[darcs-users] darcs patch: add substitution mechanism for PatchChoices (and 4 more)

[Ganesh Sittampalam]

On Sat, 19 Sep 2009, Eric Kow wrote:

>>  negTag :: Tag -> Tag
>> -negTag (TG n) = TG (-n)
>> +negTag (TG k n) = TG k (-n)
>
> I have no idea what *negating* a tag is for, but I guess it's not too
> important that I understand it for now.

For what it's worth I didn't fully understand it either. But it happened 
with the previous direct numeric representation so I just translated it 
instead of spending a lot of time figuring out what was going on.

>> +-- Running canonize twice is apparently necessary to fix issue525;
>> +-- would be nice to understand why.
>
> I'm not sure if this helps any, but in a response to my 'applied, but
> huh?' review of issue525, David remarked that 'canonize' is really
> a misnomer for what is really a patch simplification routine.
>
> http://lists.osuosl.org/pipermail/darcs-users/2008-November/015902.html

Thanks, that is useful. Something to return to later in cleaning up code, 
I think. The QuickCheck properties I submitted will also be of some use in 
understanding this area, I hope.

Ganesh