I think that the main thing that concerns people about A12 is just that there are a lot of pieces involved, and they are not sure what you do and do not have to worry about. One message that people really need to hear loud and clear is that they don't need to understand all of them since the changes DWIM. For instance all of the stuff about lexical namespaces etc can be forgotten since if you don't use it, you won't be burned by anyone else using it, and if you do use it, then it does what you'd expect.

Another major concern is confusion about how traits differ from roles and classes differ from types (really, subtypes, but type is the keyword chosen at the moment). I admit to considerable confusion on traits vs roles until Larry said that a trait is a role on the class. As soon as I knew how the mechanism is supposed to work, I could see the difference. Stated in my own words, A role grants extra methods to objects that are given that role. The role does nothing until you call the methods that it provides, so it is relatively unobtrusive. We say, 'Foo does Bar;' to indicate that you have those methods. But if there is a role on your Class, that role could easily arrange to do whatever it wanted when you were being created. In other words it could have changed what you are, which is why we say, 'Foo is Bar;'. That far more intrusive thing is called a trait. As for classes vs types, while I can describe what a type is, I'm significantly less clear on what practical role is anticipated for them.

As for my areas of actual concern, they exist but I don't think that E12 is going to be able to address them. Here are a few:

1. Having read the discussion of how roles will be implemented with multiple inheritance, how well will the role/trait stuff play with people declaring the current inheritance model?
2. My major concern about any mix-in kind of design (including inheritance and roles) is that people can wind up having methods available and have no idea where the method came from. I'm hoping that there will be a limited number of very useful traits and roles that are built in (or at least widely used) and people don't go about building too many of their own. That will let me amortize the cost of learning them over a lot of usage. This is not, however, fundamentally a language design issue but an education one.
3. I'd like to understand the proposed aliasing mechanism better than I do. I understand the need (particularly when cooperating across languages), but I'd like to understand how it will be done.
4. The proposal that we have multiple versions of modules loaded at the same time scares me. I cannot help but think that it is an inherently fragile thing to attempt, particularly if someone does anything dynamic. Problems comes in many flavours, and I'm not sure which bothers me most. Here is a sample.
   • How can the feature help if developers have not specified versions for dependencies? I'm in the habit of just saying "use X", so do most people. Most do likewise. If most of CPAN follows suit, then this great feature will never assist me in the real world because I'll depend on some module that requires some specific version of another module, and the specific version was never specified. Relying on developer behaviour is fragile.
   • How do you handle versioning of proxied modules? For instance suppose that module A dynamically loads any of a number of modules, including B, behind the scenes on behalf of C. (Consider a plugin architecture to see why someone might do this.) How do you address version dependencies between C and B?
   • When will bugs go away? If module X is used by many others, and an annoying bug in it is fixed in module X, people can keep running into that bug until every other module has been updated to say that they work with the newer version of module X. This delay can be rather frustrating for the person who is trying to remove the bug on their local system.
   • What code am I REALLY running? When debugging, one of the more frustrating bugs to track down is when the code that you are running is not the code that you are looking at. Been there, done that. When you load multiple versions of the same software at the same time, this multiplies.
   • Where's my data? When you load CGI.pm, behind the scenes it can read in all of STDIN. (It has to for a POST.) So now a poor web developer has 2 modules, which require different versions of CGI.pm. One gets the data, and the other gets confused. As does the developer who is trying to figure out why CGI.pm just broke. This becomes an issue any time a module has to interact with external resources.
   • What about C libraries? So I'm using a nice XS module, and then go to load 2 versions of it. But each wants a different version of a .dll or .so (either platform, same issue). Unless you figure out how to make the versioning trick work down to the C level, it is going to break on lots of important modules.
   • Feeling bloated? While I'm hardly the first to argue for efficiency over everything else, loading multiple versions of the same software at the same time is a problem. Particularly if the module is supposed to do something like hold my database handle so that I don't need extra database connections...
When I first heard about the idea of trying to support having multiple versions of software on your system, I thought that it was a fundamentally broken idea. No, I don't want the added complexity when I think that it will fail, and complicate my life in the process. That opinion hasn't improved with time...

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