Separate flags -XDeriveFunctor, -XDeriveFoldable, -XDeriveTraversable

See Trac #2953. This patch implements a distinct flag for each extended
class that may be automatically derived.  And I updated the user manual
to reflect the fact that we can now derive Functor, Foldable, Traversable.

compiler/main/DynFlags.hs

@@ -222,9 +222,13 @@ data DynFlag
    | Opt_ViewPatterns
    | Opt_GADTs
    | Opt_RelaxedPolyRec
+
    | Opt_StandaloneDeriving
    | Opt_DeriveDataTypeable
    | Opt_DeriveFunctor
+   | Opt_DeriveTraversable
+   | Opt_DeriveFoldable
+
    | Opt_TypeSynonymInstances
    | Opt_FlexibleContexts
    | Opt_FlexibleInstances
@@ -1792,6 +1796,8 @@ xFlags = [
   ( "StandaloneDeriving",               Opt_StandaloneDeriving, const Supported ),
   ( "DeriveDataTypeable",               Opt_DeriveDataTypeable, const Supported ),
   ( "DeriveFunctor",                    Opt_DeriveFunctor, const Supported ),
+  ( "DeriveTraversable",                Opt_DeriveTraversable, const Supported ),
+  ( "DeriveFoldable",                   Opt_DeriveFoldable, const Supported ),
   ( "TypeSynonymInstances",             Opt_TypeSynonymInstances, const Supported ),
   ( "FlexibleContexts",                 Opt_FlexibleContexts, const Supported ),
   ( "FlexibleInstances",                Opt_FlexibleInstances, const Supported ),
@@ -1833,6 +1839,8 @@ glasgowExtsFlags = [
            , Opt_StandaloneDeriving
            , Opt_DeriveDataTypeable
            , Opt_DeriveFunctor
+           , Opt_DeriveFoldable
+           , Opt_DeriveTraversable
            , Opt_FlexibleContexts
            , Opt_FlexibleInstances
            , Opt_ConstrainedClassMethods

compiler/typecheck/TcDeriv.lhs

@@ -801,11 +801,15 @@ sideConditions cls
   | cls_key == enumClassKey    	   = Just (cond_std `andCond` cond_isEnumeration)
   | cls_key == ixClassKey      	   = Just (cond_std `andCond` cond_enumOrProduct)
   | cls_key == boundedClassKey 	   = Just (cond_std `andCond` cond_enumOrProduct)
-  | cls_key == dataClassKey    	   = Just (cond_mayDeriveDataTypeable `andCond` cond_std `andCond` cond_noUnliftedArgs)
-  | cls_key == functorClassKey 	   = Just (cond_functorOK True)	 -- NB: no cond_std!
-  | cls_key == foldableClassKey	   = Just (cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
-  | cls_key == traversableClassKey = Just (cond_functorOK False)
-  | getName cls `elem` typeableClassNames = Just (cond_mayDeriveDataTypeable `andCond` cond_typeableOK)
+  | cls_key == dataClassKey    	   = Just (checkFlag Opt_DeriveDataTypeable `andCond` 
+                                           cond_std `andCond` cond_noUnliftedArgs)
+  | cls_key == functorClassKey 	   = Just (checkFlag Opt_DeriveFunctor `andCond`
+    	                                   cond_functorOK True)	 -- NB: no cond_std!
+  | cls_key == foldableClassKey	   = Just (checkFlag Opt_DeriveFoldable `andCond`
+    	                                   cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
+  | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
+    	                                   cond_functorOK False)
+  | getName cls `elem` typeableClassNames = Just (checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK)
   | otherwise = Nothing
   where
     cls_key = getUnique cls
@@ -936,12 +940,16 @@ cond_functorOK allowFunctions (dflags, rep_tc)
     functions 	= ptext (sLit "contains function types")
     wrong_arg 	= ptext (sLit "uses the type variable in an argument other than the last")
 
-cond_mayDeriveDataTypeable :: Condition
-cond_mayDeriveDataTypeable (dflags, _)
- | dopt Opt_DeriveDataTypeable dflags = Nothing
- | otherwise = Just why
+checkFlag :: DynFlag -> Condition
+checkFlag flag (dflags, _)
+  | dopt flag dflags = Nothing
+  | otherwise        = Just why
   where
-    why  = ptext (sLit "You need -XDeriveDataTypeable to derive an instance for this class")
+    why = ptext (sLit "You need -X") <> text flag_str 
+          <+> ptext (sLit "to derive an instance for this class")
+    flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
+                 [s]   -> s
+                 other -> pprPanic "checkFlag" (ppr other)
 
 std_class_via_iso :: Class -> Bool
 std_class_via_iso clas	-- These standard classes can be derived for a newtype

docs/users_guide/glasgow_exts.xml

@@ -2682,7 +2682,7 @@ GHC always treats the <emphasis>last</emphasis> parameter of the instance
 
 
 <sect2 id="deriving-typeable">
-<title>Deriving clause for classes <literal>Typeable</literal> and <literal>Data</literal></title>
+<title>Deriving clause for extra classes (<literal>Typeable</literal>, <literal>Data</literal>, etc)</title>
 
 <para>
 Haskell 98 allows the programmer to add "<literal>deriving( Eq, Ord )</literal>" to a data type 
@@ -2692,11 +2692,11 @@ classes <literal>Eq</literal>, <literal>Ord</literal>,
 <literal>Enum</literal>, <literal>Ix</literal>, <literal>Bounded</literal>, <literal>Read</literal>, and <literal>Show</literal>.
 </para>
 <para>
-GHC extends this list with two more classes that may be automatically derived 
-(provided the <option>-XDeriveDataTypeable</option> flag is specified):
-<literal>Typeable</literal>, and <literal>Data</literal>.  These classes are defined in the library
-modules <literal>Data.Typeable</literal> and <literal>Data.Generics</literal> respectively, and the
-appropriate class must be in scope before it can be mentioned in the <literal>deriving</literal> clause.
+GHC extends this list with several more classes that may be automatically derived:
+<itemizedlist>
+<listitem><para> With <option>-XDeriveDataTypeable</option>, you can derive instances of the classes
+<literal>Typeable</literal>, and <literal>Data</literal>, defined in the library
+modules <literal>Data.Typeable</literal> and <literal>Data.Generics</literal> respectively.
 </para>
 <para>An instance of <literal>Typeable</literal> can only be derived if the
 data type has seven or fewer type parameters, all of kind <literal>*</literal>.
@@ -2712,6 +2712,26 @@ In other cases, there is nothing to stop the programmer writing a <literal>Typab
 class, whose kind suits that of the data type constructor, and
 then writing the data type instance by hand.
 </para>
+</listitem>
+
+<listitem><para> With <option>-XDeriveFunctor</option>, you can derive instances of 
+the class <literal>Functor</literal>,
+defined in <literal>GHC.Base</literal>.
+</para></listitem>
+
+<listitem><para> With <option>-XDeriveFoldable</option>, you can derive instances of 
+the class <literal>Foldable</literal>,
+defined in <literal>Data.Foldable</literal>.
+</para></listitem>
+
+<listitem><para> With <option>-XDeriveTraversable</option>, you can derive instances of 
+the class <literal>Traversable</literal>,
+defined in <literal>Data.Traversable</literal>.
+</para></listitem>
+</itemizedlist>
+In each case the appropriate class must be in scope before it 
+can be mentioned in the <literal>deriving</literal> clause.
+</para>
 </sect2>
 
 <sect2 id="newtype-deriving">