Unique as special boxing type & hidden constructors

[Edward Z. Yang]

The definition dates back to 1996, so it seems plausible that
newtype is the way to go now.

Edward

Excerpts from p.k.f.holzenspies's message of 2014-08-15 11:52:47 +0100:
> Dear all,
> 
> 
> I'm working with Alan to instantiate everything for Data.Data, so that we can do better SYB-traversals (which should also help newcomers significantly to get into the GHC code base). Alan's looking at the AST types, I'm looking at the basic types in the compiler.
> 
> Right now, I'm looking at Unique and two questions come up:
> 
> > data Unique = MkUnique FastInt
> 
> 
> 1) As someone already commented: Is there a specific reason (other than history) that this isn't simply a newtype around an Int? If we're boxing anyway, we may as well use the default Int boxing and newtype-coerce to the specific purpose of Unique, no?
> 
> 
> 2) As a general question for GHC hacking style; what is the reason for hiding the constructors in the first place?
> 
> I understand about abstraction and there are reasons for hiding, but there's a "public GHC API" and then there are all these modules that people can import at their own peril. Nothing is guaranteed about their consistency from version to version of GHC. I don't really see the point about hiding constructors (getting in the way of automatically deriving things) and then giving extra functions like (in the case of Unique):
> 
> > getKeyFastInt (MkUnique x) = x
> 
> > mkUniqueGrimily x = MkUnique (iUnbox x)
> 
> 
> I would propose to just make Unique a newtype for an Int and making the constructor visible.
> 
> 
> Regards,
> 
> Philip