Module hierarchy: ByteCode and Runtime (cf #13009)

Update haddock submodule

1 files changed, 1355 insertions, 0 deletions

diff --git a/compiler/GHC/Runtime/Heap/Inspect.hs b/compiler/GHC/Runtime/Heap/Inspect.hs
new file mode 100644
index 0000000000..de6f9a7af3
--- /dev/null
+++ b/compiler/GHC/Runtime/Heap/Inspect.hs
@@ -0,0 +1,1355 @@
+{-# LANGUAGE BangPatterns, CPP, ScopedTypeVariables, MagicHash #-}
+
+-----------------------------------------------------------------------------
+--
+-- GHC Interactive support for inspecting arbitrary closures at runtime
+--
+-- Pepe Iborra (supported by Google SoC) 2006
+--
+-----------------------------------------------------------------------------
+module GHC.Runtime.Heap.Inspect(
+     -- * Entry points and types
+     cvObtainTerm,
+     cvReconstructType,
+     improveRTTIType,
+     Term(..),
+
+     -- * Utils
+     isFullyEvaluatedTerm,
+     termType, mapTermType, termTyCoVars,
+     foldTerm, TermFold(..),
+     cPprTerm, cPprTermBase,
+
+     constrClosToName -- exported to use in test T4891
+ ) where
+
+#include "HsVersions.h"
+
+import GhcPrelude
+
+import GHC.Runtime.Interpreter as GHCi
+import GHCi.RemoteTypes
+import HscTypes
+
+import DataCon
+import Type
+import GHC.Types.RepType
+import qualified Unify as U
+import Var
+import TcRnMonad
+import TcType
+import TcMType
+import TcHsSyn ( zonkTcTypeToTypeX, mkEmptyZonkEnv, ZonkFlexi( RuntimeUnkFlexi ) )
+import TcUnify
+import TcEnv
+
+import TyCon
+import Name
+import OccName
+import Module
+import GHC.Iface.Env
+import Util
+import VarSet
+import BasicTypes       ( Boxity(..) )
+import TysPrim
+import PrelNames
+import TysWiredIn
+import DynFlags
+import Outputable as Ppr
+import GHC.Char
+import GHC.Exts.Heap
+import GHC.Runtime.Heap.Layout ( roundUpTo )
+
+import Control.Monad
+import Data.Maybe
+import Data.List ((\\))
+#if defined(INTEGER_GMP)
+import GHC.Exts
+import Data.Array.Base
+import GHC.Integer.GMP.Internals
+#elif defined(INTEGER_SIMPLE)
+import GHC.Exts
+import GHC.Integer.Simple.Internals
+#endif
+import qualified Data.Sequence as Seq
+import Data.Sequence (viewl, ViewL(..))
+import Foreign
+import System.IO.Unsafe
+
+
+---------------------------------------------
+-- * A representation of semi evaluated Terms
+---------------------------------------------
+
+data Term = Term { ty        :: RttiType
+                 , dc        :: Either String DataCon
+                               -- Carries a text representation if the datacon is
+                               -- not exported by the .hi file, which is the case
+                               -- for private constructors in -O0 compiled libraries
+                 , val       :: ForeignHValue
+                 , subTerms  :: [Term] }
+
+          | Prim { ty        :: RttiType
+                 , valRaw    :: [Word] }
+
+          | Suspension { ctype    :: ClosureType
+                       , ty       :: RttiType
+                       , val      :: ForeignHValue
+                       , bound_to :: Maybe Name   -- Useful for printing
+                       }
+          | NewtypeWrap{       -- At runtime there are no newtypes, and hence no
+                               -- newtype constructors. A NewtypeWrap is just a
+                               -- made-up tag saying "heads up, there used to be
+                               -- a newtype constructor here".
+                         ty           :: RttiType
+                       , dc           :: Either String DataCon
+                       , wrapped_term :: Term }
+          | RefWrap    {       -- The contents of a reference
+                         ty           :: RttiType
+                       , wrapped_term :: Term }
+
+termType :: Term -> RttiType
+termType t = ty t
+
+isFullyEvaluatedTerm :: Term -> Bool
+isFullyEvaluatedTerm Term {subTerms=tt} = all isFullyEvaluatedTerm tt
+isFullyEvaluatedTerm Prim {}            = True
+isFullyEvaluatedTerm NewtypeWrap{wrapped_term=t} = isFullyEvaluatedTerm t
+isFullyEvaluatedTerm RefWrap{wrapped_term=t}     = isFullyEvaluatedTerm t
+isFullyEvaluatedTerm _                  = False
+
+instance Outputable (Term) where
+ ppr t | Just doc <- cPprTerm cPprTermBase t = doc
+       | otherwise = panic "Outputable Term instance"
+
+----------------------------------------
+-- Runtime Closure information functions
+----------------------------------------
+
+isThunk :: GenClosure a -> Bool
+isThunk ThunkClosure{} = True
+isThunk APClosure{} = True
+isThunk APStackClosure{} = True
+isThunk _             = False
+
+-- Lookup the name in a constructor closure
+constrClosToName :: HscEnv -> GenClosure a -> IO (Either String Name)
+constrClosToName hsc_env ConstrClosure{pkg=pkg,modl=mod,name=occ} = do
+   let occName = mkOccName OccName.dataName occ
+       modName = mkModule (stringToUnitId pkg) (mkModuleName mod)
+   Right `fmap` lookupOrigIO hsc_env modName occName
+constrClosToName _hsc_env clos =
+   return (Left ("conClosToName: Expected ConstrClosure, got " ++ show (fmap (const ()) clos)))
+
+-----------------------------------
+-- * Traversals for Terms
+-----------------------------------
+type TermProcessor a b = RttiType -> Either String DataCon -> ForeignHValue -> [a] -> b
+
+data TermFold a = TermFold { fTerm        :: TermProcessor a a
+                           , fPrim        :: RttiType -> [Word] -> a
+                           , fSuspension  :: ClosureType -> RttiType -> ForeignHValue
+                                            -> Maybe Name -> a
+                           , fNewtypeWrap :: RttiType -> Either String DataCon
+                                            -> a -> a
+                           , fRefWrap     :: RttiType -> a -> a
+                           }
+
+
+data TermFoldM m a =
+                   TermFoldM {fTermM        :: TermProcessor a (m a)
+                            , fPrimM        :: RttiType -> [Word] -> m a
+                            , fSuspensionM  :: ClosureType -> RttiType -> ForeignHValue
+                                             -> Maybe Name -> m a
