{-# OPTIONS_GHC -fno-implicit-prelude -fallow-overlapping-instances #-}

-- The -fallow-overlapping-instances flag allows the user to over-ride
-- the instances for Typeable given here.  In particular, we provide an instance
--	instance ... => Typeable (s a) 
-- But a user might want to say
--	instance ... => Typeable (MyType a b)

-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Typeable
-- Copyright   :  (c) The University of Glasgow, CWI 2001--2004
-- License     :  BSD-style (see the file libraries/base/LICENSE)
-- 
-- Maintainer  :  libraries@haskell.org
-- Stability   :  experimental
-- Portability :  portable
--
-- The 'Typeable' class reifies types to some extent by associating type
-- representations to types. These type representations can be compared,
-- and one can in turn define a type-safe cast operation. To this end,
-- an unsafe cast is guarded by a test for type (representation)
-- equivalence. The module "Data.Dynamic" uses Typeable for an
-- implementation of dynamics. The module "Data.Generics" uses Typeable
-- and type-safe cast (but not dynamics) to support the \"Scrap your
-- boilerplate\" style of generic programming.
--
-- Note, only relevant if you use dynamic linking. If you have a program
-- that is statically linked with Data.Typeable, and then dynamically link
-- a program that also uses Data.Typeable, you'll get two copies of the module.
-- That's fine, but behind the scenes, the module uses a mutable variable to
-- allocate unique Ids to type constructors.  So in the situation described,
-- there'll be two separate Id allocators, which aren't comparable to each other.
-- This can lead to chaos.  (It's a bug that we will fix.)  None of
-- this matters if you aren't using dynamic linking.
--
-----------------------------------------------------------------------------

module Data.Typeable
  (

	-- * The Typeable class
	Typeable( typeOf ),	-- :: a -> TypeRep

	-- * Type-safe cast
	cast,			-- :: (Typeable a, Typeable b) => a -> Maybe b
	gcast,			-- a generalisation of cast

	-- * Type representations
	TypeRep,	-- abstract, instance of: Eq, Show, Typeable
	TyCon,		-- abstract, instance of: Eq, Show, Typeable

	-- * Construction of type representations
	mkTyCon,	-- :: String  -> TyCon
	mkTyConApp,	-- :: TyCon   -> [TypeRep] -> TypeRep
	mkAppTy,	-- :: TypeRep -> TypeRep   -> TypeRep
	mkFunTy,	-- :: TypeRep -> TypeRep   -> TypeRep

	-- * Observation of type representations
	splitTyConApp,	-- :: TypeRep -> (TyCon, [TypeRep])
	funResultTy,	-- :: TypeRep -> TypeRep   -> Maybe TypeRep
	typeRepTyCon,	-- :: TypeRep -> TyCon
	typeRepArgs,	-- :: TypeRep -> [TypeRep]
	tyConString,	-- :: TyCon   -> String

	-- * The other Typeable classes
	-- | /Note:/ The general instances are provided for GHC only.
	Typeable1( typeOf1 ),	-- :: t a -> TypeRep
	Typeable2( typeOf2 ),	-- :: t a b -> TypeRep
	Typeable3( typeOf3 ),	-- :: t a b c -> TypeRep
	Typeable4( typeOf4 ),	-- :: t a b c d -> TypeRep
	Typeable5( typeOf5 ),	-- :: t a b c d e -> TypeRep
	Typeable6( typeOf6 ),	-- :: t a b c d e f -> TypeRep
	Typeable7( typeOf7 ),	-- :: t a b c d e f g -> TypeRep
	gcast1,			-- :: ... => c (t a) -> Maybe (c (t' a))
	gcast2,			-- :: ... => c (t a b) -> Maybe (c (t' a b))

	-- * Default instances
	-- | /Note:/ These are not needed by GHC, for which these instances
	-- are generated by general instance declarations.
	typeOfDefault,	-- :: (Typeable1 t, Typeable a) => t a -> TypeRep
	typeOf1Default,	-- :: (Typeable2 t, Typeable a) => t a b -> TypeRep
	typeOf2Default,	-- :: (Typeable3 t, Typeable a) => t a b c -> TypeRep
	typeOf3Default,	-- :: (Typeable4 t, Typeable a) => t a b c d -> TypeRep
	typeOf4Default,	-- :: (Typeable5 t, Typeable a) => t a b c d e -> TypeRep
	typeOf5Default,	-- :: (Typeable6 t, Typeable a) => t a b c d e f -> TypeRep
	typeOf6Default	-- :: (Typeable7 t, Typeable a) => t a b c d e f g -> TypeRep

