[Haskell-cafe] Re: [Haskell] Re: Global Variables and IOinitializers

Graham Klyne GK at ninebynine.org
Tue Nov 30 03:52:42 EST 2004

At 13:43 29/11/04 -0800, John Meacham wrote:
>On Mon, Nov 29, 2004 at 03:09:53PM -0000, Simon Peyton-Jones wrote:
> > | > 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.
> >
> > Take care here.  There is no guarantee that the unique number generated
> > will be the same in each run.  So if you have Ord Typeable, this program
> > may give unpredictable results:
> >
> > main = print (typeOf True < typeOf 'x')
> >
> > This unfortunate observabilty of an ordering (or hash value) that is
> > needed only for efficient finite maps, is very annoying.  I wish I knew
> > a way round it.  As it is we can pick
> >       a) expose Ord/Hash, but have unpredictable results
> >       b) not have Ord/Hash, but have inefficient maps
>I thought it would be good to have two Ord classes, one to give the
>natural ordering (Ord) if one exists, and one to give the most efficient
>one for implementing maps/sets which has the side constraint that
>nothing may observably depend on what the actual order is, just that it
>is a valid total ordering. I have come across a few types where such a
>distinction would have been nice to have. either because the ordering
>was arbitrary so exposing it via 'Ord' seemed like a white lie to the
>user or a much more efficient yet non-intuitive ordering was possible..
>of course, the side condition here is pretty vauge. I don't know how to
>enforce it within the type system, but it is a pretty straightforward
>condition which I don't think would cause too much trouble in practice
>to maintain.

This reminds me of recent discussion about multiple flavours of Show.  Is 
there a pattern here?


Graham Klyne
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