+                            , fNewtypeWrapM :: RttiType -> Either String DataCon
+                                            -> a -> m a
+                            , fRefWrapM     :: RttiType -> a -> m a
+                           }
+
+foldTerm :: TermFold a -> Term -> a
+foldTerm tf (Term ty dc v tt) = fTerm tf ty dc v (map (foldTerm tf) tt)
+foldTerm tf (Prim ty    v   ) = fPrim tf ty v
+foldTerm tf (Suspension ct ty v b) = fSuspension tf ct ty v b
+foldTerm tf (NewtypeWrap ty dc t)  = fNewtypeWrap tf ty dc (foldTerm tf t)
+foldTerm tf (RefWrap ty t)         = fRefWrap tf ty (foldTerm tf t)
+
+
+foldTermM :: Monad m => TermFoldM m a -> Term -> m a
+foldTermM tf (Term ty dc v tt) = mapM (foldTermM tf) tt >>= fTermM tf ty dc v
+foldTermM tf (Prim ty    v   ) = fPrimM tf ty v
+foldTermM tf (Suspension ct ty v b) = fSuspensionM tf ct ty v b
+foldTermM tf (NewtypeWrap ty dc t)  = foldTermM tf t >>=  fNewtypeWrapM tf ty dc
+foldTermM tf (RefWrap ty t)         = foldTermM tf t >>= fRefWrapM tf ty
+
+idTermFold :: TermFold Term
+idTermFold = TermFold {
+              fTerm = Term,
+              fPrim = Prim,
+              fSuspension  = Suspension,
+              fNewtypeWrap = NewtypeWrap,
+              fRefWrap = RefWrap
+                      }
+
+mapTermType :: (RttiType -> Type) -> Term -> Term
+mapTermType f = foldTerm idTermFold {
+          fTerm       = \ty dc hval tt -> Term (f ty) dc hval tt,
+          fSuspension = \ct ty hval n ->
+                          Suspension ct (f ty) hval n,
+          fNewtypeWrap= \ty dc t -> NewtypeWrap (f ty) dc t,
+          fRefWrap    = \ty t -> RefWrap (f ty) t}
+
+mapTermTypeM :: Monad m =>  (RttiType -> m Type) -> Term -> m Term
+mapTermTypeM f = foldTermM TermFoldM {
+          fTermM       = \ty dc hval tt -> f ty >>= \ty' -> return $ Term ty'  dc hval tt,
+          fPrimM       = (return.) . Prim,
+          fSuspensionM = \ct ty hval n ->
+                          f ty >>= \ty' -> return $ Suspension ct ty' hval n,
+          fNewtypeWrapM= \ty dc t -> f ty >>= \ty' -> return $ NewtypeWrap ty' dc t,
+          fRefWrapM    = \ty t -> f ty >>= \ty' -> return $ RefWrap ty' t}
+
+termTyCoVars :: Term -> TyCoVarSet
+termTyCoVars = foldTerm TermFold {
+            fTerm       = \ty _ _ tt   ->
+                          tyCoVarsOfType ty `unionVarSet` concatVarEnv tt,
+            fSuspension = \_ ty _ _ -> tyCoVarsOfType ty,
+            fPrim       = \ _ _ -> emptyVarSet,
+            fNewtypeWrap= \ty _ t -> tyCoVarsOfType ty `unionVarSet` t,
+            fRefWrap    = \ty t -> tyCoVarsOfType ty `unionVarSet` t}
+    where concatVarEnv = foldr unionVarSet emptyVarSet
+
+----------------------------------
+-- Pretty printing of terms
+----------------------------------
+
+type Precedence        = Int
+type TermPrinterM m    = Precedence -> Term -> m SDoc
+
+app_prec,cons_prec, max_prec ::Int
+max_prec  = 10
+app_prec  = max_prec
+cons_prec = 5 -- TODO Extract this info from GHC itself
+
+pprTermM, ppr_termM, pprNewtypeWrap :: Monad m => TermPrinterM m -> TermPrinterM m
+pprTermM y p t = pprDeeper `liftM` ppr_termM y p t
+
+ppr_termM y p Term{dc=Left dc_tag, subTerms=tt} = do
+  tt_docs <- mapM (y app_prec) tt
+  return $ cparen (not (null tt) && p >= app_prec)
+                  (text dc_tag <+> pprDeeperList fsep tt_docs)
+
+ppr_termM y p Term{dc=Right dc, subTerms=tt}
+{-  | dataConIsInfix dc, (t1:t2:tt') <- tt  --TODO fixity
+  = parens (ppr_term1 True t1 <+> ppr dc <+> ppr_term1 True ppr t2)
+    <+> hsep (map (ppr_term1 True) tt)
+-} -- TODO Printing infix constructors properly
+  = do { tt_docs' <- mapM (y app_prec) tt
+       ; return $ ifPprDebug (show_tm tt_docs')
+                             (show_tm (dropList (dataConTheta dc) tt_docs'))
+                  -- Don't show the dictionary arguments to
+                  -- constructors unless -dppr-debug is on
+       }
+  where
+    show_tm tt_docs
+      | null tt_docs = ppr dc
+      | otherwise    = cparen (p >= app_prec) $
+                       sep [ppr dc, nest 2 (pprDeeperList fsep tt_docs)]
+
+ppr_termM y p t@NewtypeWrap{} = pprNewtypeWrap y p t
+ppr_termM y p RefWrap{wrapped_term=t}  = do
+  contents <- y app_prec t
+  return$ cparen (p >= app_prec) (text "GHC.Prim.MutVar#" <+> contents)
+  -- The constructor name is wired in here ^^^ for the sake of simplicity.
+  -- I don't think mutvars are going to change in a near future.
+  -- In any case this is solely a presentation matter: MutVar# is
+  -- a datatype with no constructors, implemented by the RTS
+  -- (hence there is no way to obtain a datacon and print it).
+ppr_termM _ _ t = ppr_termM1 t
+
+
+ppr_termM1 :: Monad m => Term -> m SDoc
+ppr_termM1 Prim{valRaw=words, ty=ty} =
+    return $ repPrim (tyConAppTyCon ty) words
+ppr_termM1 Suspension{ty=ty, bound_to=Nothing} =
+    return (char '_' <+> whenPprDebug (text "::" <> ppr ty))
+ppr_termM1 Suspension{ty=ty, bound_to=Just n}
+--  | Just _ <- splitFunTy_maybe ty = return$ ptext (sLit("<function>")
+  | otherwise = return$ parens$ ppr n <> text "::" <> ppr ty
+ppr_termM1 Term{}        = panic "ppr_termM1 - Term"
+ppr_termM1 RefWrap{}     = panic "ppr_termM1 - RefWrap"
+ppr_termM1 NewtypeWrap{} = panic "ppr_termM1 - NewtypeWrap"
+
+pprNewtypeWrap y p NewtypeWrap{ty=ty, wrapped_term=t}
+  | Just (tc,_) <- tcSplitTyConApp_maybe ty
+  , ASSERT(isNewTyCon tc) True
+  , Just new_dc <- tyConSingleDataCon_maybe tc = do
+             real_term <- y max_prec t
+             return $ cparen (p >= app_prec) (ppr new_dc <+> real_term)
+pprNewtypeWrap _ _ _ = panic "pprNewtypeWrap"
+
+-------------------------------------------------------
+-- Custom Term Pretty Printers
+-------------------------------------------------------
+
+-- We can want to customize the representation of a
+--  term depending on its type.
+-- However, note that custom printers have to work with
+--  type representations, instead of directly with types.
+-- We cannot use type classes here, unless we employ some
+--  typerep trickery (e.g. Weirich's RepLib tricks),
+--  which I didn't. Therefore, this code replicates a lot
+--  of what type classes provide for free.
