[Haskell-cafe] Re: [Haskell] Re: Global Variables and IO
benjamin.franksen at bessy.de
Mon Nov 29 05:57:31 EST 2004
On Monday 29 November 2004 11:35, George Russell wrote:
> (indexing with TypeRep)
> > This is yet another incidence where Robert Will's ByMaps would be
> > very useful
> In fact GHC at least *already* generates a unique integer for each
> TypeRep. A good idea, since it means comparisons can be done in unit
> time. Thus indexing can be done trivially using this integer as a
> hash function.
Yes, I have seen this in the code, too. The Ord and Typeable instances
should be trivial.
There was a recent discussion about allowing to derive an instance from
anywhere at the top-level, and not only in the type definition. This is
one more example where such a feature would be very useful.
Another related example is the class Typeable itself. It has been noted
by others that the current interface is not type safe, since mkTyCon
gets an arbitrary string as argument. (Unfortunately this means that
GlobalVariables.hs and ExecutionContext.hs aren't really type safe
Typeable would be completely safe if the only way to declare instances
would be to derive them, but this is only practical if it can be done
from anywhere outside the data type definition.
Can anyone think of a situation where adding a derived instance to an
abstract data type breaks one of its invariants?
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