isaac jones
Ross has asked for use cases for functional dependencies and so far has only two replies. Surely there are those on this list who have use of functional dependencies?
Personally, I have never used FDs, but I recall some discussion we had in the Hat tracing project about the possibility of using them to solve a problem we had. (In the end, I think we decided it was either too complex, or the particular FD use case was not supported by any compiler.) The idea was to drop from traced computations down to the original computations (unwrap), and lift them back up again (wrap), in order to run "trusted" code at the original speed. This class looks something like class Wrap a b | a -> b, b -> a where wrap :: a -> R b unwrap :: R b -> a Nothing too unusual I'm sure. But there are some subtleties to do with the way this class interacts with other classes. A sketch of these is given in a short memo, attached to the wiki under "Use Cases" at http://hackage.haskell.org/trac/haskell-prime/wiki/FunctionalDependencies The relevant section is 3.2. One interesting thing to note for the FD discussion is the need/possibility of defining the same multi-parameter class at several different kinds. Regards, Malcolm