[project @ 2003-12-16 16:23:25 by simonpj]

Fix newtype deriving for Enum

diff --git a/ghc/compiler/typecheck/TcDeriv.lhs b/ghc/compiler/typecheck/TcDeriv.lhs
index 85f0688..da42749 100644 (file)
--- a/ghc/compiler/typecheck/TcDeriv.lhs
+++ b/ghc/compiler/typecheck/TcDeriv.lhs
@@ -358,13 +358,13 @@ makeDerivEqns tycl_decls
            ]
 
     mk_eqn_help gla_exts NewType tycon clas tys
-      | can_derive_via_isomorphism && (gla_exts || standard_class gla_exts clas)
+      | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
       =                -- Go ahead and use the isomorphism
           traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)     `thenM_`
                   new_dfun_name clas tycon             `thenM` \ dfun_name ->
           returnM (Nothing, Just (InstInfo { iDFunId = mk_dfun dfun_name,
                                              iBinds = NewTypeDerived rep_tys }))
-      | standard_class gla_exts clas
+      | std_class gla_exts clas
       = mk_eqn_help gla_exts DataType tycon clas tys   -- Go via bale-out route
 
       | otherwise                              -- Non-standard instance
@@ -509,12 +509,18 @@ makeDerivEqns tycl_decls
                                       ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
 
     bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing) 
-    standard_class gla_exts clas =  key `elem` derivableClassKeys
-                                || (gla_exts && (key == typeableClassKey || key == dataClassKey))
-       where
-         key = classKey clas
-
 
+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]
+       -- Not Read/Show because they respect the type
+       -- Not Enum, becuase newtypes are never in Enum
 
 
 new_dfun_name clas tycon       -- Just a simple wrapper