+
+type CustomTermPrinter m = TermPrinterM m
+                         -> [Precedence -> Term -> (m (Maybe SDoc))]
+
+-- | Takes a list of custom printers with a explicit recursion knot and a term,
+-- and returns the output of the first successful printer, or the default printer
+cPprTerm :: Monad m => CustomTermPrinter m -> Term -> m SDoc
+cPprTerm printers_ = go 0 where
+  printers = printers_ go
+  go prec t = do
+    let default_ = Just `liftM` pprTermM go prec t
+        mb_customDocs = [pp prec t | pp <- printers] ++ [default_]
+    mdoc <- firstJustM mb_customDocs
+    case mdoc of
+      Nothing -> panic "cPprTerm"
+      Just doc -> return $ cparen (prec>app_prec+1) doc
+
+  firstJustM (mb:mbs) = mb >>= maybe (firstJustM mbs) (return . Just)
+  firstJustM [] = return Nothing
+
+-- Default set of custom printers. Note that the recursion knot is explicit
+cPprTermBase :: forall m. Monad m => CustomTermPrinter m
+cPprTermBase y =
+  [ ifTerm (isTupleTy.ty) (\_p -> liftM (parens . hcat . punctuate comma)
+                                      . mapM (y (-1))
+                                      . subTerms)
+  , ifTerm (\t -> isTyCon listTyCon (ty t) && subTerms t `lengthIs` 2)
+           ppr_list
+  , ifTerm' (isTyCon intTyCon    . ty) ppr_int
+  , ifTerm' (isTyCon charTyCon   . ty) ppr_char
+  , ifTerm' (isTyCon floatTyCon  . ty) ppr_float
+  , ifTerm' (isTyCon doubleTyCon . ty) ppr_double
+  , ifTerm' (isIntegerTy         . ty) ppr_integer
+  ]
+ where
+   ifTerm :: (Term -> Bool)
+          -> (Precedence -> Term -> m SDoc)
+          -> Precedence -> Term -> m (Maybe SDoc)
+   ifTerm pred f = ifTerm' pred (\prec t -> Just <$> f prec t)
+
+   ifTerm' :: (Term -> Bool)
+          -> (Precedence -> Term -> m (Maybe SDoc))
+          -> Precedence -> Term -> m (Maybe SDoc)
+   ifTerm' pred f prec t@Term{}
+       | pred t    = f prec t
+   ifTerm' _ _ _ _  = return Nothing
+
+   isTupleTy ty    = fromMaybe False $ do
+     (tc,_) <- tcSplitTyConApp_maybe ty
+     return (isBoxedTupleTyCon tc)
+
+   isTyCon a_tc ty = fromMaybe False $ do
+     (tc,_) <- tcSplitTyConApp_maybe ty
+     return (a_tc == tc)
+
+   isIntegerTy ty = fromMaybe False $ do
+     (tc,_) <- tcSplitTyConApp_maybe ty
+     return (tyConName tc == integerTyConName)
+
+   ppr_int, ppr_char, ppr_float, ppr_double
+      :: Precedence -> Term -> m (Maybe SDoc)
+   ppr_int _ Term{subTerms=[Prim{valRaw=[w]}]} =
+      return (Just (Ppr.int (fromIntegral w)))
+   ppr_int _ _ = return Nothing
+
+   ppr_char _ Term{subTerms=[Prim{valRaw=[w]}]} =
+      return (Just (Ppr.pprHsChar (chr (fromIntegral w))))
+   ppr_char _ _ = return Nothing
+
+   ppr_float   _ Term{subTerms=[Prim{valRaw=[w]}]} = do
+      let f = unsafeDupablePerformIO $
+                alloca $ \p -> poke p w >> peek (castPtr p)
+      return (Just (Ppr.float f))
+   ppr_float _ _ = return Nothing
+
+   ppr_double  _ Term{subTerms=[Prim{valRaw=[w]}]} = do
+      let f = unsafeDupablePerformIO $
+                alloca $ \p -> poke p w >> peek (castPtr p)
+      return (Just (Ppr.double f))
+   -- let's assume that if we get two words, we're on a 32-bit
+   -- machine. There's no good way to get a DynFlags to check the word
+   -- size here.
+   ppr_double  _ Term{subTerms=[Prim{valRaw=[w1,w2]}]} = do
+      let f = unsafeDupablePerformIO $
+                alloca $ \p -> do
+                  poke p (fromIntegral w1 :: Word32)
+                  poke (p `plusPtr` 4) (fromIntegral w2 :: Word32)
+                  peek (castPtr p)
+      return (Just (Ppr.double f))
+   ppr_double _ _ = return Nothing
+
+   ppr_integer :: Precedence -> Term -> m (Maybe SDoc)
+#if defined(INTEGER_GMP)
+   -- Reconstructing Integers is a bit of a pain. This depends deeply
+   -- on the integer-gmp representation, so it'll break if that
+   -- changes (but there are several tests in
+   -- tests/ghci.debugger/scripts that will tell us if this is wrong).
+   --
+   --   data Integer
+   --     = S# Int#
+   --     | Jp# {-# UNPACK #-} !BigNat
+   --     | Jn# {-# UNPACK #-} !BigNat
+   --
+   --   data BigNat = BN# ByteArray#
+   --
+   ppr_integer _ Term{subTerms=[Prim{valRaw=[W# w]}]} =
+      return (Just (Ppr.integer (S# (word2Int# w))))
+   ppr_integer _ Term{dc=Right con,
+                      subTerms=[Term{subTerms=[Prim{valRaw=ws}]}]} = do
+      -- We don't need to worry about sizes that are not an integral
+      -- number of words, because luckily GMP uses arrays of words
+      -- (see GMP_LIMB_SHIFT).
+      let
+        !(UArray _ _ _ arr#) = listArray (0,length ws-1) ws
+        constr
+          | "Jp#" <- getOccString (dataConName con) = Jp#
+          | otherwise = Jn#
+      return (Just (Ppr.integer (constr (BN# arr#))))
+#elif defined(INTEGER_SIMPLE)
+   -- As with the GMP case, this depends deeply on the integer-simple
+   -- representation.
+   --
+   -- @
+   -- data Integer = Positive !Digits | Negative !Digits | Naught
+   --
+   -- data Digits = Some !Word# !Digits
+   --             | None
+   -- @
+   --
+   -- NB: the above has some type synonyms expanded out for the sake of brevity
+   ppr_integer _ Term{subTerms=[]} =
+      return (Just (Ppr.integer Naught))
+   ppr_integer _ Term{dc=Right con, subTerms=[digitTerm]}
+        | Just digits <- get_digits digitTerm
+        = return (Just (Ppr.integer (constr digits)))
+      where
+        get_digits :: Term -> Maybe Digits
+        get_digits Term{subTerms=[]} = Just None
+        get_digits Term{subTerms=[Prim{valRaw=[W# w]},t]}
+          = Some w <$> get_digits t
+        get_digits _ = Nothing
+
+        constr
+          | "Positive" <- getOccString (dataConName con) = Positive
+          | otherwise = Negative
+#endif
+   ppr_integer _ _ = return Nothing
+
+   --Note pprinting of list terms is not lazy
+   ppr_list :: Precedence -> Term -> m SDoc
+   ppr_list p (Term{subTerms=[h,t]}) = do
+       let elems      = h : getListTerms t
+           isConsLast = not (termType (last elems) `eqType` termType h)
+           is_string  = all (isCharTy . ty) elems
+           chars = [ chr (fromIntegral w)
+                   | Term{subTerms=[Prim{valRaw=[w]}]} <- elems ]
+
+       print_elems <- mapM (y cons_prec) elems
+       if is_string
+        then return (Ppr.doubleQuotes (Ppr.text chars))
+        else if isConsLast
+        then return $ cparen (p >= cons_prec)
+                    $ pprDeeperList fsep
+                    $ punctuate (space<>colon) print_elems
+        else return $ brackets
+                    $ pprDeeperList fcat
+                    $ punctuate comma print_elems
+
+        where getListTerms Term{subTerms=[h,t]} = h : getListTerms t
+              getListTerms Term{subTerms=[]}    = []
+              getListTerms t@Suspension{}       = [t]
+              getListTerms t = pprPanic "getListTerms" (ppr t)
+   ppr_list _ _ = panic "doList"
+
+
+repPrim :: TyCon -> [Word] -> SDoc
+repPrim t = rep where
+   rep x
+    -- Char# uses native machine words, whereas Char's Storable instance uses
+    -- Int32, so we have to read it as an Int.
+    | t == charPrimTyCon             = text $ show (chr (build x :: Int))
+    | t == intPrimTyCon              = text $ show (build x :: Int)
+    | t == wordPrimTyCon             = text $ show (build x :: Word)
+    | t == floatPrimTyCon            = text $ show (build x :: Float)
+    | t == doublePrimTyCon           = text $ show (build x :: Double)
+    | t == int32PrimTyCon            = text $ show (build x :: Int32)
+    | t == word32PrimTyCon           = text $ show (build x :: Word32)
+    | t == int64PrimTyCon            = text $ show (build x :: Int64)
+    | t == word64PrimTyCon           = text $ show (build x :: Word64)
+    | t == addrPrimTyCon             = text $ show (nullPtr `plusPtr` build x)
+    | t == stablePtrPrimTyCon        = text "<stablePtr>"
+    | t == stableNamePrimTyCon       = text "<stableName>"
+    | t == statePrimTyCon            = text "<statethread>"
+    | t == proxyPrimTyCon            = text "<proxy>"
+    | t == realWorldTyCon            = text "<realworld>"
+    | t == threadIdPrimTyCon         = text "<ThreadId>"
+    | t == weakPrimTyCon             = text "<Weak>"
+    | t == arrayPrimTyCon            = text "<array>"
+    | t == smallArrayPrimTyCon       = text "<smallArray>"
+    | t == byteArrayPrimTyCon        = text "<bytearray>"
+    | t == mutableArrayPrimTyCon     = text "<mutableArray>"
+    | t == smallMutableArrayPrimTyCon = text "<smallMutableArray>"
+    | t == mutableByteArrayPrimTyCon = text "<mutableByteArray>"
+    | t == mutVarPrimTyCon           = text "<mutVar>"
+    | t == mVarPrimTyCon             = text "<mVar>"
+    | t == tVarPrimTyCon             = text "<tVar>"
+    | otherwise                      = char '<' <> ppr t <> char '>'
+    where build ww = unsafePerformIO $ withArray ww (peek . castPtr)
+--   This ^^^ relies on the representation of Haskell heap values being
+--   the same as in a C array.