  ) where

import qualified Data.HashTable as HT
import Data.Maybe
import Data.Either
import Data.Int
import Data.Word
import Data.List( foldl )

























import Hugs.Prelude	( Key(..), TypeRep(..), TyCon(..), Ratio,
			  Exception, ArithException, IOException,
			  ArrayException, AsyncException, Handle,
			  Ptr, FunPtr, ForeignPtr, StablePtr )
import Hugs.IORef	( IORef, newIORef, readIORef, writeIORef )
import Hugs.IOExts	( unsafePerformIO, unsafeCoerce )
	-- For the Typeable instance
import Hugs.Array	( Array )
import Hugs.ConcBase	( MVar )

















                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

















































































	-- 
	-- let fTy = mkTyCon "Foo" in show (mkTyConApp (mkTyCon ",,")
	--                                 [fTy,fTy,fTy])
	-- 
	-- returns "(Foo,Foo,Foo)"
	--
	-- The TypeRep Show instance promises to print tuple types
	-- correctly. Tuple type constructors are specified by a 
	-- sequence of commas, e.g., (mkTyCon ",,,,") returns
	-- the 5-tuple tycon.

----------------- Construction --------------------

-- | Applies a type constructor to a sequence of types
mkTyConApp  :: TyCon -> [TypeRep] -> TypeRep
mkTyConApp tc@(TyCon tc_k _) args 
  = TypeRep (appKeys tc_k arg_ks) tc args
  where
    arg_ks = [k | TypeRep k _ _ <- args]

-- | A special case of 'mkTyConApp', which applies the function 
-- type constructor to a pair of types.
mkFunTy  :: TypeRep -> TypeRep -> TypeRep
mkFunTy f a = mkTyConApp funTc [f,a]

-- | Splits a type constructor application
splitTyConApp :: TypeRep -> (TyCon,[TypeRep])
splitTyConApp (TypeRep _ tc trs) = (tc,trs)

-- | Applies a type to a function type.  Returns: @'Just' u@ if the
-- first argument represents a function of type @t -> u@ and the
-- second argument represents a function of type @t@.  Otherwise,
-- returns 'Nothing'.
funResultTy :: TypeRep -> TypeRep -> Maybe TypeRep
funResultTy trFun trArg
  = case splitTyConApp trFun of
      (tc, [t1,t2]) | tc == funTc && t1 == trArg -> Just t2
      _ -> Nothing

-- | Adds a TypeRep argument to a TypeRep.
mkAppTy :: TypeRep -> TypeRep -> TypeRep
mkAppTy (TypeRep tr_k tc trs) arg_tr
  = let (TypeRep arg_k _ _) = arg_tr
     in  TypeRep (appKey tr_k arg_k) tc (trs++[arg_tr])

-- If we enforce the restriction that there is only one
-- @TyCon@ for a type & it is shared among all its uses,
-- we can map them onto Ints very simply. The benefit is,
-- of course, that @TyCon@s can then be compared efficiently.

-- Provided the implementor of other @Typeable@ instances
-- takes care of making all the @TyCon@s CAFs (toplevel constants),
-- this will work. 

-- If this constraint does turn out to be a sore thumb, changing
-- the Eq instance for TyCons is trivial.

-- | Builds a 'TyCon' object representing a type constructor.  An
-- implementation of "Data.Typeable" should ensure that the following holds:
--
-- >  mkTyCon "a" == mkTyCon "a"
--

mkTyCon :: String	-- ^ the name of the type constructor (should be unique
			-- in the program, so it might be wise to use the
			-- fully qualified name).
	-> TyCon	-- ^ A unique 'TyCon' object
mkTyCon str = TyCon (mkTyConKey str) str

----------------- Observation ---------------------

-- | Observe the type constructor of a type representation
typeRepTyCon :: TypeRep -> TyCon
typeRepTyCon (TypeRep _ tc _) = tc

-- | Observe the argument types of a type representation
typeRepArgs :: TypeRep -> [TypeRep]
typeRepArgs (TypeRep _ _ args) = args

