From: simonpj@microsoft.com <unknown>
Date: Thu, 7 Jan 2010 15:11:13 +0000 (+0000)
Subject: A little refactoring, plus improve error locations
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=0884a2cb09cd5f609b6163a225ca3b8cce942250

A little refactoring, plus improve error locations

Fixes some sub-items of Trac #597
---

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index 3466cbf..5d2b829 100644
--- a/compiler/typecheck/TcDeriv.lhs
+++ b/compiler/typecheck/TcDeriv.lhs
@@ -440,8 +440,7 @@ deriveStandalone (L loc (DerivDecl deriv_ty))
               <+> text "tvs:" <+> ppr tvs
               <+> text "theta:" <+> ppr theta
               <+> text "tau:" <+> ppr tau)
-       ; (cls, inst_tys) <- checkValidInstHead tau
-       ; checkValidInstance tvs theta cls inst_tys
+       ; (cls, inst_tys) <- checkValidInstance deriv_ty tvs theta tau
 		-- C.f. TcInstDcls.tcLocalInstDecl1
 
        ; let cls_tys = take (length inst_tys - 1) inst_tys
diff --git a/compiler/typecheck/TcInstDcls.lhs b/compiler/typecheck/TcInstDcls.lhs
index c4c5d58..6ffa9d9 100644
--- a/compiler/typecheck/TcInstDcls.lhs
+++ b/compiler/typecheck/TcInstDcls.lhs
@@ -410,8 +410,7 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
         ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
 
         -- Now, check the validity of the instance.
-        ; (clas, inst_tys) <- checkValidInstHead tau
-        ; checkValidInstance tyvars theta clas inst_tys
+        ; (clas, inst_tys) <- checkValidInstance poly_ty tyvars theta tau
 
         -- Next, process any associated types.
         ; idx_tycons <- recoverM (return []) $
diff --git a/compiler/typecheck/TcMType.lhs b/compiler/typecheck/TcMType.lhs
index 6d6d102..492cbf9 100644
--- a/compiler/typecheck/TcMType.lhs
+++ b/compiler/typecheck/TcMType.lhs
@@ -70,7 +70,8 @@ import TyCon
 import Var
 
 -- others:
-import TcRnMonad          -- TcType, amongst others
+import HsSyn		-- HsType
+import TcRnMonad        -- TcType, amongst others
 import FunDeps
 import Name
 import VarEnv
@@ -1638,11 +1639,15 @@ instTypeErr pp_ty msg
 %*									*
 %************************************************************************
 
-
 \begin{code}
-checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
-checkValidInstance tyvars theta clas inst_tys
-  = do	{ undecidable_ok <- doptM Opt_UndecidableInstances
+checkValidInstance :: LHsType Name -> [TyVar] -> ThetaType -> Type 
+                   -> TcM (Class, [TcType])
+checkValidInstance hs_type tyvars theta tau
+  = setSrcSpan (getLoc hs_type) $
+    do	{ (clas, inst_tys) <- setSrcSpan head_loc $
+                              checkValidInstHead tau
+
+        ; undecidable_ok <- doptM Opt_UndecidableInstances
 
 	; checkValidTheta InstThetaCtxt theta
 	; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
@@ -1656,10 +1661,17 @@ checkValidInstance tyvars theta clas inst_tys
 	-- The Coverage Condition
 	; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
 	  	  (instTypeErr (pprClassPred clas inst_tys) msg)
+
+        ; return (clas, inst_tys)
 	}
   where
     msg  = parens (vcat [ptext (sLit "the Coverage Condition fails for one of the functional dependencies;"),
 			 undecidableMsg])
+
+	-- The location of the "head" of the instance
+    head_loc = case hs_type of
+                 L _ (HsForAllTy _ _ _ (L loc _)) -> loc
+                 L loc _                          -> loc
 \end{code}
 
 Termination test: the so-called "Paterson conditions" (see Section 5 of
@@ -1727,7 +1739,6 @@ Notice that this instance (just) satisfies the Paterson termination
 conditions.  Then we *could* derive an instance decl like this:
 
 	instance (C Int a, Eq b, Eq c) => Eq (T a b c) 
-
 even though there is no instance for (C Int a), because there just
 *might* be an instance for, say, (C Int Bool) at a site where we
 need the equality instance for T's.