deriving-instances.md

@@ -49,3 +49,14 @@ The only real change is that some of the default declarations are provided gener
 [ http://www.haskell.org/ghc/dist/current/docs/users_guide/type-extensions.html\#stand-alone-deriving](http://www.haskell.org/ghc/dist/current/docs/users_guide/type-extensions.html#stand-alone-deriving) describes an implementation of something very like this in GHC.
 
 [ http://haskell.org/haskellwiki/GHC/Stand-alone_deriving_declarations](http://haskell.org/haskellwiki/GHC/Stand-alone_deriving_declarations) contains a great discussion of some of these ideas. (NB. Section 3 is misleading: leaving the clauses out of an instance declaration *does not* leave all the methods undefined; rather it gives the default methods of the class).
+
+
+As there are only a few fixed methods/classes that can be derived, we may choose not to have any special syntax for the few derived methods, but just put a comment in the code:
+
+```wiki
+class Eq a where
+  (==) :: a -> a -> Bool
+  (/=) :: a -> a -> Bool
+  -- (==) has a magic way of be derived for datatypes 
+  x /= y = not (x == y)
+```
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