On Thu, Apr 25, 2013 at 5:35 PM, Matthias Felleisen <matth...@ccs.neu.edu> wrote: > eqv? is Scheme's attempt at incorporating an approximation to the so-called > notion of observational equivalence. Two values are observationally > equivalent if they are interchangeable in all contexts -- wherever you see > one value, you can replace it with another value in an expression position. > (If these values could show up inside of strings it would be trivial to > distinguish all values.) You can easily show that this mathematically defined > function is not computable. Therefore eqv? must be an approximation. And the > rationale must be that it is useful in some programs. From a microscopic > perspective, eqv? is mostly like equal? but when its recursive traversal > reaches compound data it falls back on pointer equality. From a program > perspective, I will admit that I fail to have an example handy.
When eqv? was first defined, it made a little more sense than that -- it traversed all of Scheme's immutable "data structures": numbers and characters, making it a lot like Henry Baker's `egal`. In the intervening years, Racket has added lots of other immutable data structures, but we've kept `eqv?` the way it originally was. Sam ____________________ Racket Users list: http://lists.racket-lang.org/users
