On 10/10/07, Michael Vanier
Is there an implementation of a symbol type in Haskell i.e. a string which has a constant-time comparison operation?
To borrow Prolog terminology, it sounds like you're looking for an "atom" data type. I've not done it, but I've plotted to implement a module according to the following sketch: module Data.Atom where data Atom .... atom :: String -> Atom -- or ByteString name :: Atom -> String -- or ByteString instance Eq Atom where ... instance Ord Atom where ... The constructor function would do hash-consing using unsafePerformIO internally to build a [hash] table of extant Atoms. If ByteString is used for the internal "name", the hash consing means that you can use the Ptr for O(1) equality tests. The implementation of compare would still need to do a normal string comparison, after doing an initial equality test. If you do the O(1) equality test before doing a full compare, the performance will be very good in many situations, since non-equal comparisons tend to terminate quickly. The exception of course is strings with long common prefixes (e.g. URLs). For symbol names in a compiler, this is unlikely to be a significant problem. cheers, T. -- Thomas Conway drtomc@gmail.com Silence is the perfectest herald of joy: I were but little happy, if I could say how much.