Type checking for type synonym families

[ghc-hetmet.git] / compiler / typecheck / TcClassDcl.lhs

diff --git a/compiler/typecheck/TcClassDcl.lhs b/compiler/typecheck/TcClassDcl.lhs
index f4c7058..67f2945 100644 (file)
--- a/compiler/typecheck/TcClassDcl.lhs
+++ b/compiler/typecheck/TcClassDcl.lhs
@@ -425,7 +425,7 @@ mkMethId origin clas sel_id inst_tys
        rho_ty       = ASSERT( length tyvars == length inst_tys )
                       substTyWith tyvars inst_tys rho
        (preds,tau)  = tcSplitPhiTy rho_ty
-        first_pred   = head preds
+        first_pred   = ASSERT( not (null preds)) head preds
     in
        -- The first predicate should be of form (C a b)
        -- where C is the class in question
@@ -528,7 +528,7 @@ mkDefMethRhs origin clas inst_tys sel_id loc GenDefMeth
          -- case we require that the instance decl is for a single-parameter
          -- type class with type variable arguments:
          --    instance (...) => C (T a b)
-    clas_tyvar    = head (classTyVars clas)
+    clas_tyvar    = ASSERT (not (null (classTyVars clas))) head (classTyVars clas)
     Just tycon   = maybe_tycon
     maybe_tycon   = case inst_tys of 
                        [ty] -> case tcSplitTyConApp_maybe ty of
@@ -733,7 +733,7 @@ tcAddDeclCtxt decl thing_inside
                                 else "data type" ++ maybeInst
           | isFamilyDecl decl = "family"
 
-     maybeInst | isFamInstDecl decl = " family"
+     maybeInst | isFamInstDecl decl = " instance"
               | otherwise          = ""
 
      ctxt = hsep [ptext SLIT("In the"), text thing,