-- | Observe string encoding of a type representation
tyConString :: TyCon   -> String
tyConString  (TyCon _ str) = str

----------------- Showing TypeReps --------------------

instance Show TypeRep where
  showsPrec p (TypeRep _ tycon tys) =
    case tys of
      [] -> showsPrec p tycon
      [x]   | tycon == listTc -> showChar '[' . shows x . showChar ']'
      [a,r] | tycon == funTc  -> showParen (p > 8) $
			         showsPrec 9 a .
                                 showString " -> " .
                                 showsPrec 8 r
      xs | isTupleTyCon tycon -> showTuple tycon xs
	 | otherwise	     ->
	    showParen (p > 9) $
   	    showsPrec p tycon . 
	    showChar ' '      . 
	    showArgs tys

instance Show TyCon where
  showsPrec _ (TyCon _ s) = showString s

isTupleTyCon :: TyCon -> Bool
isTupleTyCon (TyCon _ (',':_)) = True
isTupleTyCon _		       = False

-- Some (Show.TypeRep) helpers:

showArgs :: Show a => [a] -> ShowS
showArgs [] = id
showArgs [a] = showsPrec 10 a
showArgs (a:as) = showsPrec 10 a . showString " " . showArgs as 

showTuple :: TyCon -> [TypeRep] -> ShowS
showTuple (TyCon _ str) args = showChar '(' . go str args
 where
  go [] [a] = showsPrec 10 a . showChar ')'
  go _  []  = showChar ')' -- a failure condition, really.
  go (',':xs) (a:as) = showsPrec 10 a . showChar ',' . go xs as
  go _ _   = showChar ')'

-------------------------------------------------------------
--
--	The Typeable class and friends
--
-------------------------------------------------------------

-- | The class 'Typeable' allows a concrete representation of a type to
-- be calculated.
class Typeable a where
  typeOf :: a -> TypeRep
  -- ^ Takes a value of type @a@ and returns a concrete representation
  -- of that type.  The /value/ of the argument should be ignored by
  -- any instance of 'Typeable', so that it is safe to pass 'undefined' as
  -- the argument.

-- | Variant for unary type constructors
class Typeable1 t where
  typeOf1 :: t a -> TypeRep

-- | For defining a 'Typeable' instance from any 'Typeable1' instance.
typeOfDefault :: (Typeable1 t, Typeable a) => t a -> TypeRep
typeOfDefault x = typeOf1 x `mkAppTy` typeOf (argType x)
 where
   argType :: t a -> a
   argType =  undefined

-- | Variant for binary type constructors
class Typeable2 t where
  typeOf2 :: t a b -> TypeRep

-- | For defining a 'Typeable1' instance from any 'Typeable2' instance.
typeOf1Default :: (Typeable2 t, Typeable a) => t a b -> TypeRep
typeOf1Default x = typeOf2 x `mkAppTy` typeOf (argType x)
 where
   argType :: t a b -> a
   argType =  undefined

-- | Variant for 3-ary type constructors
class Typeable3 t where
  typeOf3 :: t a b c -> TypeRep

-- | For defining a 'Typeable2' instance from any 'Typeable3' instance.
typeOf2Default :: (Typeable3 t, Typeable a) => t a b c -> TypeRep
typeOf2Default x = typeOf3 x `mkAppTy` typeOf (argType x)
 where
   argType :: t a b c -> a
   argType =  undefined

-- | Variant for 4-ary type constructors
class Typeable4 t where
  typeOf4 :: t a b c d -> TypeRep

-- | For defining a 'Typeable3' instance from any 'Typeable4' instance.
typeOf3Default :: (Typeable4 t, Typeable a) => t a b c d -> TypeRep
typeOf3Default x = typeOf4 x `mkAppTy` typeOf (argType x)
 where
   argType :: t a b c d -> a
   argType =  undefined

-- | Variant for 5-ary type constructors
class Typeable5 t where
  typeOf5 :: t a b c d e -> TypeRep

-- | For defining a 'Typeable4' instance from any 'Typeable5' instance.
typeOf4Default :: (Typeable5 t, Typeable a) => t a b c d e -> TypeRep
typeOf4Default x = typeOf5 x `mkAppTy` typeOf (argType x)
 where
   argType :: t a b c d e -> a
   argType =  undefined

-- | Variant for 6-ary type constructors
class Typeable6 t where
  typeOf6 :: t a b c d e f -> TypeRep

