On Sat, Jun 7, 2008 at 3:03 PM, Dan Doel
On Friday 06 June 2008, Andrew Coppin wrote:
It's really quite frustrating that it is 100% impossible to write a single function that will process lists, arrays, sets, maps, byte strings, etc. You have to write several different versions. OK, so some functions really don't make sense for a set because it's unordered, and some functions don't make sense for a map, and so forth. But for example, if I write some complicated algorithm that uses a list to store data and I change that to a set instead, I now have to wade through the function changing every operation from a list-op into a set-op. It's really very annoying!
It's not 100% impossible, depending on what exactly you're doing. For instance...
Set a, ByteStrings, Seq a, Map k a and [a] are Monoids Set, Array i, Map k, Tree, Seq and [] are Foldable functors Array i, Map k, Tree, Seq, Tree and [] are Traversable functors
Those can get you lots of operations (folds, maps, unions...) although there may be gaps that could be filled (Foldable is sufficient to define null, for instance, but it isn't in Data.Foldable).
These do help in the case you want to traverse data structures that share a set of properties (e.g. are Functors). However, it doesn't address the problem of sharing an interface among Set, IntSet, ByteSet, etc. In other words there's a need for a Set type class. Hopefully type families will help us write such an interface and keep it efficient. Cheers, Johan