On 24/10/12 12:08, Jon Fairbairn wrote:
Is there a convenient way of handling a data structure with lots of fields of different types that may or may not be filled in?
Not sure about convenience, but here is a type safe solution with O(log n) lookups and updates. The idea is to define a GADT tree type with a fixed layout: -- define the structure type MyT = TBranch (TLeaf A) (TBranch (TLeaf B) (TLeaf C)) -- a value level tree that uses that structure type My = GTree MyT You still have to define the paths to the members pa = GL GH pb = GR (GL GH) pc = GR (GR GH) But once you have that you can perform lookups and updates: *Main> glookup pc (gupdate pa (Just A) (gupdate pc (Just C) gempty)) Just C It shouldn't be too hard to make a template haskell function that generates these paths. Or perhaps the corresponding lenses. Twan