-- | For defining a 'Typeable5' instance from any 'Typeable6' instance.
typeOf5Default :: (Typeable6 t, Typeable a) => t a b c d e f -> TypeRep
typeOf5Default x = typeOf6 x `mkAppTy` typeOf (argType x)
 where
   argType :: t a b c d e f -> a
   argType =  undefined

-- | Variant for 7-ary type constructors
class Typeable7 t where
  typeOf7 :: t a b c d e f g -> TypeRep

-- | For defining a 'Typeable6' instance from any 'Typeable7' instance.
typeOf6Default :: (Typeable7 t, Typeable a) => t a b c d e f g -> TypeRep
typeOf6Default x = typeOf7 x `mkAppTy` typeOf (argType x)
 where
   argType :: t a b c d e f g -> a
   argType =  undefined













































-------------------------------------------------------------
--
--		Type-safe cast
--
-------------------------------------------------------------

-- | The type-safe cast operation
cast :: (Typeable a, Typeable b) => a -> Maybe b
cast x = r
       where
	 r = if typeOf x == typeOf (fromJust r)
               then Just $ unsafeCoerce x
	       else Nothing

-- | A flexible variation parameterised in a type constructor
gcast :: (Typeable a, Typeable b) => c a -> Maybe (c b)
gcast x = r
 where
  r = if typeOf (getArg x) == typeOf (getArg (fromJust r))
        then Just $ unsafeCoerce x
        else Nothing
  getArg :: c x -> x 
  getArg = undefined

-- | Cast for * -> *
gcast1 :: (Typeable1 t, Typeable1 t') => c (t a) -> Maybe (c (t' a)) 
gcast1 x = r
 where
  r = if typeOf1 (getArg x) == typeOf1 (getArg (fromJust r))
       then Just $ unsafeCoerce x
       else Nothing
  getArg :: c x -> x 
  getArg = undefined

-- | Cast for * -> * -> *
gcast2 :: (Typeable2 t, Typeable2 t') => c (t a b) -> Maybe (c (t' a b)) 
gcast2 x = r
 where
  r = if typeOf2 (getArg x) == typeOf2 (getArg (fromJust r))
       then Just $ unsafeCoerce x
       else Nothing
  getArg :: c x -> x 
  getArg = undefined

-------------------------------------------------------------
--
--	Instances of the Typeable classes for Prelude types
--
-------------------------------------------------------------

unitTc = mkTyCon "()"; instance Typeable () where { typeOf _ = mkTyConApp unitTc [] }
listTc = mkTyCon "[]"; instance Typeable1 [] where { typeOf1 _ = mkTyConApp listTc [] }; instance Typeable a => Typeable ([] a) where { typeOf = typeOfDefault }
maybeTc = mkTyCon "Maybe"; instance Typeable1 Maybe where { typeOf1 _ = mkTyConApp maybeTc [] }; instance Typeable a => Typeable (Maybe a) where { typeOf = typeOfDefault }
ratioTc = mkTyCon "Ratio"; instance Typeable1 Ratio where { typeOf1 _ = mkTyConApp ratioTc [] }; instance Typeable a => Typeable (Ratio a) where { typeOf = typeOfDefault }
eitherTc = mkTyCon "Either"; instance Typeable2 Either where { typeOf2 _ = mkTyConApp eitherTc [] }; instance Typeable a => Typeable1 (Either a) where {   typeOf1 = typeOf1Default }; instance (Typeable a, Typeable b) => Typeable (Either a b) where {   typeOf = typeOfDefault }
funTc = mkTyCon "->"; instance Typeable2 (->) where { typeOf2 _ = mkTyConApp funTc [] }; instance Typeable a => Typeable1 ((->) a) where {   typeOf1 = typeOf1Default }; instance (Typeable a, Typeable b) => Typeable ((->) a b) where {   typeOf = typeOfDefault }
ioTc = mkTyCon "IO"; instance Typeable1 IO where { typeOf1 _ = mkTyConApp ioTc [] }; instance Typeable a => Typeable (IO a) where { typeOf = typeOfDefault }


