For a contrary point of view, there is a footnote at the bottom of page
20 in "Parsec, a fast combinator parser" by Daan Leijen, the creator of
Parsec:
"I have to warn the reader though that experience with the HaskellLight
compiler has shown that it hardly pays off in practice to use special
identifier representations instead of normal strings"
Thomas Conway wrote:
On 10/10/07, Michael Vanier <[EMAIL PROTECTED]> wrote:
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.
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
