[Haskell-cafe] Hashing over equivalence classes

[Ryan Ingram]

For the second case you might be able to come up with a commutative
hash-combiner function for && and ||.

For the lambda-term situation, I can think of a couple ways to hash
that give what you want.

(1) Ignore variable names altogether while hashing; this gives you
what you want but has the disadvantage that (\a b. a) and (\a b. b)
hash to the same value.
(2) Hash the term with de Bruijn indices.  But this is the same as
"hash the canonical element".

I don't see that you have much other choice, though.  Fortunately, due
to laziness, hash . canonicalize should not have much worse space
behavior than just hash.

Did you have something else in mind?

  -- ryan

On Sat, Mar 14, 2009 at 3:51 AM, Roman Cheplyaka <roma at ro-che.info> wrote:
> Are there some known ways to define hashing (or any other) functions over
> equivalence classes? I.e.
>
>  a ~ b => hash(a) == hash(b)
>
> where (~) is some equivalence relation. For example, you might want to
> hash lambda terms modulo alpha-equivalence or hash logical terms with
> respect to commutativity of (&&) and (||).
>
> Often we can choose 'canonical' element from each class and hash it.
> But (at least, in theory) it's not necessary. So, are there (practical)
> ways to define hash function without it?
>
> --
> Roman I. Cheplyaka :: http://ro-che.info/
> "Don't let school get in the way of your education." - Mark Twain
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