+
+-----------------------------------
+-- Type Reconstruction
+-----------------------------------
+{-
+Type Reconstruction is type inference done on heap closures.
+The algorithm walks the heap generating a set of equations, which
+are solved with syntactic unification.
+A type reconstruction equation looks like:
+
+  <datacon reptype>  =  <actual heap contents>
+
+The full equation set is generated by traversing all the subterms, starting
+from a given term.
+
+The only difficult part is that newtypes are only found in the lhs of equations.
+Right hand sides are missing them. We can either (a) drop them from the lhs, or
+(b) reconstruct them in the rhs when possible.
+
+The function congruenceNewtypes takes a shot at (b)
+-}
+
+
+-- A (non-mutable) tau type containing
+-- existentially quantified tyvars.
+--    (since GHC type language currently does not support
+--     existentials, we leave these variables unquantified)
+type RttiType = Type
+
+-- An incomplete type as stored in GHCi:
+--  no polymorphism: no quantifiers & all tyvars are skolem.
+type GhciType = Type
+
+
+-- The Type Reconstruction monad
+--------------------------------
+type TR a = TcM a
+
+runTR :: HscEnv -> TR a -> IO a
+runTR hsc_env thing = do
+  mb_val <- runTR_maybe hsc_env thing
+  case mb_val of
+    Nothing -> error "unable to :print the term"
+    Just x  -> return x
+
+runTR_maybe :: HscEnv -> TR a -> IO (Maybe a)
+runTR_maybe hsc_env thing_inside
+  = do { (_errs, res) <- initTcInteractive hsc_env thing_inside
+       ; return res }
+
+-- | Term Reconstruction trace
+traceTR :: SDoc -> TR ()
+traceTR = liftTcM . traceOptTcRn Opt_D_dump_rtti
+
+
+-- Semantically different to recoverM in TcRnMonad
+-- recoverM retains the errors in the first action,
+--  whereas recoverTc here does not
+recoverTR :: TR a -> TR a -> TR a
+recoverTR = tryTcDiscardingErrs
+
+trIO :: IO a -> TR a
+trIO = liftTcM . liftIO
+
+liftTcM :: TcM a -> TR a
+liftTcM = id
+
+newVar :: Kind -> TR TcType
+newVar = liftTcM . newFlexiTyVarTy
+
+newOpenVar :: TR TcType
+newOpenVar = liftTcM newOpenFlexiTyVarTy
+
+instTyVars :: [TyVar] -> TR (TCvSubst, [TcTyVar])
+-- Instantiate fresh mutable type variables from some TyVars
+-- This function preserves the print-name, which helps error messages
+instTyVars tvs
+  = liftTcM $ fst <$> captureConstraints (newMetaTyVars tvs)
+
+type RttiInstantiation = [(TcTyVar, TyVar)]
+   -- Associates the typechecker-world meta type variables
+   -- (which are mutable and may be refined), to their
+   -- debugger-world RuntimeUnk counterparts.
+   -- If the TcTyVar has not been refined by the runtime type
+   -- elaboration, then we want to turn it back into the
+   -- original RuntimeUnk
+
+-- | Returns the instantiated type scheme ty', and the
+--   mapping from new (instantiated) -to- old (skolem) type variables
+instScheme :: QuantifiedType -> TR (TcType, RttiInstantiation)
+instScheme (tvs, ty)
+  = do { (subst, tvs') <- instTyVars tvs
+       ; let rtti_inst = [(tv',tv) | (tv',tv) <- tvs' `zip` tvs]
+       ; return (substTy subst ty, rtti_inst) }
+
+applyRevSubst :: RttiInstantiation -> TR ()
+-- Apply the *reverse* substitution in-place to any un-filled-in
+-- meta tyvars.  This recovers the original debugger-world variable
+-- unless it has been refined by new information from the heap
+applyRevSubst pairs = liftTcM (mapM_ do_pair pairs)
+  where
+    do_pair (tc_tv, rtti_tv)
+      = do { tc_ty <- zonkTcTyVar tc_tv
+           ; case tcGetTyVar_maybe tc_ty of
+               Just tv | isMetaTyVar tv -> writeMetaTyVar tv (mkTyVarTy rtti_tv)
+               _                        -> return () }
+
+-- Adds a constraint of the form t1 == t2
+-- t1 is expected to come from walking the heap
+-- t2 is expected to come from a datacon signature
+-- Before unification, congruenceNewtypes needs to
+-- do its magic.
+addConstraint :: TcType -> TcType -> TR ()
+addConstraint actual expected = do
+    traceTR (text "add constraint:" <+> fsep [ppr actual, equals, ppr expected])
+    recoverTR (traceTR $ fsep [text "Failed to unify", ppr actual,
+                                    text "with", ppr expected]) $
+      discardResult $
+      captureConstraints $
+      do { (ty1, ty2) <- congruenceNewtypes actual expected
+         ; unifyType Nothing ty1 ty2 }
+     -- TOMDO: what about the coercion?
+     -- we should consider family instances
+
+
+-- | Term reconstruction
+--
+-- Given a pointer to a heap object (`HValue`) and its type, build a `Term`
+-- representation of the object. Subterms (objects in the payload) are also
+-- built up to the given `max_depth`. After `max_depth` any subterms will appear
+-- as `Suspension`s. Any thunks found while traversing the object will be forced
+-- based on `force` parameter.
+--
+-- Types of terms will be refined based on constructors we find during term
+-- reconstruction. See `cvReconstructType` for an overview of how type
+-- reconstruction works.