-- Types defined in GHC.IOBase
mvarTc = mkTyCon "MVar"; instance Typeable1 MVar where { typeOf1 _ = mkTyConApp mvarTc [] }; instance Typeable a => Typeable (MVar a) where { typeOf = typeOfDefault }
exceptionTc = mkTyCon "Exception"; instance Typeable Exception where { typeOf _ = mkTyConApp exceptionTc [] }
ioExceptionTc = mkTyCon "IOException"; instance Typeable IOException where { typeOf _ = mkTyConApp ioExceptionTc [] }
arithExceptionTc = mkTyCon "ArithException"; instance Typeable ArithException where { typeOf _ = mkTyConApp arithExceptionTc [] }
arrayExceptionTc = mkTyCon "ArrayException"; instance Typeable ArrayException where { typeOf _ = mkTyConApp arrayExceptionTc [] }
asyncExceptionTc = mkTyCon "AsyncException"; instance Typeable AsyncException where { typeOf _ = mkTyConApp asyncExceptionTc [] }


-- Types defined in GHC.Arr
arrayTc = mkTyCon "Array"; instance Typeable2 Array where { typeOf2 _ = mkTyConApp arrayTc [] }; instance Typeable a => Typeable1 (Array a) where {   typeOf1 = typeOf1Default }; instance (Typeable a, Typeable b) => Typeable (Array a b) where {   typeOf = typeOfDefault }












pairTc = mkTyCon ","; instance Typeable2 (,) where { typeOf2 _ = mkTyConApp pairTc [] }; instance Typeable a => Typeable1 ((,) a) where {   typeOf1 = typeOf1Default }; instance (Typeable a, Typeable b) => Typeable ((,) a b) where {   typeOf = typeOfDefault }
tup3Tc = mkTyCon ",,"; instance Typeable3 (,,) where { typeOf3 _ = mkTyConApp tup3Tc [] }; instance Typeable a => Typeable2 ((,,) a) where {   typeOf2 = typeOf2Default }; instance (Typeable a, Typeable b) => Typeable1 ((,,) a b) where {   typeOf1 = typeOf1Default }; instance (Typeable a, Typeable b, Typeable c) => Typeable ((,,) a b c) where {   typeOf = typeOfDefault }

tup4Tc :: TyCon
tup4Tc = mkTyCon ",,,"

instance Typeable4 (,,,) where
  typeOf4 tu = mkTyConApp tup4Tc []

tup5Tc :: TyCon
tup5Tc = mkTyCon ",,,,"

instance Typeable5 (,,,,) where
  typeOf5 tu = mkTyConApp tup5Tc []

tup6Tc :: TyCon
tup6Tc = mkTyCon ",,,,,"

instance Typeable6 (,,,,,) where
  typeOf6 tu = mkTyConApp tup6Tc []

tup7Tc :: TyCon
tup7Tc = mkTyCon ",,,,,,"

instance Typeable7 (,,,,,,) where
  typeOf7 tu = mkTyConApp tup7Tc []


ptrTc = mkTyCon "Ptr"; instance Typeable1 Ptr where { typeOf1 _ = mkTyConApp ptrTc [] }; instance Typeable a => Typeable (Ptr a) where { typeOf = typeOfDefault }
funPtrTc = mkTyCon "FunPtr"; instance Typeable1 FunPtr where { typeOf1 _ = mkTyConApp funPtrTc [] }; instance Typeable a => Typeable (FunPtr a) where { typeOf = typeOfDefault }
foreignPtrTc = mkTyCon "ForeignPtr"; instance Typeable1 ForeignPtr where { typeOf1 _ = mkTyConApp foreignPtrTc [] }; instance Typeable a => Typeable (ForeignPtr a) where { typeOf = typeOfDefault }
stablePtrTc = mkTyCon "StablePtr"; instance Typeable1 StablePtr where { typeOf1 _ = mkTyConApp stablePtrTc [] }; instance Typeable a => Typeable (StablePtr a) where { typeOf = typeOfDefault }
iORefTc = mkTyCon "IORef"; instance Typeable1 IORef where { typeOf1 _ = mkTyConApp iORefTc [] }; instance Typeable a => Typeable (IORef a) where { typeOf = typeOfDefault }

