Data/Dynamic.hs

   1 {-# OPTIONS -fno-implicit-prelude #-}
   2 -----------------------------------------------------------------------------
   3 --
   4 -- Module      :  Data.Dynamic
   5 -- Copyright   :  (c) The University of Glasgow 2001
   6 -- License     :  BSD-style (see the file libraries/core/LICENSE)
   7 --
   8 -- Maintainer  :  libraries@haskell.org
   9 -- Stability   :  experimental
  10 -- Portability :  non-portable
  11 --
  12 -- $Id: Dynamic.hs,v 1.1 2001/06/28 14:15:02 simonmar Exp $
  13 --
  14 -- The Dynamic interface provides basic support for dynamic types.
  15 --
  16 -- Operations for injecting values of arbitrary type into
  17 -- a dynamically typed value, Dynamic, are provided, together
  18 -- with operations for converting dynamic values into a concrete
  19 -- (monomorphic) type.
  20 --
  21 -- The Dynamic implementation provided is closely based on code
  22 -- contained in Hugs library of the same name.
  23 --
  24 -----------------------------------------------------------------------------
  25
  26 module Data.Dynamic
  27         (
  28         -- dynamic type
  29           Dynamic       -- abstract, instance of: Show, Typeable
  30         , toDyn         -- :: Typeable a => a -> Dynamic
  31         , fromDyn       -- :: Typeable a => Dynamic -> a -> a
  32         , fromDynamic   -- :: Typeable a => Dynamic -> Maybe a
  33
  34         -- type representation
  35
  36         , Typeable(
  37              typeOf)    -- :: a -> TypeRep
  38
  39           -- Dynamic defines Typeable instances for the following
  40         -- Prelude types: [a], (), (a,b), (a,b,c), (a,b,c,d),
  41         -- (a,b,c,d,e), (a->b), (Array a b), Bool, Char,
  42         -- (Complex a), Double, (Either a b), Float, Handle,
  43         -- Int, Integer, (IO a), (Maybe a), Ordering
  44
  45         , TypeRep       -- abstract, instance of: Eq, Show, Typeable
  46         , TyCon         -- abstract, instance of: Eq, Show, Typeable
  47
  48         -- type representation constructors/operators:
  49         , mkTyCon       -- :: String  -> TyCon
  50         , mkAppTy       -- :: TyCon   -> [TypeRep] -> TypeRep
  51         , mkFunTy       -- :: TypeRep -> TypeRep   -> TypeRep
  52         , applyTy       -- :: TypeRep -> TypeRep   -> Maybe TypeRep
  53
  54         --
  55         -- let fTy = mkTyCon "Foo" in show (mkAppTy (mkTyCon ",,")
  56         --                                 [fTy,fTy,fTy])
  57         --
  58         -- returns "(Foo,Foo,Foo)"
  59         --
  60         -- The TypeRep Show instance promises to print tuple types
  61         -- correctly. Tuple type constructors are specified by a
  62         -- sequence of commas, e.g., (mkTyCon ",,,,") returns
  63         -- the 5-tuple tycon.
  64         ) where
  65
  66
  67 #ifdef __GLASGOW_HASKELL__
  68 import GHC.Base
  69 import GHC.Maybe
  70 import GHC.Show
  71 import GHC.Err
  72 import GHC.Num
  73 import GHC.Float
  74 import GHC.IOBase
  75 import GHC.Dynamic
  76 #endif
  77
  78 #ifdef __GLASGOW_HASKELL__
  79 import GHC.Prim                 ( unsafeCoerce# )
  80
  81 unsafeCoerce :: a -> b
  82 unsafeCoerce = unsafeCoerce#
  83 #endif
  84
  85 #include "Dynamic.h"
  86
  87 -- The dynamic type is represented by Dynamic, carrying
  88 -- the dynamic value along with its type representation:
  89
  90 -- the instance just prints the type representation.
  91 instance Show Dynamic where
  92    showsPrec _ (Dynamic t _) =
  93           showString "<<" .
  94           showsPrec 0 t   .
  95           showString ">>"
  96
  97 -- Operations for going to and from Dynamic:
  98
  99 toDyn :: Typeable a => a -> Dynamic
 100 toDyn v = Dynamic (typeOf v) (unsafeCoerce v)
 101
 102 fromDyn :: Typeable a => Dynamic -> a -> a
 103 fromDyn (Dynamic t v) def
 104   | typeOf def == t = unsafeCoerce v
 105   | otherwise       = def
 106
 107 fromDynamic :: Typeable a => Dynamic -> Maybe a
 108 fromDynamic (Dynamic t v) =
 109   case unsafeCoerce v of
 110     r | t == typeOf r -> Just r
 111       | otherwise     -> Nothing
 112
 113 -- (Abstract) universal datatype:
 114
 115 instance Show TypeRep where
 116   showsPrec p (App tycon tys) =
 117     case tys of
 118       [] -> showsPrec p tycon
 119       [x] | tycon == listTc    -> showChar '[' . shows x . showChar ']'
 120       xs
 121         | isTupleTyCon tycon -> showTuple tycon xs
 122         | otherwise          ->
 123             showParen (p > 9) $
 124             showsPrec p tycon .
 125             showChar ' '      .
