On Wed, Sep 24, 2008 at 5:18 PM, Jonathan Cast
Right. I (of course) take the global view as axiomatic, and I view Haskell 98's implementation something like this:
If an instance is defined (somewhere in existence), then it should be in scope everywhere. This is indistinguishable, in practice, from being in scope everywhere the class and type are both in scope. However, implementing this would require the implementation to search *every* module with an orphan instance when looking for instances. (Conceptually, this should include all of Hackage; if you use an instance from a package, but nothing else, under --make (or -e or -i) that package should (in principle) still be found and linked in.) Presumably, it's easier to only search the modules you've imported for instances. So a compromise is adopted: modules imported (directly or indirectly) have to be searched, but other modules can safely be ignored.
So I view refusing to import one module so you can define/import a conflicting instance as deliberate exploitation of a mis-feature/limitation of the language.
Another reason to prefer the global interpretation of instances is
that it allows for more efficient, non-dictionary-passing
implementations of type classes. Both JHC's type-case and Mark Jones's
partial evaluation techniques require instances that are the same
everywhere.
--
Dave Menendez