[project @ 2003-04-08 13:03:58 by simonpj]

Better error messages

diff --git a/ghc/compiler/typecheck/TcDeriv.lhs b/ghc/compiler/typecheck/TcDeriv.lhs
index 1e178fa..bd9383f 100644 (file)
--- a/ghc/compiler/typecheck/TcDeriv.lhs
+++ b/ghc/compiler/typecheck/TcDeriv.lhs
@@ -466,9 +466,9 @@ makeDerivEqns tycl_decls
           && right_arity                       -- Well kinded;
                                                -- eg not: newtype T ... deriving( ST )
                                                --      because ST needs *2* type params
-          && n_tyvars_to_keep >= 0             -- Well kinded; 
+          && n_tyvars_to_keep >= 0             -- Type constructor has right kind:
                                                -- eg not: newtype T = T Int deriving( Monad )
-          && n_args_to_keep   >= 0             -- Well kinded: 
+          && n_args_to_keep   >= 0             -- Rep type has right kind: 
                                                -- eg not: newtype T a = T Int deriving( Monad )
           && eta_ok                            -- Eta reduction works
           && not (isRecursiveTyCon tycon)      -- Does not work for recursive tycons:
@@ -485,16 +485,19 @@ makeDerivEqns tycl_decls
 
        cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
                                (vcat [ptext SLIT("even with cunning newtype deriving:"),
-                                       if right_arity then empty else
-                                       quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1"),
-                                       if n_tyvars_to_keep >= 0 && n_args_to_keep >= 0 then empty else
-                                         ptext SLIT("the type constructor has wrong kind"),
-                                       if n_args_to_keep >= 0 then empty else
-                                         ptext SLIT("representation type has wrong kind"),
-                                       if eta_ok then empty else 
-                                         ptext SLIT("the eta-reduction property does not hold"),
-                                       if not (isRecursiveTyCon tycon) then empty else
+                                       if isRecursiveTyCon tycon then
                                          ptext SLIT("the newtype is recursive")
+                                       else empty,
+                                       if not right_arity then 
+                                         quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
+                                       else empty,
+                                       if not (n_tyvars_to_keep >= 0) then 
+                                         ptext SLIT("the type constructor has wrong kind")
+                                       else if not (n_args_to_keep >= 0) then
+                                         ptext SLIT("the representation type has wrong kind")
+                                       else if not eta_ok then 
+                                         ptext SLIT("the eta-reduction property does not hold")
+                                       else empty
                                      ])
 
        non_std_err = derivingThingErr clas tys tycon tyvars_to_keep