instance visibility (was: Re: The base library and GHC 6.10)

[David Roundy]

On Wed, Sep 24, 2008 at 09:28:04AM -0700, Jonathan Cast wrote:
> On Wed, 2008-09-24 at 17:30 +0100, Claus Reinke wrote:
> > > (sorry for the delay in replying to this...)
> > 
> > No problem, though you seem to be restating your opinion 
> > intead of addressing my concrete points? For those who, like
> > me, have lost the thread in the meantime, here is a link to the 
> > message you reply to:
> > 
> > http://www.haskell.org/pipermail/libraries/2008-September/010623.html
> >  
> > >>> .., the only sensible way to think about instances is as global properties.
> > > ..it has nothing to do with bugs or misfeatures in GHC, it's a 
> > > fact of Haskell 98.
> > 
> > I thought my example demonstrated quite clearly that instances 
> > are *not* global in Haskell.
> > 
> > >> A type class specifies a relation between types. Both the types
> > >> and the class are named, and if instances are placed in separate
> > >> modules, the modules are named as well. The combination of
> > >> module, class and type names gives quite a bit of control over instance 
> > >> import/export, even if it is terribly cumbersome and
> > >> limited (and easily defeated by just one library importing all
> > >> instances "for convenience"). Neither the relation (class), nor
> > >> its domain (types), nor its extent (instances) are "global".
> > > 
> > > The point is that instances are unconditionally re-exported, 
> > 
> > Yes, and I'm not disputing that point. What I am disputing are
> > its consequences/interpretation. As I said, instances accumulate 
> > upwards along the import hierarchy. But they do not propagate 
> > downwards, so they do not have global scope, and one can exert 
> > some control over all of type relation (class), domain (types), and 
> > extent (instances).
> > 
> > What one cannot do (in Haskell 98) is to have two instances of
> > the same class, for the same types, in the same import hierarchy.
> 
> Of course, when you consider that Main, by definition, imports every
> module in the program directly or indirectly, this is pretty close to
> saying `instances are global'.  Isn't it?

No, because the Main module has a choice to only import certain
modules.  Thus when orphan instances are defined, we may have a choice
of which instances we want to use.

David