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.