Improve error messages for 'deriving' clauses

This patch improves the misleading error message reported in Trac #1133.

Please merge the patch to the 6.6.1 branch.

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index f86dd64..7a584cd 100644 (file)
--- a/compiler/typecheck/TcDeriv.lhs
+++ b/compiler/typecheck/TcDeriv.lhs
@@ -568,12 +568,6 @@ cond_glaExts (gla_exts, _rep_tc) | gla_exts  = Nothing
   where
     why  = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
 
-std_class gla_exts clas 
-  =  key `elem` derivableClassKeys
-  || (gla_exts && (key == typeableClassKey || key == dataClassKey))
-  where
-     key = classKey clas
-    
 std_class_via_iso clas -- These standard classes can be derived for a newtype
                        -- using the isomorphism trick *even if no -fglasgow-exts*
   = classKey clas `elem`  [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
@@ -599,18 +593,26 @@ mkNewTypeEqn orig gla_exts overlap_flag tvs cls cls_tys
             tycon tc_args 
             rep_tycon rep_tc_args
   | can_derive_via_isomorphism && (gla_exts || std_class_via_iso cls)
-  =    do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
-          ;    -- Go ahead and use the isomorphism
-            dfun_name <- new_dfun_name cls tycon
-          ; return (Nothing, Just (InstInfo { iSpec  = mk_inst_spec dfun_name,
-                                              iBinds = NewTypeDerived ntd_info })) }
-  | std_class gla_exts cls
-  = mkDataTypeEqn orig gla_exts tvs cls cls_tys tycon tc_args rep_tycon rep_tc_args    -- Go via bale-out route
-
-       -- Otherwise its a non-standard instance
+  = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
+       ;       -- Go ahead and use the isomorphism
+          dfun_name <- new_dfun_name cls tycon
+       ; return (Nothing, Just (InstInfo { iSpec  = mk_inst_spec dfun_name,
+                                           iBinds = NewTypeDerived ntd_info })) }
+
+  | isNothing mb_std_err       -- Use the standard H98 method
+  = do { loc <- getSrcSpanM
+       ; eqn <- mk_data_eqn loc orig tvs cls tycon tc_args rep_tycon rep_tc_args
+       ; return (Just eqn, Nothing) }
+
+       -- Otherwise we can't derive
   | gla_exts  = baleOut cant_derive_err        -- Too hard
-  | otherwise = baleOut non_std_err    -- Just complain about being a non-std instance
+  | otherwise = baleOut std_err                -- Just complain about being a non-std instance
   where
+       mb_std_err = checkSideConditions gla_exts cls cls_tys rep_tycon
+       std_err = derivingThingErr cls cls_tys tc_app $
+                 vcat [fromJust mb_std_err,
+                       ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")]
+
        -- Here is the plan for newtype derivings.  We see
        --        newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
        -- where t is a type,
@@ -752,7 +754,7 @@ mkNewTypeEqn orig gla_exts overlap_flag tvs cls cls_tys
        cant_derive_err = derivingThingErr cls cls_tys tc_app
                                (vcat [ptext SLIT("even with cunning newtype deriving:"),
                                        if isRecursiveTyCon tycon then
-                                         ptext SLIT("the newtype is recursive")
+                                         ptext SLIT("the newtype may be recursive")
                                        else empty,
                                        if not right_arity then 
                                          quotes (ppr (mkClassPred cls cls_tys)) <+> ptext SLIT("does not have arity 1")
@@ -765,10 +767,6 @@ mkNewTypeEqn orig gla_exts overlap_flag tvs cls cls_tys
                                          ptext SLIT("the eta-reduction property does not hold")
                                        else empty
                                      ])
-
-       non_std_err = derivingThingErr cls cls_tys tc_app
-                               (vcat [non_std_why cls,
-                                      ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
 \end{code}
 
 
@@ -1122,3 +1120,4 @@ badDerivedPred pred
          nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
 \end{code}
 
+ 
\ No newline at end of file