-------------------------------------------------------
--
-- Generate Typeable instances for standard datatypes
--
-------------------------------------------------------

boolTc = mkTyCon "Bool"; instance Typeable Bool where { typeOf _ = mkTyConApp boolTc [] }
charTc = mkTyCon "Char"; instance Typeable Char where { typeOf _ = mkTyConApp charTc [] }
floatTc = mkTyCon "Float"; instance Typeable Float where { typeOf _ = mkTyConApp floatTc [] }
doubleTc = mkTyCon "Double"; instance Typeable Double where { typeOf _ = mkTyConApp doubleTc [] }
intTc = mkTyCon "Int"; instance Typeable Int where { typeOf _ = mkTyConApp intTc [] }

wordTc = mkTyCon "Word"; instance Typeable Word where { typeOf _ = mkTyConApp wordTc [] }

integerTc = mkTyCon "Integer"; instance Typeable Integer where { typeOf _ = mkTyConApp integerTc [] }
orderingTc = mkTyCon "Ordering"; instance Typeable Ordering where { typeOf _ = mkTyConApp orderingTc [] }
handleTc = mkTyCon "Handle"; instance Typeable Handle where { typeOf _ = mkTyConApp handleTc [] }

int8Tc = mkTyCon "Int8"; instance Typeable Int8 where { typeOf _ = mkTyConApp int8Tc [] }
int16Tc = mkTyCon "Int16"; instance Typeable Int16 where { typeOf _ = mkTyConApp int16Tc [] }
int32Tc = mkTyCon "Int32"; instance Typeable Int32 where { typeOf _ = mkTyConApp int32Tc [] }
int64Tc = mkTyCon "Int64"; instance Typeable Int64 where { typeOf _ = mkTyConApp int64Tc [] }

word8Tc = mkTyCon "Word8"; instance Typeable Word8 where { typeOf _ = mkTyConApp word8Tc [] }
word16Tc = mkTyCon "Word16"; instance Typeable Word16 where { typeOf _ = mkTyConApp word16Tc [] }
word32Tc = mkTyCon "Word32"; instance Typeable Word32 where { typeOf _ = mkTyConApp word32Tc [] }
word64Tc = mkTyCon "Word64"; instance Typeable Word64 where { typeOf _ = mkTyConApp word64Tc [] }

tyconTc = mkTyCon "TyCon"; instance Typeable TyCon where { typeOf _ = mkTyConApp tyconTc [] }
typeRepTc = mkTyCon "TypeRep"; instance Typeable TypeRep where { typeOf _ = mkTyConApp typeRepTc [] }





---------------------------------------------
--
--		Internals 
--
---------------------------------------------





data KeyPr = KeyPr !Key !Key deriving( Eq )

hashKP :: KeyPr -> Int32
hashKP (KeyPr (Key k1) (Key k2)) = (HT.hashInt k1 + HT.hashInt k2) `rem` HT.prime

data Cache = Cache { next_key :: !(IORef Key),	-- Not used by GHC (calls genSym instead)
		     tc_tbl   :: !(HT.HashTable String Key),
		     ap_tbl   :: !(HT.HashTable KeyPr Key) }

{-# NOINLINE cache #-}
cache :: Cache
cache = unsafePerformIO $ do
		empty_tc_tbl <- HT.new (==) HT.hashString
		empty_ap_tbl <- HT.new (==) hashKP
		key_loc      <- newIORef (Key 1) 
		return (Cache { next_key = key_loc,
				tc_tbl = empty_tc_tbl, 
				ap_tbl = empty_ap_tbl })

newKey :: IORef Key -> IO Key



newKey kloc = do { k@(Key i) <- readIORef kloc ;
		   writeIORef kloc (Key (i+1)) ;
		   return k }

















mkTyConKey :: String -> Key
mkTyConKey str 
  = unsafePerformIO $ do
	let Cache {next_key = kloc, tc_tbl = tbl} = cache
	mb_k <- HT.lookup tbl str
	case mb_k of
	  Just k  -> return k
	  Nothing -> do { k <- newKey kloc ;
			  HT.insert tbl str k ;
			  return k }

appKey :: Key -> Key -> Key
appKey k1 k2
  = unsafePerformIO $ do
	let Cache {next_key = kloc, ap_tbl = tbl} = cache
	mb_k <- HT.lookup tbl kpr
	case mb_k of
	  Just k  -> return k
	  Nothing -> do { k <- newKey kloc ;
			  HT.insert tbl kpr k ;
			  return k }
  where
    kpr = KeyPr k1 k2

appKeys :: Key -> [Key] -> Key
appKeys k ks = foldl appKey k ks
