Fix Trac #2597 (first bug): correct type checking for empty list

The GHC front end never generates (ExplicitList []), but TH can.
This patch makes the typechecker robust to such programs.

diff --git a/compiler/typecheck/TcExpr.lhs b/compiler/typecheck/TcExpr.lhs
index d7708b3..6260b89 100644 (file)
--- a/compiler/typecheck/TcExpr.lhs
+++ b/compiler/typecheck/TcExpr.lhs
@@ -315,9 +315,18 @@ tcExpr (HsIf pred b1 b2) res_ty
 tcExpr (HsDo do_or_lc stmts body _) res_ty
   = tcDoStmts do_or_lc stmts body res_ty
 
-tcExpr in_expr@(ExplicitList _ exprs) res_ty   -- Non-empty list
+tcExpr in_expr@(ExplicitList _ exprs) res_ty
   = do         { (elt_ty, coi) <- boxySplitListTy res_ty
        ; exprs' <- mapM (tc_elt elt_ty) exprs
+       ; when (null exprs) (zapToMonotype elt_ty >> return ())
+               -- If there are no expressions in the comprehension
+               -- we must still fill in the box
+               --
+               -- The GHC front end never generates an empty ExplicitList
+               -- (instead it generates the [] data constructor) but
+               -- Template Haskell might.  We could fix the bit of 
+               -- TH that generates ExplicitList, but it seems less
+               -- fragile to just handle the case here.
        ; return $ mkHsWrapCoI coi (ExplicitList elt_ty exprs') }
   where
     tc_elt elt_ty expr = tcPolyExpr expr elt_ty