Understanding Darcs/Undoing mistakes
There are many ways to get rid of things: remove, rollback, revert, obliterate, unpull, unrecord... One would almost think too many. Only three of these are very important. In order of gravity, they are revert, unrecord, obliterate. You can also see them as being part of this table of symmetries:
(i.e: add, remove, mv, editing a file)
|less recent||pull||unpull (aka obliterate)|
Don't worry too much about this table. Surely it will make sense later on.
Anyway, here is a brief comparison of all the different ways you can get rid of stuff. Each one has its place, even the weird ones, like rollback.
The big three 
revert only removes what's new 
The simplest of these commands is revert. All reverting does is to get rid of stuff you have not recorded yet. You can think of revert as being the "opposite" of whatsnew. Revert gets rid of stuff that is new.
unrecord makes things new again 
Unrecord does something quite different from revert. Whereas revert gets rid of stuff that is new, unrecord removes a patch, but here's the important part, makes the stuff in it new again so that you can choose to re-record or revert it at your leisure.
Unrecord should only be used if you recorded something, realised you made a mistake, and want to record it differently (note also amend-record). Note: only use unrecord if you are sure that your repository is the only one that has that patch in it!
Note that the picture above gives a somewhat more accurate depiction of what unrecord does - it removes a patch and the corresponding modifications from the pristine tree. The fact that something is new again is just a natural consequence of this fact.
obliterate is unrecord + revert 
Obliterate was deliberately named to be scary. Obliterate can be seen as unrecording a patch (thus making its stuff new again) and then reverting it. In other words, obliterate totally wipes a patch out! Typically: you would use obliterate to go really far back in time. Say, "hmm, all that stuff I've been working on for the past three months was pretty stupid". Obliterate is the answer there.
|It is probably a good idea to darcs get a backup copy of your repository before obliterating things. This way, if you accidentally remove something by mistake, you can pull it back in|
Other ways to get rid of things 
unpull is obliterate 
Unpull and obliterate are exactly the same command and is only named this way to reflect its usefulness for undoing a pull.
Rollback is not a command that you can expect to use very often. The situation is this. You've got a patch you want to get rid of, but people have been telling you that you shouldn't obliterate or unrecord patches that are already in other people's repositories. So what do you do? One solution is to fire up your text editor and make exact opposite changes as the ones in the patch (and then record, etc). Another solution is to generate a rollback patch, which does the same thing. It creates a patch that does exactly the opposite of another patch.
Some users just find it easier to go the text-editor route.
remove doesn't belong here 
Despite its getting-rid-of-things style name, remove is not really an undo kind of command. Its job is the opposite of add's: it tells darcs not to pay attention to a file any longer. But as we mentioned in the previous chapter, most of the time you don't even need darcs remove. Simply telling your computer to get rid of the file in your working directory is good enough for darcs to notice it is gone.
Questions and objections 
- But... but... I just want to go back to the state of my repository from two weeks ago!
- obliterate is probably what you want. See above.