(By the way, sorry about cross-posting again. People have already replied to just 'haskell@' and just 'libraries@' but I'll try to stick to 'libraries@' after this since it seems that some users' mail clients show them two copies otherwise)
This idea has been raised before, but it was a while back, and we called it "grafting". Here's the start of the thread, which went on for quite some time:
http://www.haskell.org/pipermail/libraries/2003-August/001310.html
Actually it looks like that is slightly different from my idea, perhaps I should have expounded a bit more in my original post. Correct me if I'm wrong, but in my proposal: - grafting/mounting would be done per compilation unit. In yours it seems it would be done per (user, system). - configuration of graft/mount points would be done at compile time, zero or one times per package import option. In yours it would be at package install time. I think my proposal is better. In it there would be no cross-system compatibility issues: since each program would specify where its imported packages get mounted itself, you could write a Haskell program on one system and be assured that someone else could use it on another system without problems. I think this is a rather important property and we shouldn't allow it to be broken - a warning that, as you say, "This wouldn't be recommended though: any source code you write won't compile on someone else's system that doesn't have the same convention" is quite insufficient in my opinion! Plus, I think the ability to remount packages at non-standard locations is an important one, but for the above reason your proposal makes it too dangerous and therefore unusable in practice. Our proposals agree on this point:
The implementation must obey the following rule: When compiling a module belonging to a package, that package is temporarily grafted into the root of the module hierarchy.
It was kind of "implicit" in my proposal though. I think the "alternative design" which is mentioned in your proposal is interesting:
Alternative design: modules in the current package could be specified explicitly, perhaps by prefixing them with '.'. This would avoid the possibility of overlap between the current package and the global hierarchy, at the expense of having to add lots of extra '.'s.
I think that might be a useful feature. Obviously one could introduce the '.' syntax and still allow the present sytax to be used for backward compatibility. Frederik -- http://ofb.net/~frederik/