Easy fix for Trac #2409

[ghc-hetmet.git] / compiler / deSugar / DsUtils.lhs

diff --git a/compiler/deSugar/DsUtils.lhs b/compiler/deSugar/DsUtils.lhs
index cf171ce..2254ec2 100644 (file)
--- a/compiler/deSugar/DsUtils.lhs
+++ b/compiler/deSugar/DsUtils.lhs
@@ -168,8 +168,8 @@ mk_val_app (Var f `App` Type ty1 `App` Type _ `App` arg1) arg2 _ res_ty
   = Case arg1 case_bndr res_ty [(DEFAULT,[],arg2)]
   where
     case_bndr = case arg1 of
-                  Var v1 -> v1 -- Note [Desugaring seq (2)]
-                  _      -> mkWildId ty1
+                  Var v1 | isLocalId v1 -> v1  -- Note [Desugaring seq (2) and (3)]
+                  _                     -> mkWildId ty1
 
 mk_val_app fun arg arg_ty _    -- See Note [CoreSyn let/app invariant]
   | not (isUnLiftedType arg_ty) || exprOkForSpeculation arg
@@ -220,7 +220,7 @@ the seq, by re-introducing the space leak:
 We can try to avoid doing this by ensuring that the binder-swap in the
 case happens, so we get his at an early stage:
    case chp of chp2 { I# -> ...chp2... }
-But this is fragile.  The real culprit is the source program.  Perhpas we
+But this is fragile.  The real culprit is the source program.  Perhaps we
 should have said explicitly
    let !chp2 = chp in ...chp2...
 
@@ -234,6 +234,15 @@ Notice the shadowing of the case binder! And now all is well.
 The reason it's a hack is because if you define mySeq=seq, the hack
 won't work on mySeq.  
 
+Note [Desugaring seq (3)] cf Trac #2409
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The isLocalId ensures that we don't turn 
+       True `seq` e
+into
+       case True of True { ... }
+which stupidly tries to bind the datacon 'True'. 
+
+
 %************************************************************************
 %*                                                                     *
 \subsection{ Selecting match variables}