+--
+cvObtainTerm
+    :: HscEnv
+    -> Int      -- ^ How many times to recurse for subterms
+    -> Bool     -- ^ Force thunks
+    -> RttiType -- ^ Type of the object to reconstruct
+    -> ForeignHValue   -- ^ Object to reconstruct
+    -> IO Term
+cvObtainTerm hsc_env max_depth force old_ty hval = runTR hsc_env $ do
+  -- we quantify existential tyvars as universal,
+  -- as this is needed to be able to manipulate
+  -- them properly
+   let quant_old_ty@(old_tvs, old_tau) = quantifyType old_ty
+       sigma_old_ty = mkInvForAllTys old_tvs old_tau
+   traceTR (text "Term reconstruction started with initial type " <> ppr old_ty)
+   term <-
+     if null old_tvs
+      then do
+        term  <- go max_depth sigma_old_ty sigma_old_ty hval
+        term' <- zonkTerm term
+        return $ fixFunDictionaries $ expandNewtypes term'
+      else do
+              (old_ty', rev_subst) <- instScheme quant_old_ty
+              my_ty <- newOpenVar
+              when (check1 quant_old_ty) (traceTR (text "check1 passed") >>
+                                          addConstraint my_ty old_ty')
+              term  <- go max_depth my_ty sigma_old_ty hval
+              new_ty <- zonkTcType (termType term)
+              if isMonomorphic new_ty || check2 (quantifyType new_ty) quant_old_ty
+                 then do
+                      traceTR (text "check2 passed")
+                      addConstraint new_ty old_ty'
+                      applyRevSubst rev_subst
+                      zterm' <- zonkTerm term
+                      return ((fixFunDictionaries . expandNewtypes) zterm')
+                 else do
+                      traceTR (text "check2 failed" <+> parens
+                                       (ppr term <+> text "::" <+> ppr new_ty))
+                      -- we have unsound types. Replace constructor types in
+                      -- subterms with tyvars
+                      zterm' <- mapTermTypeM
+                                 (\ty -> case tcSplitTyConApp_maybe ty of
+                                           Just (tc, _:_) | tc /= funTyCon
+                                               -> newOpenVar
+                                           _   -> return ty)
+                                 term
+                      zonkTerm zterm'
+   traceTR (text "Term reconstruction completed." $$
+            text "Term obtained: " <> ppr term $$
+            text "Type obtained: " <> ppr (termType term))
+   return term
+    where
+  go :: Int -> Type -> Type -> ForeignHValue -> TcM Term
+   -- I believe that my_ty should not have any enclosing
+   -- foralls, nor any free RuntimeUnk skolems;
+   -- that is partly what the quantifyType stuff achieved
+   --
+   -- [SPJ May 11] I don't understand the difference between my_ty and old_ty
+
+  go 0 my_ty _old_ty a = do
+    traceTR (text "Gave up reconstructing a term after" <>
+                  int max_depth <> text " steps")
+    clos <- trIO $ GHCi.getClosure hsc_env a
+    return (Suspension (tipe (info clos)) my_ty a Nothing)
+  go !max_depth my_ty old_ty a = do
+    let monomorphic = not(isTyVarTy my_ty)
+    -- This ^^^ is a convention. The ancestor tests for
+    -- monomorphism and passes a type instead of a tv
+    clos <- trIO $ GHCi.getClosure hsc_env a
+    case clos of
+-- Thunks we may want to force
+      t | isThunk t && force -> do
+         traceTR (text "Forcing a " <> text (show (fmap (const ()) t)))
+         liftIO $ GHCi.seqHValue hsc_env a
+         go (pred max_depth) my_ty old_ty a
+-- Blackholes are indirections iff the payload is not TSO or BLOCKING_QUEUE. If
+-- the indirection is a TSO or BLOCKING_QUEUE, we return the BLACKHOLE itself as
+-- the suspension so that entering it in GHCi will enter the BLACKHOLE instead
+-- of entering the TSO or BLOCKING_QUEUE (which leads to runtime panic).
+      BlackholeClosure{indirectee=ind} -> do
+         traceTR (text "Following a BLACKHOLE")
+         ind_clos <- trIO (GHCi.getClosure hsc_env ind)
+         let return_bh_value = return (Suspension BLACKHOLE my_ty a Nothing)
+         case ind_clos of
+           -- TSO and BLOCKING_QUEUE cases
+           BlockingQueueClosure{} -> return_bh_value
+           OtherClosure info _ _
+             | tipe info == TSO -> return_bh_value
+           UnsupportedClosure info
+             | tipe info == TSO -> return_bh_value
+           -- Otherwise follow the indirectee
+           -- (NOTE: This code will break if we support TSO in ghc-heap one day)
+           _ -> go max_depth my_ty old_ty ind
+-- We always follow indirections
+      IndClosure{indirectee=ind} -> do
+         traceTR (text "Following an indirection" )
+         go max_depth my_ty old_ty ind
+-- We also follow references
+      MutVarClosure{var=contents}
+         | Just (tycon,[world,contents_ty]) <- tcSplitTyConApp_maybe old_ty
+             -> do
+                  -- Deal with the MutVar# primitive
+                  -- It does not have a constructor at all,
+                  -- so we simulate the following one
+                  -- MutVar# :: contents_ty -> MutVar# s contents_ty
+         traceTR (text "Following a MutVar")
+         contents_tv <- newVar liftedTypeKind
+         MASSERT(isUnliftedType my_ty)
+         (mutvar_ty,_) <- instScheme $ quantifyType $ mkVisFunTy
+                            contents_ty (mkTyConApp tycon [world,contents_ty])
+         addConstraint (mkVisFunTy contents_tv my_ty) mutvar_ty
+         x <- go (pred max_depth) contents_tv contents_ty contents
+         return (RefWrap my_ty x)
+
+ -- The interesting case
+      ConstrClosure{ptrArgs=pArgs,dataArgs=dArgs} -> do
+        traceTR (text "entering a constructor " <> ppr dArgs <+>
+                      if monomorphic
+                        then parens (text "already monomorphic: " <> ppr my_ty)
+                        else Ppr.empty)
+        Right dcname <- liftIO $ constrClosToName hsc_env clos
+        (mb_dc, _)   <- tryTc (tcLookupDataCon dcname)
+        case mb_dc of
+          Nothing -> do -- This can happen for private constructors compiled -O0
+                        -- where the .hi descriptor does not export them
+                        -- In such case, we return a best approximation:
+                        --  ignore the unpointed args, and recover the pointeds
+                        -- This preserves laziness, and should be safe.
+                       traceTR (text "Not constructor" <+> ppr dcname)
+                       let dflags = hsc_dflags hsc_env
+                           tag = showPpr dflags dcname
+                       vars     <- replicateM (length pArgs)
+                                              (newVar liftedTypeKind)
+                       subTerms <- sequence $ zipWith (\x tv ->
+                           go (pred max_depth) tv tv x) pArgs vars
+                       return (Term my_ty (Left ('<' : tag ++ ">")) a subTerms)
+          Just dc -> do
+            traceTR (text "Is constructor" <+> (ppr dc $$ ppr my_ty))
+            subTtypes <- getDataConArgTys dc my_ty
+            subTerms <- extractSubTerms (\ty -> go (pred max_depth) ty ty) clos subTtypes
+            return (Term my_ty (Right dc) a subTerms)
+
+      -- This is to support printing of Integers. It's not a general
+      -- mechanism by any means; in particular we lose the size in
+      -- bytes of the array.
+      ArrWordsClosure{bytes=b, arrWords=ws} -> do
+         traceTR (text "ByteArray# closure, size " <> ppr b)
+         return (Term my_ty (Left "ByteArray#") a [Prim my_ty ws])
+
+-- The otherwise case: can be a Thunk,AP,PAP,etc.
+      _ -> do
+         traceTR (text "Unknown closure:" <+>
+                  text (show (fmap (const ()) clos)))
+         return (Suspension (tipe (info clos)) my_ty a Nothing)
+
+  -- insert NewtypeWraps around newtypes
+  expandNewtypes = foldTerm idTermFold { fTerm = worker } where
+   worker ty dc hval tt
+     | Just (tc, args) <- tcSplitTyConApp_maybe ty
+     , isNewTyCon tc
+     , wrapped_type    <- newTyConInstRhs tc args
+     , Just dc'        <- tyConSingleDataCon_maybe tc
+     , t'              <- worker wrapped_type dc hval tt
+     = NewtypeWrap ty (Right dc') t'
+     | otherwise = Term ty dc hval tt
+
+
+   -- Avoid returning types where predicates have been expanded to dictionaries.
