X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcExpr.lhs;h=784d466bde1ea9eca0e34cb5e358c84db3f3aff3;hb=389cca214f33a29646e08d57e3dca862140007b2;hp=b101cb5ad0b6eaacfa4c7dfc5c779db2b809199c;hpb=80d071f68134bf3ad89d4de0d83807e2f0ec32c0;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcExpr.lhs b/compiler/typecheck/TcExpr.lhs
index b101cb5..784d466 100644
--- a/compiler/typecheck/TcExpr.lhs
+++ b/compiler/typecheck/TcExpr.lhs
@@ -551,8 +551,8 @@ tcExpr expr@(RecordUpd record_expr rbinds _ _ _) res_ty
 	-- Figure out types for the scrutinee and result
 	-- Both are of form (T a b c), with fresh type variables, but with
 	-- common variables where the scrutinee and result must have the same type
-	-- These are variables that appear in *any* arg of *any* of the relevant constructors
-        -- *except* in the updated fields
+	-- These are variables that appear in *any* arg of *any* of the
+	-- relevant constructors *except* in the updated fields
 	-- 
 	; let fixed_tvs = getFixedTyVars con1_tvs relevant_cons
 	      is_fixed_tv tv = tv `elemVarSet` fixed_tvs
@@ -601,8 +601,9 @@ tcExpr expr@(RecordUpd record_expr rbinds _ _ _) res_ty
     			    fixed_tvs = exactTyVarsOfTypes fixed_tys
 			    	    -- fixed_tys: See Note [Type of a record update]
 			    	        `unionVarSet` tyVarsOfTheta theta 
-				    -- Universally-quantified tyvars that appear in any of the 
-				    -- *implicit* arguments to the constructor are fixed
+				    -- Universally-quantified tyvars that
+				    -- appear in any of the *implicit*
+				    -- arguments to the constructor are fixed
 			    	    -- See Note [Implict type sharing]
 			    	        
 		            fixed_tys = [ty | (fld,ty) <- zip flds arg_tys
@@ -1151,18 +1152,33 @@ thBrackId orig id ps_var lie_var
 		-- so we zap it to a LiftedTypeKind monotype
 		-- C.f. the call in TcPat.newLitInst
 
-	; setLIEVar lie_var	$ do
-	{ lift <- newMethodFromName orig id_ty' DsMeta.liftName
-		   -- Put the 'lift' constraint into the right LIE
+	; lift <- if isStringTy id_ty' then
+	       	     tcLookupId DsMeta.liftStringName
+		     -- See Note [Lifting strings]
+	          else
+                     setLIEVar lie_var	$ do  -- Put the 'lift' constraint into the right LIE
+                     newMethodFromName orig id_ty' DsMeta.liftName
 	   
 		   -- Update the pending splices
 	; ps <- readMutVar ps_var
 	; writeMutVar ps_var ((idName id, nlHsApp (nlHsVar lift) (nlHsVar id)) : ps)
 
-	; return id } }
+	; return id }
 #endif /* GHCI */
 \end{code}
 
+Note [Lifting strings]
+~~~~~~~~~~~~~~~~~~~~~~
+If we see $(... [| s |] ...) where s::String, we don't want to
+generate a mass of Cons (CharL 'x') (Cons (CharL 'y') ...)) etc.
+So this conditional short-circuits the lifting mechanism to generate
+(liftString "xy") in that case.  I didn't want to use overlapping instances
+for the Lift class in TH.Syntax, because that can lead to overlapping-instance
+errors in a polymorphic situation.  
+
+If this check fails (which isn't impossible) we get another chance; see
+Note [Converting strings] in Convert.lhs 
+
 Local record selectors
 ~~~~~~~~~~~~~~~~~~~~~~
 Record selectors for TyCons in this module are ordinary local bindings,