On 29/01/2013, at 12:43 PM, Bob Hutchison <hutch-li...@recursive.ca> wrote:
> > The immediate problem is mapping an input to the system, some json message > containing a reference to the 'thing' (like a key of some kind). I have to > take that reference and find the thing and operate on it. All operations are > easily accommodated by a type class. However, since I can't have a collection > with mixed types even if the types satisfy a type class, I can't quite see > how to actually store the things so I can find them. > > So there are a couple of obvious ways to handle this. > > I could use an ADT and a sum type of all the known kinds of thing, but I > already know that this has to be extended and that's going to be problematic > with users doing this on their own. And the type signatures look ugly. So I > think that's not the best. > > I could use an ADT that contains functions that correspond to the functions > of the type class, and that close over the 'thing' in question. I think this > could be made to work, but I'm concerned with walking into more nasty > surprises… > My advice is to go for the latter option. I'm not sure what nasty surprises you are expecting, but this "record of functions" approach is the one that I normally take when I am building a system that needs new types added without requiring global changes. I know that existentials and GADTs are possible solutions, but I've not needed the extra complexity here. Cheers, Tim _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
