[Haskell-cafe] Type classes, collections, sum types, closures, and a massive headache

[Tim Docker]

On 29/01/2013, at 12:43 PM, Bob Hutchison <hutch-lists at 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