On Sat, Aug 18, 2007 at 03:33:19AM +0200, Benjamin Franksen wrote:
Adrian Hey wrote:
Benjamin Franksen wrote:
Adrian Hey wrote:
Is there a type that is an instance of Eq, but not Ord (or could not reasonably be made an instance of Ord)?
Data.Typeable.TypeRep
Interesting problem, but I don't see any reason why this could not be an instance of Ord.
I asked this question once and got as answer (from one of the Simons, IIRC) that exposing the obvious Ord instance would break referential transparency: there is no guarantee that the integer keys and therefore the order among TypeReps is the same for different runs, even of the same program.
This (counter-)example may be irrelevant for the discussion at hand or not. Anyway, you asked for one...
There's nothing stoping you from writing instance Ord TypeRep where compare = comparing show STRef could also be cheaply made Ord, but at the cost of a bit more work; the idea is to notice that GHC's compacting garbage collector never reorders "abstract heap addreses", a concept I just made up which starts at 0 and increases as new blocks are added to the heap; so one must create an inverse map from physical blocks (which are aligned, so finding metadata is just a bitmask) to abstract block numbers, and then use that information to implement a compareAddress# primop suitable for {ST,IO}Reff, TVar, and {ST,IO}{U,}Array (in the latter case, you would do address comparison for pinned arrays and arbitrarily declare all pinned arrays larger than all unpinned arrays). Stefan