Re: [Haskell-cafe] Data.Typeable TypeRep Ord instance.
2010/12/30 Andreas Baldeau andr...@baldeau.net: instance Ord TypeRep where compare t1 t2 = compare (unsafePerformIO (typeRepKey t1)) (unsafePerformIO (typeRepKey t2)) I think it would suffice. Thank you for a tip. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data.Typeable TypeRep Ord instance.
On 01:08 Sun 05 Dec , Serguey Zefirov wrote: Why TypeRep does have equality and doesn't have ordering? It would be good to have that. I think the problem is, that it's hard to give an ordering that stays the same for all runs of your program. If you don't need this property you could use typeRepKey to give an instance as follows: instance Ord TypeRep where compare t1 t2 = compare (unsafePerformIO (typeRepKey t1)) (unsafePerformIO (typeRepKey t2)) I know it's not good style to use unsafePerformIO, but if you look at how typeRepKey is implemented I think it should be okay: typeRepKey :: TypeRep - IO Int typeRepKey (TypeRep (Key i) _ _) = return i (The Eq instance also uses the key for comparison.) Right now when I have to order two type representations I convert them to string and then compare. This is somewhat inefficient and not quite straightforward. The implementation above should be efficient but should not be used when data between multiple runs since the ordering may change. Andreas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data.Typeable TypeRep Ord instance.
On Sat, Dec 4, 2010 at 2:08 PM, Serguey Zefirov sergu...@gmail.com wrote: Why TypeRep does have equality and doesn't have ordering? It would be good to have that. Yes, I have wanted that too. It would make maps from types to values possible/efficient. There is a very critical path in jhc that use type-indexed data structures that I have to implement a very hacky workaround for no Ord instance for TypeRep John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data.Typeable TypeRep Ord instance.
Why should they? You can compare them in whatever way you like. And there isn't a natural/inherent sense of total order between types. On Sun, Dec 5, 2010 at 6:08 AM, Serguey Zefirov sergu...@gmail.com wrote: Why TypeRep does have equality and doesn't have ordering? It would be good to have that. Right now when I have to order two type representations I convert them to string and then compare. This is somewhat inefficient and not quite straightforward. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tianyi Cui ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data.Typeable TypeRep Ord instance.
2010/12/5 Tianyi Cui tianyi...@gmail.com:
Why should they? You can compare them in whatever way you like. And there
isn't a natural/inherent sense of total order between types.
I cannot compare then the way I'd like. ;)
Consider the following:
data BiMap a = BiMap {
values :: Map Int a
,indices :: Map a Int
}
It will serve well for Int's, Bool's and Expr's.
Then I decided to store typed Expr's, based on GADTs. Those Expr's
contains type indices and it would be natural to classify BiMaps by
their type indices and look up in there. If I require type indices to
be Typeable, all I need is ordering on TypeRep.
Also, I prototyped hypergraph library with hyperedges as types with
type family as HList of types denoting labels. There I needed ordering
on TypeRep too, again, for efficiency reasons.
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
