So I spent a few hours trying to modify my JSON code to work around this issue, to no avail (well I can get the serialization all right, but the deserialization seems to be tricky). So I've got another approach to work: since the default for Data instances is not right when your objects contains Map structures, lets rewrite these... Using the source for Data.Map and the documentation for Data.Generics.Data, I managed to get it to work, but of course the price is loads (loads, loads) of boilerplate code.
So here goes: I have the simple object: (assume import qualified Data.Map as M)
data MapObjF=MapObjF (M.Map String Int) String
So MapObjF contains a Map String Integer and a String
And here's the Data instance definition (big breath):
instance Data MapObjF where
gfoldl k z (MapObjF m s) = k (k (z (MapObjF . M.fromList)) (M.toList m)) s
gunfold k z _ = k (k (z (MapObjF . M.fromList)))
toConstr (MapObjF _ _) = con_MapObjF
dataTypeOf _ = ty_MapObjF
con_MapObjF = mkConstr ty_MapObjF "MapObjF"  Prefix
ty_MapObjF = mkDataType "MyModule.MapObjF" [con_MapObjF]
I define the constructor, the datatype, and implement gfoldl and gunfold transforming the map into a list of tuples. Of course if you have several constructors, loads of fields it soon becomes unwieldy. Now, is there a Haskell macro system so I can easily generate all that boilerplate for all my data types? Noooo I don't want to leave Haskell for LISP...