>>>
>>> Yes, the GADT stuff makes refactoring quite nice.
>>>
>>> I looked at this patch bundle, I don't really see any problems with
>>> it, but I didn't look that closely.  Do you think it would be hard to
>>> write QuickCheck properties that compare the new implementation with
>>> the old one?  My reasoning is this: the old implementation was simpler
>>> and easier to inspect, so we can use it as a test answer oracle with
>>> reasonable confidence.
>>
>> Yes, I think it would be quite easy to write properties - the tricky bit
>> is generating the selection function. I don't think the old implementation
>> was actually simpler though, as it relied on commuteWhatWeCan which isn't
>> really very simply.
>>
>> For what it's worth I'm reasonably confident that I've reproduced the old
>> behaviour, though obviously QuickCheck properties wouldn't hurt. I think
>> my preferred approach to doing that would be to write them for the
>> specification I claim in the comments, and then to check both old and new
>> against the properties.
>
> It looks good to me, and is going in.  I agree with your suggestion of how
> to implement tests.  Can't coarbitrary be used to generate the selection
> function?

Probably, but coarbitrary always confuses the hell out of me until I think 
about it for a while, so I don't know off the top of my head. Now you 
mention it, it certainly fits with what I understand it is for, but I'd 
have to check into the details to be sure.

Ganesh