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

Claus Reinke claus.reinke at
Wed Sep 24 13:48:15 EDT 2008

>> 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?

Close, but no cigar!-) The global view accepts strictly more
programs than the Haskell 98 view (see the mesage I linked 
to for an example). It would also refuse some programs that
Ghc (in non-Haskell 98 mode) accepts. And would stand in
the way of looking for more precise instance import/export
control (see both of my messages;-).


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