Compiler versions

[Simon Marlow]

> > > I prefer George's idea of an abstract version type which is 
> > > an instance of Show, Read and Ord.  I'd probably also add 
> majorVersion and 
> > > minorVersion functions, which could return perhaps the 
> same abstract type. 
> 
> Sounds good to me.
> 
> > Using an abstract version type seems to be the best choice for the
> > reasons you give.  However, there's a compromise here:
> > 
> >   - If you have an abstract version type for the version of
> >     entity X, then you need one version type for each X, because
> >     they have different versioning policies.  (e.g. we can't
> >     use the same version type for the version of the 
> Haskell language
> >     and the version of the compiler, or versions of libraries).
> 
> Can't we solve that problem with classes?
> 
>     class (Show a, Read a, Ord a) => Version a where
>         version      :: a
>         majorVersion :: a -> a
>         minorVersion :: a -> a
> 
> Each entity (language, compiler, library) can indeed define its own
> versioning type and policy, with this abstract interface.  It just
> needs to publish the name of the concrete type that implements it.

There isn't really a "problem" here, just that having lots of different
version types might be unnecessary.  I'm not sure that having a unified
majorVersion/minorVersion interface buys very much - it implies a kind
of binary tree structure to versions, but then I have to say how that is
overlaid on the actual structure of each version type, and what it means
to compose majorVersion/minorVersion.

What would you use 'version' for, BTW?

For packages at least, we have to pick a single versioning policy for
all packages so that the library infrastructure can manage dependencies
without having to know a separate versioning policy for each individual
package.

Ok, well it's not that big a deal.  I guess we could have

  module System.Info -- or something
  
  data ImplementationVersion -- instacne Eq, Ord, Read, Show
  implementationVersion :: ImplementationVersion

  data HaskellVersion -- instance Eq, Ord, Read, Show
  haskellVersion :: HaskellVersion
  haskellExtensions :: [Extension]

and then have a separate PackageVersion type for packages.  I'll
probably re-use the PackageVersion type for GHC's version.

Cheers,
	Simon