[Haskell-cafe] RE: Efficient or predictable Ord for Typeable

[Simon Peyton-Jones]

The trouble is that *any* "function" can now deliver unpredictable
results.  Can I rely on the fact that
	foo :: Int -> Int
will always give the same answer given the same input.  Not any more.

I think the strongest argument here is that it's like a more benign
version of unsafePerformIO, whose existence also threatens foo's
predictability.

| -----Original Message-----
| From: George Russell [mailto:ger at informatik.uni-bremen.de]
| Sent: 30 November 2004 09:55
| To: haskell-cafe at haskell.org; Simon Peyton-Jones; John Meacham
| Subject: Efficient or predictable Ord for Typeable
| 
| Simon PJ wrote (snipped):
|  > 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
| 
| In this case there is a simple solution.  Simply define
| 
|     newtype FastUnpredictableInstances a = FastUnpredictableInstances
a
| 
| and define the fast Ord and Hash only for FastUnpredictableInstances.
| People who manage to type all that then have no grounds to grumble
| if they get bitten.  For something as basic as implementing execution
| contexts, FastUnpredictableInstances is acceptable.