On Thu, Feb 21, 2008 at 9:09 PM, John Meacham
On Thu, Feb 21, 2008 at 08:32:47PM +0100, Lemmih wrote:
I tried to implement this but, unfortunately, atoms are frequently being relied on for unique ids. Some cases are easy to fix, others less so. John, do you have time to document the intended behavior in the difficult cases?
I am not sure what you mean by difficult cases or what you are trying to fix, atoms are exactly an implementation of the standard atom type in computing as used in prolog, lisp, and the X11 protocol among other things. Uninterpreted strings with a very fast identity operation.
Atoms in general are fine but they're not the right tool for this particular job. We rarely use atoms for keys (I have the profiling information to back this up) and, when we do, using (hash,string) is ridiculously fast. The costs of using atoms include: broken properties (get . put = id), inefficiencies and obfuscated code. Given that I have significantly improved the performance of Jhc, I hope it carries some weight when I say that there are NO performance reasons for using atoms, neither in CPU time or memory usage.
Things got a lot better in terms of space once I made them a custom binary implementation, that saved 7 bytes an atom which is often as long as the string itself.
That's like giving ice cream to a kid with tuberculosis: It's a a nice thing to do but curing the disease would be better. Those 7 bytes would be completely irrelevant if we only saved unique strings once. There are about 9000 unique strings. A 7 byte overhead would be 63k. We currently waste 10,000k by saving ~100 copies of each unique string.
In any case, I would want any solution to be completely independent of the fact that atoms are being used as identifiers in a programming intermediate langauge.
I couldn't agree more. I say, let's take it one step further and assign unique ids to named variables in the same way we do with unnamed variables. Each variable would have a 'Anonymous | Named Name' tag that would be used for pretty-printing.
My Atom type is a generally useful library I use other places. I would think this would involve a custom Binary monad that distributed and collected an 'atom environment' of sorts that would then be stored in a different chunk in the file, then whenever one wanted to store/retrieve an atom, they would just add an index into the table.
The only other usage I've found was in Stats.hs. And that's definitely not justified. I was referring to cases like DataConstructors.hs and LambdaLift.hs. In DataConstructor.hs it is trivial to assign a new id since it only has to be locally unique. In LambdaLift.hs, on the other hand, I can't tell whether shadowing is OK, whether reusing the old id is OK, or where to get a set free variables if a unique id is required. Some documentation about what the code tries to do would be very helpful. -- Cheers, Lemmih