[Haskell-cafe] Hashing over equivalence classes

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