Improved error messages for higher-rank equality contexts

diff --git a/compiler/typecheck/TcMType.lhs b/compiler/typecheck/TcMType.lhs
index c574820..1ae1b2b 100644 (file)
--- a/compiler/typecheck/TcMType.lhs
+++ b/compiler/typecheck/TcMType.lhs
@@ -1071,10 +1071,13 @@ checkFreeness forall_tyvars theta
     complain pred    = addErrTc (freeErr pred)
 
 freeErr pred
-  = sep [ptext SLIT("All of the type variables in the constraint") <+> quotes (pprPred pred) <+>
-                  ptext SLIT("are already in scope"),
-        nest 4 (ptext SLIT("(at least one must be universally quantified here)"))
-    ]
+  = sep [ ptext SLIT("All of the type variables in the constraint") <+> 
+          quotes (pprPred pred)
+       , ptext SLIT("are already in scope") <+>
+          ptext SLIT("(at least one must be universally quantified here)")
+       , nest 4 $
+            ptext SLIT("(Use -XFlexibleContexts to lift this restriction)")
+        ]
 \end{code}
 
 \begin{code}
@@ -1085,7 +1088,7 @@ checkThetaCtxt ctxt theta
 badPredTyErr sty = ptext SLIT("Illegal constraint") <+> pprPred sty
 eqPredTyErr  sty = ptext SLIT("Illegal equational constraint") <+> pprPred sty
                   $$
-                  parens (ptext SLIT("Use -ftype-families to permit this"))
+                  parens (ptext SLIT("Use -XTypeFamilies to permit this"))
 predTyVarErr pred  = sep [ptext SLIT("Non type-variable argument"),
                          nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
 dupPredWarn dups   = ptext SLIT("Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)

diff --git a/compiler/types/TypeRep.lhs b/compiler/types/TypeRep.lhs
index a3a46b3..1b2f56d 100644 (file)
--- a/compiler/types/TypeRep.lhs
+++ b/compiler/types/TypeRep.lhs
@@ -501,14 +501,22 @@ ppr_type p (FunTy ty1 ty2)
 ppr_forall_type :: Prec -> Type -> SDoc
 ppr_forall_type p ty
   = maybeParen p FunPrec $
-    sep [pprForAll tvs, pprThetaArrow ctxt, pprType tau]
+    sep [pprForAll tvs, pprThetaArrow (ctxt1 ++ ctxt2), pprType tau]
   where
-    (tvs,  rho) = split1 [] ty
-    (ctxt, tau) = split2 [] rho
-
-    split1 tvs (ForAllTy tv ty) = split1 (tv:tvs) ty
-    split1 tvs (NoteTy _ ty)    = split1 tvs ty
-    split1 tvs ty              = (reverse tvs, ty)
+    (tvs, ctxt1, rho) = split1 [] [] ty
+    (ctxt2, tau)      = split2 [] rho
+
+    -- We need to be extra careful here as equality constraints will occur as
+    -- type variables with an equality kind.  So, while collecting quantified
+    -- variables, we separate the coercion variables out and turn them into
+    -- equality predicates.
+    split1 tvs eqs (ForAllTy tv ty) 
+      | isCoVar tv               = split1 tvs (eq:eqs) ty
+      | otherwise                = split1 (tv:tvs) eqs ty
+      where
+        PredTy eq = tyVarKind tv
+    split1 tvs eqs (NoteTy _ ty) = split1 tvs eqs ty
+    split1 tvs eqs ty           = (reverse tvs, reverse eqs, ty)
  
     split2 ps (NoteTy _ arg    -- Rather a disgusting case
               `FunTy` res)           = split2 ps (arg `FunTy` res)