[Haskell-cafe] problems with derive/TH

Simon Peyton-Jones simonpj at microsoft.com
Wed Jun 4 05:09:47 EDT 2008

| World.hs:42:0:
|    No instances for (Eq (a (M.Map String Player)),
|                      Eq (a (M.Map ItemId Item)),
|                      Eq (a (M.Map PlayerId Player)),
|                      Eq (a (M.Map RoomId Room)),
|                      Eq (a RoomId))
|      arising from the 'deriving' clause of a data type declaration
|                   at World.hs:(42,0)-(50,14)

Yes, automatic 'deriving' in GHC became a little more restrictive in GHC 6.8 (I think), for very good reasons.

I think that's what your problem is.  The darcs repo http://cdsmith.twu.net/demos/mud seems to be offline today, so I can't look, but I think that's highly likely.  The solution is invariably to add a manual instance declaration, as you have done in your subsequent message.


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