Comparing hash, ptr, str gives you a pretty good acceptance/rejection test. hash for the quick rejection, ptr for quick acceptance, str for accuracy. Especially since the particular fingerprints for Typeable at least are usually made up of 3 bytestrings that were just stuffed in and forgotten about.
That said, this would seem to bring ByteString or at least Ptr in at a pretty low level for the level of generality folks seem to be suddenly seeking. -Edward On Wed, Feb 20, 2013 at 12:12 PM, Ian Lynagh <i...@well-typed.com> wrote: > On Fri, Feb 15, 2013 at 02:45:19PM +0000, Simon Marlow wrote: > > > > Remember that fingerprinting is not hashing. For fingerprinting we > > need to have a realistic expectation of no collisions. I don't > > think FNV is suitable. > > > > I'm sure it would be possible to replace the C md5 code with some > > Haskell. Performance *is* important here though - Typeable is in > > the inner loop of certain generic programming libraries, like SYB. > > We currently just compare > hash(str) > for equality, right? Could we instead compare > (hash str, str) > ? That would be even more correct, even if a bad/cheap hash function is > used, and would only be slower for the case where the types match > (unless you're unlucky and get a hash collision). > > In fact, we may be able to arrange it so that in the equal case the > strings are normally exactly the same string, so we can do a cheap > pointer equality test (like ByteString already does) to make the equal > case fast too (falling back to actually checking the strings are equal, > if they aren't the same string). > > > Thanks > Ian > > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >
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