Add flags for TypeSynonymInstances and FlexibleInstances

compiler/main/DynFlags.hs

@@ -183,6 +183,8 @@ data DynFlag
    | Opt_RecordPuns
    | Opt_GADTs
    | Opt_RelaxedPolyRec			-- -X=RelaxedPolyRec
+   | Opt_TypeSynonymInstances
+   | Opt_FlexibleInstances
    | Opt_MultiParamTypeClasses
    | Opt_FunctionalDependencies
    | Opt_MagicHash
@@ -1133,6 +1135,8 @@ xFlags = [
   ( "ExtendedDefaultRules",		Opt_ExtendedDefaultRules ),
   ( "ImplicitParams",			Opt_ImplicitParams ),
   ( "ScopedTypeVariables",  		Opt_ScopedTypeVariables ),
+  ( "TypeSynonymInstances",         Opt_TypeSynonymInstances ),
+  ( "FlexibleInstances",            Opt_FlexibleInstances ),
   ( "MultiParamTypeClasses",        Opt_MultiParamTypeClasses ),
   ( "FunctionalDependencies",        Opt_FunctionalDependencies ),
   ( "GeneralizedNewtypeDeriving",   Opt_GeneralizedNewtypeDeriving ),
@@ -1151,6 +1155,8 @@ glasgowExtsFlags = [ Opt_GlasgowExts
 		   , Opt_GADTs
 		   , Opt_ImplicitParams 
 		   , Opt_ScopedTypeVariables
+           , Opt_TypeSynonymInstances
+           , Opt_FlexibleInstances
            , Opt_MultiParamTypeClasses
            , Opt_FunctionalDependencies
 		   , Opt_MagicHash

compiler/typecheck/TcMType.lhs

@@ -1125,19 +1125,30 @@ checkValidInstHead ty	-- Should be a source type
 
 check_inst_head dflags clas tys
 	-- If GlasgowExts then check at least one isn't a type variable
-  | dopt Opt_GlasgowExts dflags
-  = mapM_ check_one tys
-
-	-- WITH HASKELL 98, MUST HAVE C (T a b c)
-  | otherwise
-  = checkTc (isSingleton tys && tcValidInstHeadTy first_ty)
-	    (instTypeErr (pprClassPred clas tys) head_shape_msg)
-
+  = do checkTc (dopt Opt_TypeSynonymInstances dflags ||
+                all tcInstHeadTyNotSynonym tys)
+               (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
+       checkTc (dopt Opt_FlexibleInstances dflags ||
+                all tcInstHeadTyAppAllTyVars tys)
+               (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
+       checkTc (dopt Opt_MultiParamTypeClasses dflags ||
+                isSingleton tys)
+               (instTypeErr (pprClassPred clas tys) head_one_type_msg)
+       mapM_ check_one tys
   where
-    (first_ty : _) = tys
-
-    head_shape_msg = parens (text "The instance type must be of form (T a1 ... an)" $$
-			     text "where T is not a synonym, and a1 ... an are distinct type *variables*")
+    head_type_synonym_msg = parens (
+                text "All instance types must be of the form (T t1 ... tn)" $$
+                text "where T is not a synonym." $$
+                text "Use -XTypeSynonymInstances if you want to disable this.")
+
+    head_type_args_tyvars_msg = parens (
+                text "All instance types must be of the form (T a1 ... an)" $$
+                text "where a1 ... an are distinct type *variables*" $$
+                text "Use -XFlexibleInstances if you want to disable this.")
+
+    head_one_type_msg = parens (
+                text "Only one type can be given in an instance head." $$
+                text "Use -XMultiParamTypeClasses if you want to allow more.")
 
 	-- For now, I only allow tau-types (not polytypes) in 
 	-- the head of an instance decl.  

compiler/typecheck/TcType.lhs

@@ -45,7 +45,8 @@ module TcType (
   tcSplitFunTy_maybe, tcSplitFunTys, tcFunArgTy, tcFunResultTy, tcSplitFunTysN,
   tcSplitTyConApp, tcSplitTyConApp_maybe, tcTyConAppTyCon, tcTyConAppArgs,
   tcSplitAppTy_maybe, tcSplitAppTy, tcSplitAppTys, repSplitAppTy_maybe,
-  tcValidInstHeadTy, tcGetTyVar_maybe, tcGetTyVar,
+  tcInstHeadTyNotSynonym, tcInstHeadTyAppAllTyVars,
+  tcGetTyVar_maybe, tcGetTyVar,
   tcSplitSigmaTy, tcMultiSplitSigmaTy, 
 
   ---------------------------------
@@ -790,14 +791,24 @@ tcSplitDFunHead tau
 	Just (ClassP clas tys) -> (clas, tys)
 	other -> panic "tcSplitDFunHead"
 
-tcValidInstHeadTy :: Type -> Bool
+tcInstHeadTyNotSynonym :: Type -> Bool
 -- Used in Haskell-98 mode, for the argument types of an instance head
 -- These must not be type synonyms, but everywhere else type synonyms
 -- are transparent, so we need a special function here
-tcValidInstHeadTy ty
+tcInstHeadTyNotSynonym ty
   = case ty of
-	NoteTy _ ty     -> tcValidInstHeadTy ty
-	TyConApp tc tys -> not (isSynTyCon tc) && ok tys
+	NoteTy _ ty     -> tcInstHeadTyNotSynonym ty
+	TyConApp tc tys -> not (isSynTyCon tc)
+	FunTy arg res   -> True
+	other		-> False
+
+tcInstHeadTyAppAllTyVars :: Type -> Bool
+-- Used in Haskell-98 mode, for the argument types of an instance head
+-- These must be a constructor applied to type variable arguments
+tcInstHeadTyAppAllTyVars ty
+  = case ty of
+	NoteTy _ ty     -> tcInstHeadTyAppAllTyVars ty
+	TyConApp _ tys  -> ok tys
 	FunTy arg res   -> ok [arg, res]
 	other		-> False
   where