FIX #2845: Allow breakpoints on expressions with unlifted type

It turns out we can easily support breakpoints on expressions with
unlifted types, by translating

  case tick# of _ -> e

into

  let f = \s . case tick# of _ -> e
  in  f realWorld#

instead of just a plain let-binding.  This is the same trick that GHC
uses for abstracting join points of unlifted type.

In #2845, GHC has eta-expanded the tick expression, changing the
result type from IO a to (# State#, a #), which was the reason the
tick was suddenly being ignored.  By supporting ticks on unlifted
expressions we can make it work again, although some confusion might
arise because _result will no longer be available (it now has
unboxed-tuple type, so we can't bind it in the environment).  The
underlying problem here is that GHC does transformations like
eta-expanding the tick expressions, and there's nothing we can do to
prevent that.

diff --git a/compiler/ghci/ByteCodeGen.lhs b/compiler/ghci/ByteCodeGen.lhs
index 95aae77..888aeec 100644 (file)
--- a/compiler/ghci/ByteCodeGen.lhs
+++ b/compiler/ghci/ByteCodeGen.lhs
@@ -17,6 +17,7 @@ import LibFFI
 
 import Outputable
 import Name
+import MkId
 import Id
 import FiniteMap
 import ForeignCall
@@ -454,9 +455,21 @@ schemeE d s p (AnnLet binds (_,body))
 -- best way to calculate the free vars but it seemed like the least
 -- intrusive thing to do
 schemeE d s p exp@(AnnCase {})
-   | Just (_tickInfo, rhs) <- isTickedExp' exp
+   | Just (_tickInfo, _rhs) <- isTickedExp' exp
    = if isUnLiftedType ty
-        then schemeE d s p (snd rhs)
+        then do
+          -- If the result type is unlifted, then we must generate
+          --   let f = \s . case tick# of _ -> e 
+          --   in  f realWorld#
+          -- When we stop at the breakpoint, _result will have an unlifted
+          -- type and hence won't be bound in the environment, but the
+          -- breakpoint will otherwise work fine.
+          id <- newId (mkFunTy realWorldStatePrimTy ty)
+          st <- newId realWorldStatePrimTy
+          let letExp = AnnLet (AnnNonRec id (fvs, AnnLam st (emptyVarSet, exp)))
+                              (emptyVarSet, (AnnApp (emptyVarSet, AnnVar id) 
+                                                    (emptyVarSet, AnnVar realWorldPrimId)))
+          schemeE d s p letExp
         else do
           id <- newId ty
           -- Todo: is emptyVarSet correct on the next line?

diff --git a/compiler/main/InteractiveEval.hs b/compiler/main/InteractiveEval.hs
index 50eae9f..794459c 100644 (file)
--- a/compiler/main/InteractiveEval.hs
+++ b/compiler/main/InteractiveEval.hs
@@ -609,18 +609,22 @@ bindLocalsAtBreakpoint hsc_env apStack (Just info) = do
    --    - tidy the type variables
    --    - globalise the Id (Ids are supposed to be Global, apparently).
    --
-   let all_ids | isPointer result_id = result_id : new_ids
-               | otherwise           = new_ids
+   let result_ok = isPointer result_id
+                    && not (isUnboxedTupleType (idType result_id))
+
+       all_ids | result_ok = result_id : new_ids
+               | otherwise = new_ids
        (id_tys, tyvarss) = mapAndUnzip (skolemiseTy.idType) all_ids
        (_,tidy_tys) = tidyOpenTypes emptyTidyEnv id_tys
        new_tyvars = unionVarSets tyvarss             
-   let final_ids = zipWith setIdType all_ids tidy_tys
+       final_ids = zipWith setIdType all_ids tidy_tys
        ictxt0 = hsc_IC hsc_env
        ictxt1 = extendInteractiveContext ictxt0 final_ids new_tyvars
+
    Linker.extendLinkEnv [ (name,hval) | (name, Just hval) <- zip names mb_hValues ]
-   Linker.extendLinkEnv [(result_name, unsafeCoerce# apStack)]
+   when result_ok $ Linker.extendLinkEnv [(result_name, unsafeCoerce# apStack)]
    hsc_env1 <- rttiEnvironment hsc_env{ hsc_IC = ictxt1 }
-   return (hsc_env1, result_name:names, span)
+   return (hsc_env1, if result_ok then result_name:names else names, span)
   where
    mkNewId :: OccName -> Id -> IO Id
    mkNewId occ id = do