+  fixFunDictionaries = foldTerm idTermFold {fSuspension = worker} where
+      worker ct ty hval n | isFunTy ty = Suspension ct (dictsView ty) hval n
+                          | otherwise  = Suspension ct ty hval n
+
+extractSubTerms :: (Type -> ForeignHValue -> TcM Term)
+                -> GenClosure ForeignHValue -> [Type] -> TcM [Term]
+extractSubTerms recurse clos = liftM thdOf3 . go 0 0
+  where
+    array = dataArgs clos
+
+    go ptr_i arr_i [] = return (ptr_i, arr_i, [])
+    go ptr_i arr_i (ty:tys)
+      | Just (tc, elem_tys) <- tcSplitTyConApp_maybe ty
+      , isUnboxedTupleTyCon tc
+                -- See Note [Unboxed tuple RuntimeRep vars] in TyCon
+      = do (ptr_i, arr_i, terms0) <-
+               go ptr_i arr_i (dropRuntimeRepArgs elem_tys)
+           (ptr_i, arr_i, terms1) <- go ptr_i arr_i tys
+           return (ptr_i, arr_i, unboxedTupleTerm ty terms0 : terms1)
+      | otherwise
+      = case typePrimRepArgs ty of
+          [rep_ty] ->  do
+            (ptr_i, arr_i, term0)  <- go_rep ptr_i arr_i ty rep_ty
+            (ptr_i, arr_i, terms1) <- go ptr_i arr_i tys
+            return (ptr_i, arr_i, term0 : terms1)
+          rep_tys -> do
+           (ptr_i, arr_i, terms0) <- go_unary_types ptr_i arr_i rep_tys
+           (ptr_i, arr_i, terms1) <- go ptr_i arr_i tys
+           return (ptr_i, arr_i, unboxedTupleTerm ty terms0 : terms1)
+
+    go_unary_types ptr_i arr_i [] = return (ptr_i, arr_i, [])
+    go_unary_types ptr_i arr_i (rep_ty:rep_tys) = do
+      tv <- newVar liftedTypeKind
+      (ptr_i, arr_i, term0)  <- go_rep ptr_i arr_i tv rep_ty
+      (ptr_i, arr_i, terms1) <- go_unary_types ptr_i arr_i rep_tys
+      return (ptr_i, arr_i, term0 : terms1)
+
+    go_rep ptr_i arr_i ty rep
+      | isGcPtrRep rep = do
+          t <- recurse ty $ (ptrArgs clos)!!ptr_i
+          return (ptr_i + 1, arr_i, t)
+      | otherwise = do
+          -- This is a bit involved since we allow packing multiple fields
+          -- within a single word. See also
+          -- GHC.StgToCmm.Layout.mkVirtHeapOffsetsWithPadding
+          dflags <- getDynFlags
+          let word_size = wORD_SIZE dflags
+              big_endian = wORDS_BIGENDIAN dflags
+              size_b = primRepSizeB dflags rep
+              -- Align the start offset (eg, 2-byte value should be 2-byte
+              -- aligned). But not more than to a word. The offset calculation
+              -- should be the same with the offset calculation in
+              -- GHC.StgToCmm.Layout.mkVirtHeapOffsetsWithPadding.
+              !aligned_idx = roundUpTo arr_i (min word_size size_b)
+              !new_arr_i = aligned_idx + size_b
+              ws | size_b < word_size =
+                     [index size_b aligned_idx word_size big_endian]
+                 | otherwise =
+                     let (q, r) = size_b `quotRem` word_size
+                     in ASSERT( r == 0 )
+                        [ array!!i
+                        | o <- [0.. q - 1]
+                        , let i = (aligned_idx `quot` word_size) + o
+                        ]
+          return (ptr_i, new_arr_i, Prim ty ws)
+
+    unboxedTupleTerm ty terms
+      = Term ty (Right (tupleDataCon Unboxed (length terms)))
+                (error "unboxedTupleTerm: no HValue for unboxed tuple") terms
+
+    -- Extract a sub-word sized field from a word
+    index item_size_b index_b word_size big_endian =
+        (word .&. (mask `shiftL` moveBytes)) `shiftR` moveBytes
+      where
+        mask :: Word
+        mask = case item_size_b of
+            1 -> 0xFF
+            2 -> 0xFFFF
+            4 -> 0xFFFFFFFF
+            _ -> panic ("Weird byte-index: " ++ show index_b)
+        (q,r) = index_b `quotRem` word_size
+        word = array!!q
+        moveBytes = if big_endian
+                    then word_size - (r + item_size_b) * 8
+                    else r * 8
+
+
+-- | Fast, breadth-first Type reconstruction
+--
+-- Given a heap object (`HValue`) and its (possibly polymorphic) type (usually
+-- obtained in GHCi), try to reconstruct a more monomorphic type of the object.
+-- This is used for improving type information in debugger. For example, if we
+-- have a polymorphic function:
+--
+--     sumNumList :: Num a => [a] -> a
+--     sumNumList [] = 0
+--     sumNumList (x : xs) = x + sumList xs
+--
+-- and add a breakpoint to it:
+--
+--     ghci> break sumNumList
+--     ghci> sumNumList ([0 .. 9] :: [Int])
+--
+-- ghci shows us more precise types than just `a`s:
+--
+--     Stopped in Main.sumNumList, debugger.hs:3:23-39
+--     _result :: Int = _
+--     x :: Int = 0
+--     xs :: [Int] = _
+--
+cvReconstructType
+    :: HscEnv
+    -> Int       -- ^ How many times to recurse for subterms
+    -> GhciType  -- ^ Type to refine
+    -> ForeignHValue  -- ^ Refine the type using this value
+    -> IO (Maybe Type)
+cvReconstructType hsc_env max_depth old_ty hval = runTR_maybe hsc_env $ do
+   traceTR (text "RTTI started with initial type " <> ppr old_ty)
+   let sigma_old_ty@(old_tvs, _) = quantifyType old_ty
+   new_ty <-
+       if null old_tvs
+        then return old_ty
+        else do
+          (old_ty', rev_subst) <- instScheme sigma_old_ty
+          my_ty <- newOpenVar
+          when (check1 sigma_old_ty) (traceTR (text "check1 passed") >>
+                                      addConstraint my_ty old_ty')
+          search (isMonomorphic `fmap` zonkTcType my_ty)
+                 (\(ty,a) -> go ty a)
+                 (Seq.singleton (my_ty, hval))
+                 max_depth
+          new_ty <- zonkTcType my_ty
+          if isMonomorphic new_ty || check2 (quantifyType new_ty) sigma_old_ty
+            then do
+                 traceTR (text "check2 passed" <+> ppr old_ty $$ ppr new_ty)
+                 addConstraint my_ty old_ty'
+                 applyRevSubst rev_subst
+                 zonkRttiType new_ty
+            else traceTR (text "check2 failed" <+> parens (ppr new_ty)) >>
+                 return old_ty
+   traceTR (text "RTTI completed. Type obtained:" <+> ppr new_ty)
+   return new_ty
+    where
+--  search :: m Bool -> ([a] -> [a] -> [a]) -> [a] -> m ()
+  search _ _ _ 0 = traceTR (text "Failed to reconstruct a type after " <>
+                                int max_depth <> text " steps")
+  search stop expand l d =
+    case viewl l of
+      EmptyL  -> return ()
+      x :< xx -> unlessM stop $ do
+                  new <- expand x
+                  search stop expand (xx `mappend` Seq.fromList new) $! (pred d)
+
+   -- returns unification tasks,since we are going to want a breadth-first search
+  go :: Type -> ForeignHValue -> TR [(Type, ForeignHValue)]
+  go my_ty a = do
+    traceTR (text "go" <+> ppr my_ty)
+    clos <- trIO $ GHCi.getClosure hsc_env a
+    case clos of
+      BlackholeClosure{indirectee=ind} -> go my_ty ind
+      IndClosure{indirectee=ind} -> go my_ty ind
+      MutVarClosure{var=contents} -> do
+         tv'   <- newVar liftedTypeKind
+         world <- newVar liftedTypeKind
+         addConstraint my_ty (mkTyConApp mutVarPrimTyCon [world,tv'])
+         return [(tv', contents)]
+      ConstrClosure{ptrArgs=pArgs} -> do
+        Right dcname <- liftIO $ constrClosToName hsc_env clos
+        traceTR (text "Constr1" <+> ppr dcname)
+        (mb_dc, _) <- tryTc (tcLookupDataCon dcname)
+        case mb_dc of
+          Nothing-> do
+            forM pArgs $ \x -> do
+              tv <- newVar liftedTypeKind
+              return (tv, x)
+
+          Just dc -> do
+            arg_tys <- getDataConArgTys dc my_ty
+            (_, itys) <- findPtrTyss 0 arg_tys
+            traceTR (text "Constr2" <+> ppr dcname <+> ppr arg_tys)
+            return $ zipWith (\(_,ty) x -> (ty, x)) itys pArgs
+      _ -> return []
+
+findPtrTys :: Int  -- Current pointer index
+           -> Type -- Type
+           -> TR (Int, [(Int, Type)])
+findPtrTys i ty
+  | Just (tc, elem_tys) <- tcSplitTyConApp_maybe ty
+  , isUnboxedTupleTyCon tc
+  = findPtrTyss i elem_tys
+
+  | otherwise
+  = case typePrimRep ty of
+      [rep] | isGcPtrRep rep -> return (i + 1, [(i, ty)])
+            | otherwise      -> return (i,     [])
+      prim_reps              ->
+        foldM (\(i, extras) prim_rep ->
+                if isGcPtrRep prim_rep
+                  then newVar liftedTypeKind >>= \tv -> return (i + 1, extras ++ [(i, tv)])
+                  else return (i, extras))
+              (i, []) prim_reps
+
+findPtrTyss :: Int
+            -> [Type]
+            -> TR (Int, [(Int, Type)])
+findPtrTyss i tys = foldM step (i, []) tys
+  where step (i, discovered) elem_ty = do
+          (i, extras) <- findPtrTys i elem_ty
+          return (i, discovered ++ extras)
+
+
+-- Compute the difference between a base type and the type found by RTTI
+-- improveType <base_type> <rtti_type>
+-- The types can contain skolem type variables, which need to be treated as normal vars.
