X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcMType.lhs;h=1b7bb64c0d0c2711747a45c761b755f55d5594ad;hb=cf48cf640cc96fc0cb50b5c683cf16bbede064a0;hp=ca5ca0e409c2e7f687fbbeab8aaa976689f0e8af;hpb=69defef14097006a8fb35d9a03bb3bb2d842855f;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcMType.lhs b/compiler/typecheck/TcMType.lhs
index ca5ca0e..1b7bb64 100644
--- a/compiler/typecheck/TcMType.lhs
+++ b/compiler/typecheck/TcMType.lhs
@@ -1136,8 +1136,8 @@ check_inst_head dflags clas tys
   where
     (first_ty : _) = tys
 
-    head_shape_msg = parens (text "The instance type must be of form (T a b c)" $$
-			     text "where T is not a synonym, and a,b,c are distinct type variables")
+    head_shape_msg = parens (text "The instance type must be of form (T a1 ... an)" $$
+			     text "where T is not a synonym, and a1 ... an are distinct type *variables*")
 
 	-- For now, I only allow tau-types (not polytypes) in 
 	-- the head of an instance decl.  
@@ -1183,7 +1183,11 @@ checkValidInstance tyvars theta clas inst_tys
 			 undecidableMsg])
 \end{code}
 
-Termination test: each assertion in the context satisfies
+Termination test: the so-called "Paterson conditions" (see Section 5 of
+"Understanding functionsl dependencies via Constraint Handling Rules, 
+JFP Jan 2007).
+
+We check that each assertion in the context satisfies:
  (1) no variable has more occurrences in the assertion than in the head, and
  (2) the assertion has fewer constructors and variables (taken together
      and counting repetitions) than the head.