+-- In particular, we want them to unify with things.
+improveRTTIType :: HscEnv -> RttiType -> RttiType -> Maybe TCvSubst
+improveRTTIType _ base_ty new_ty = U.tcUnifyTyKi base_ty new_ty
+
+getDataConArgTys :: DataCon -> Type -> TR [Type]
+-- Given the result type ty of a constructor application (D a b c :: ty)
+-- return the types of the arguments.  This is RTTI-land, so 'ty' might
+-- not be fully known.  Moreover, the arg types might involve existentials;
+-- if so, make up fresh RTTI type variables for them
+--
+-- I believe that con_app_ty should not have any enclosing foralls
+getDataConArgTys dc con_app_ty
+  = do { let rep_con_app_ty = unwrapType con_app_ty
+       ; traceTR (text "getDataConArgTys 1" <+> (ppr con_app_ty $$ ppr rep_con_app_ty
+                   $$ ppr (tcSplitTyConApp_maybe rep_con_app_ty)))
+       ; ASSERT( all isTyVar ex_tvs ) return ()
+                 -- ex_tvs can only be tyvars as data types in source
+                 -- Haskell cannot mention covar yet (Aug 2018)
+       ; (subst, _) <- instTyVars (univ_tvs ++ ex_tvs)
+       ; addConstraint rep_con_app_ty (substTy subst (dataConOrigResTy dc))
+              -- See Note [Constructor arg types]
+       ; let con_arg_tys = substTys subst (dataConRepArgTys dc)
+       ; traceTR (text "getDataConArgTys 2" <+> (ppr rep_con_app_ty $$ ppr con_arg_tys $$ ppr subst))
+       ; return con_arg_tys }
+  where
+    univ_tvs = dataConUnivTyVars dc
+    ex_tvs   = dataConExTyCoVars dc
+
+{- Note [Constructor arg types]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider a GADT (cf #7386)
+   data family D a b
+   data instance D [a] a where
+     MkT :: a -> D [a] (Maybe a)
+     ...
+
+In getDataConArgTys
+* con_app_ty is the known type (from outside) of the constructor application,
+  say D [Int] Int
+
+* The data constructor MkT has a (representation) dataConTyCon = DList,
+  say where
+    data DList a where
+      MkT :: a -> DList a (Maybe a)
+      ...
+
+So the dataConTyCon of the data constructor, DList, differs from
+the "outside" type, D. So we can't straightforwardly decompose the
+"outside" type, and we end up in the "_" branch of the case.
+
+Then we match the dataConOrigResTy of the data constructor against the
+outside type, hoping to get a substitution that tells how to instantiate
+the *representation* type constructor.   This looks a bit delicate to
+me, but it seems to work.
+-}
+
+-- Soundness checks
+--------------------
+{-
+This is not formalized anywhere, so hold to your seats!
+RTTI in the presence of newtypes can be a tricky and unsound business.
+
+Example:
+~~~~~~~~~
+Suppose we are doing RTTI for a partially evaluated
+closure t, the real type of which is t :: MkT Int, for
+
+   newtype MkT a = MkT [Maybe a]
+
+The table below shows the results of RTTI and the improvement
+calculated for different combinations of evaluatedness and :type t.
+Regard the two first columns as input and the next two as output.
+
+  # |     t     |  :type t  | rtti(t)  | improv.    | result
+    ------------------------------------------------------------
+  1 |     _     |    t b    |    a     | none       | OK
+  2 |     _     |   MkT b   |    a     | none       | OK
+  3 |     _     |   t Int   |    a     | none       | OK
+
+  If t is not evaluated at *all*, we are safe.
+
+  4 |  (_ : _)  |    t b    |   [a]    | t = []     | UNSOUND
+  5 |  (_ : _)  |   MkT b   |  MkT a   | none       | OK (compensating for the missing newtype)
+  6 |  (_ : _)  |   t Int   |  [Int]   | t = []     | UNSOUND
+
+  If a is a minimal whnf, we run into trouble. Note that
+  row 5 above does newtype enrichment on the ty_rtty parameter.
+
+  7 | (Just _:_)|    t b    |[Maybe a] | t = [],    | UNSOUND
+    |                       |          | b = Maybe a|
+
+  8 | (Just _:_)|   MkT b   |  MkT a   |  none      | OK
+  9 | (Just _:_)|   t Int   |   FAIL   |  none      | OK
+
+  And if t is any more evaluated than whnf, we are still in trouble.
+  Because constraints are solved in top-down order, when we reach the
+  Maybe subterm what we got is already unsound. This explains why the
+  row 9 fails to complete.
+
+  10 | (Just _:_)|  t Int  | [Maybe a]   |  FAIL    | OK
+  11 | (Just 1:_)|  t Int  | [Maybe Int] |  FAIL    | OK
+
+  We can undo the failure in row 9 by leaving out the constraint
+  coming from the type signature of t (i.e., the 2nd column).
+  Note that this type information is still used
+  to calculate the improvement. But we fail
+  when trying to calculate the improvement, as there is no unifier for
+  t Int = [Maybe a] or t Int = [Maybe Int].
+
+
+  Another set of examples with t :: [MkT (Maybe Int)]  \equiv  [[Maybe (Maybe Int)]]
+
+  # |     t     |    :type t    |  rtti(t)    | improvement | result
+    ---------------------------------------------------------------------
+  1 |(Just _:_) | [t (Maybe a)] | [[Maybe b]] | t = []      |
+    |           |               |             | b = Maybe a |
+
+The checks:
+~~~~~~~~~~~
+Consider a function obtainType that takes a value and a type and produces
+the Term representation and a substitution (the improvement).
+Assume an auxiliar rtti' function which does the actual job if recovering
+the type, but which may produce a false type.
+
+In pseudocode:
+
+  rtti' :: a -> IO Type  -- Does not use the static type information
+
+  obtainType :: a -> Type -> IO (Maybe (Term, Improvement))
+  obtainType v old_ty = do
+       rtti_ty <- rtti' v
+       if monomorphic rtti_ty || (check rtti_ty old_ty)
+        then ...
+         else return Nothing
+  where check rtti_ty old_ty = check1 rtti_ty &&
+                              check2 rtti_ty old_ty
+
+  check1 :: Type -> Bool
+  check2 :: Type -> Type -> Bool
+
+Now, if rtti' returns a monomorphic type, we are safe.
+If that is not the case, then we consider two conditions.
+
+
+1. To prevent the class of unsoundness displayed by
+   rows 4 and 7 in the example: no higher kind tyvars
+   accepted.
+
+  check1 (t a)   = NO
+  check1 (t Int) = NO
+  check1 ([] a)  = YES
+
+2. To prevent the class of unsoundness shown by row 6,
+   the rtti type should be structurally more
+   defined than the old type we are comparing it to.
+  check2 :: NewType -> OldType -> Bool
+  check2 a  _        = True
+  check2 [a] a       = True
+  check2 [a] (t Int) = False
+  check2 [a] (t a)   = False  -- By check1 we never reach this equation
+  check2 [Int] a     = True
+  check2 [Int] (t Int) = True
+  check2 [Maybe a]   (t Int) = False
+  check2 [Maybe Int] (t Int) = True
+  check2 (Maybe [a])   (m [Int]) = False
+  check2 (Maybe [Int]) (m [Int]) = True
+
+-}
+
+check1 :: QuantifiedType -> Bool
+check1 (tvs, _) = not $ any isHigherKind (map tyVarKind tvs)
+ where
+   isHigherKind = not . null . fst . splitPiTys
+
+check2 :: QuantifiedType -> QuantifiedType -> Bool
+check2 (_, rtti_ty) (_, old_ty)
+  | Just (_, rttis) <- tcSplitTyConApp_maybe rtti_ty
+  = case () of
+      _ | Just (_,olds) <- tcSplitTyConApp_maybe old_ty
+        -> and$ zipWith check2 (map quantifyType rttis) (map quantifyType olds)
+      _ | Just _ <- splitAppTy_maybe old_ty
+        -> isMonomorphicOnNonPhantomArgs rtti_ty
+      _ -> True
+  | otherwise = True
+
+-- Dealing with newtypes
+--------------------------
+{-
+ congruenceNewtypes does a parallel fold over two Type values,
+ compensating for missing newtypes on both sides.
+ This is necessary because newtypes are not present
+ in runtime, but sometimes there is evidence available.
+   Evidence can come from DataCon signatures or
+ from compile-time type inference.
+ What we are doing here is an approximation
+ of unification modulo a set of equations derived
+ from newtype definitions. These equations should be the
+ same as the equality coercions generated for newtypes
+ in System Fc. The idea is to perform a sort of rewriting,
+ taking those equations as rules, before launching unification.
+
+ The caller must ensure the following.
+ The 1st type (lhs) comes from the heap structure of ptrs,nptrs.
+ The 2nd type (rhs) comes from a DataCon type signature.
+ Rewriting (i.e. adding/removing a newtype wrapper) can happen
+ in both types, but in the rhs it is restricted to the result type.
+
+   Note that it is very tricky to make this 'rewriting'
+ work with the unification implemented by TcM, where
+ substitutions are operationally inlined. The order in which
+ constraints are unified is vital as we cannot modify
+ anything that has been touched by a previous unification step.
+Therefore, congruenceNewtypes is sound only if the types
+recovered by the RTTI mechanism are unified Top-Down.
+-}
+congruenceNewtypes ::  TcType -> TcType -> TR (TcType,TcType)
+congruenceNewtypes lhs rhs = go lhs rhs >>= \rhs' -> return (lhs,rhs')
+ where
+   go l r
+ -- TyVar lhs inductive case
+    | Just tv <- getTyVar_maybe l
+    , isTcTyVar tv
+    , isMetaTyVar tv
+    = recoverTR (return r) $ do
+         Indirect ty_v <- readMetaTyVar tv
+         traceTR $ fsep [text "(congruence) Following indirect tyvar:",
+                          ppr tv, equals, ppr ty_v]
+         go ty_v r
+-- FunTy inductive case
+    | Just (l1,l2) <- splitFunTy_maybe l
+    , Just (r1,r2) <- splitFunTy_maybe r
+    = do r2' <- go l2 r2
+         r1' <- go l1 r1
+         return (mkVisFunTy r1' r2')
+-- TyconApp Inductive case; this is the interesting bit.
+    | Just (tycon_l, _) <- tcSplitTyConApp_maybe lhs
+    , Just (tycon_r, _) <- tcSplitTyConApp_maybe rhs
+    , tycon_l /= tycon_r
+    = upgrade tycon_l r
+
+    | otherwise = return r
+
+    where upgrade :: TyCon -> Type -> TR Type
+          upgrade new_tycon ty
+            | not (isNewTyCon new_tycon) = do
+              traceTR (text "(Upgrade) Not matching newtype evidence: " <>
+                       ppr new_tycon <> text " for " <> ppr ty)
+              return ty
+            | otherwise = do
+               traceTR (text "(Upgrade) upgraded " <> ppr ty <>
+                        text " in presence of newtype evidence " <> ppr new_tycon)
+               (_, vars) <- instTyVars (tyConTyVars new_tycon)
+               let ty' = mkTyConApp new_tycon (mkTyVarTys vars)
+                   rep_ty = unwrapType ty'
+               _ <- liftTcM (unifyType Nothing ty rep_ty)
+        -- assumes that reptype doesn't ^^^^ touch tyconApp args
+               return ty'
+
+
+zonkTerm :: Term -> TcM Term
+zonkTerm = foldTermM (TermFoldM
+             { fTermM = \ty dc v tt -> zonkRttiType ty    >>= \ty' ->
+                                       return (Term ty' dc v tt)
+             , fSuspensionM  = \ct ty v b -> zonkRttiType ty >>= \ty ->
+                                             return (Suspension ct ty v b)
+             , fNewtypeWrapM = \ty dc t -> zonkRttiType ty >>= \ty' ->
+                                           return$ NewtypeWrap ty' dc t
+             , fRefWrapM     = \ty t -> return RefWrap  `ap`
+                                        zonkRttiType ty `ap` return t
+             , fPrimM        = (return.) . Prim })
+
+zonkRttiType :: TcType -> TcM Type
+-- Zonk the type, replacing any unbound Meta tyvars
+-- by RuntimeUnk skolems, safely out of Meta-tyvar-land
+zonkRttiType ty= do { ze <- mkEmptyZonkEnv RuntimeUnkFlexi
+                    ; zonkTcTypeToTypeX ze ty }
+
+--------------------------------------------------------------------------------
+-- Restore Class predicates out of a representation type
+dictsView :: Type -> Type
+dictsView ty = ty
+
+
+-- Use only for RTTI types
+isMonomorphic :: RttiType -> Bool
+isMonomorphic ty = noExistentials && noUniversals
+ where (tvs, _, ty')  = tcSplitSigmaTy ty
+       noExistentials = noFreeVarsOfType ty'
+       noUniversals   = null tvs
+
+-- Use only for RTTI types
+isMonomorphicOnNonPhantomArgs :: RttiType -> Bool
+isMonomorphicOnNonPhantomArgs ty
+  | Just (tc, all_args) <- tcSplitTyConApp_maybe (unwrapType ty)
+  , phantom_vars  <- tyConPhantomTyVars tc
+  , concrete_args <- [ arg | (tyv,arg) <- tyConTyVars tc `zip` all_args
+                           , tyv `notElem` phantom_vars]
+  = all isMonomorphicOnNonPhantomArgs concrete_args
+  | Just (ty1, ty2) <- splitFunTy_maybe ty
+  = all isMonomorphicOnNonPhantomArgs [ty1,ty2]
+  | otherwise = isMonomorphic ty
+
+tyConPhantomTyVars :: TyCon -> [TyVar]
+tyConPhantomTyVars tc
+  | isAlgTyCon tc
+  , Just dcs <- tyConDataCons_maybe tc
+  , dc_vars  <- concatMap dataConUnivTyVars dcs
+  = tyConTyVars tc \\ dc_vars
+tyConPhantomTyVars _ = []
+
+type QuantifiedType = ([TyVar], Type)
+   -- Make the free type variables explicit
+   -- The returned Type should have no top-level foralls (I believe)
+
+quantifyType :: Type -> QuantifiedType
+-- Generalize the type: find all free and forall'd tyvars
+-- and return them, together with the type inside, which
+-- should not be a forall type.
+--
+-- Thus (quantifyType (forall a. a->[b]))
+-- returns ([a,b], a -> [b])
+
+quantifyType ty = ( filter isTyVar $
+                    tyCoVarsOfTypeWellScoped rho
+                  , rho)
+  where
+    (_tvs, rho) = tcSplitForAllTys ty