 126             showArgs tys
 127
 128   showsPrec p (Fun f a) =
 129      showParen (p > 8) $
 130      showsPrec 9 f . showString " -> " . showsPrec 8 a
 131
 132 -- To make it possible to convert values with user-defined types
 133 -- into type Dynamic, we need a systematic way of getting
 134 -- the type representation of an arbitrary type. A type
 135 -- class provides just the ticket,
 136
 137 class Typeable a where
 138   typeOf :: a -> TypeRep
 139
 140 -- NOTE: The argument to the overloaded `typeOf' is only
 141 -- used to carry type information, and Typeable instances
 142 -- should *never* *ever* look at its value.
 143
 144 isTupleTyCon :: TyCon -> Bool
 145 isTupleTyCon (TyCon _ (',':_)) = True
 146 isTupleTyCon _                 = False
 147
 148 instance Show TyCon where
 149   showsPrec _ (TyCon _ s) = showString s
 150
 151 -- If we enforce the restriction that there is only one
 152 -- @TyCon@ for a type & it is shared among all its uses,
 153 -- we can map them onto Ints very simply. The benefit is,
 154 -- of course, that @TyCon@s can then be compared efficiently.
 155
 156 -- Provided the implementor of other @Typeable@ instances
 157 -- takes care of making all the @TyCon@s CAFs (toplevel constants),
 158 -- this will work.
 159
 160 -- If this constraint does turn out to be a sore thumb, changing
 161 -- the Eq instance for TyCons is trivial.
 162
 163 mkTyCon :: String -> TyCon
 164 mkTyCon str = unsafePerformIO $ do
 165    v <- readIORef uni
 166    writeIORef uni (v+1)
 167    return (TyCon v str)
 168
 169 {-# NOINLINE uni #-}
 170 uni :: IORef Int
 171 uni = unsafePerformIO ( newIORef 0 )
 172
 173 -- Some (Show.TypeRep) helpers:
 174
 175 showArgs :: Show a => [a] -> ShowS
 176 showArgs [] = id
 177 showArgs [a] = showsPrec 10 a
 178 showArgs (a:as) = showsPrec 10 a . showString " " . showArgs as
 179
 180 showTuple :: TyCon -> [TypeRep] -> ShowS
 181 showTuple (TyCon _ str) args = showChar '(' . go str args
 182  where
 183   go [] [a] = showsPrec 10 a . showChar ')'
 184   go _  []  = showChar ')' -- a failure condition, really.
 185   go (',':xs) (a:as) = showsPrec 10 a . showChar ',' . go xs as
 186   go _ _   = showChar ')'
 187
 188
 189 mkAppTy  :: TyCon   -> [TypeRep] -> TypeRep
 190 mkAppTy tyc args = App tyc args
 191
 192 mkFunTy  :: TypeRep -> TypeRep   -> TypeRep
 193 mkFunTy f a = Fun f a
 194
 195 -- Auxillary functions
 196
 197 -- (f::(a->b)) `dynApply` (x::a) = (f a)::b
 198 dynApply :: Dynamic -> Dynamic -> Maybe Dynamic
 199 dynApply (Dynamic t1 f) (Dynamic t2 x) =
 200   case applyTy t1 t2 of
 201     Just t3 -> Just (Dynamic t3 ((unsafeCoerce f) x))
 202     Nothing -> Nothing
 203
 204 dynApp :: Dynamic -> Dynamic -> Dynamic
 205 dynApp f x = case dynApply f x of
 206              Just r -> r
 207              Nothing -> error ("Type error in dynamic application.\n" ++
 208                                "Can't apply function " ++ show f ++
 209                                " to argument " ++ show x)
 210
 211 applyTy :: TypeRep -> TypeRep -> Maybe TypeRep
 212 applyTy (Fun t1 t2) t3
 213   | t1 == t3    = Just t2
 214 applyTy _ _     = Nothing
 215
 216 -- Prelude types
 217
 218 listTc :: TyCon
 219 listTc = mkTyCon "[]"
 220
 221 instance Typeable a => Typeable [a] where
 222   typeOf ls = mkAppTy listTc [typeOf ((undefined:: [a] -> a) ls)]
 223
 224 unitTc :: TyCon
 225 unitTc = mkTyCon "()"
 226
 227 instance Typeable () where
 228   typeOf _ = mkAppTy unitTc []
 229
 230 tup2Tc :: TyCon
 231 tup2Tc = mkTyCon ","
 232
 233 instance (Typeable a, Typeable b) => Typeable (a,b) where
 234   typeOf tu = mkAppTy tup2Tc [typeOf ((undefined :: (a,b) -> a) tu),
 235                               typeOf ((undefined :: (a,b) -> b) tu)]
 236
 237 tup3Tc :: TyCon
 238 tup3Tc = mkTyCon ",,"
 239
 240 instance ( Typeable a , Typeable b , Typeable c) => Typeable (a,b,c) where
 241   typeOf tu = mkAppTy tup3Tc [typeOf ((undefined :: (a,b,c) -> a) tu),
 242                               typeOf ((undefined :: (a,b,c) -> b) tu),
 243                               typeOf ((undefined :: (a,b,c) -> c) tu)]
 244
 245 tup4Tc :: TyCon
 246 tup4Tc = mkTyCon ",,,"
 247
 248 instance ( Typeable a
 249          , Typeable b
 250          , Typeable c
 251          , Typeable d) => Typeable (a,b,c,d) where
 252   typeOf tu = mkAppTy tup4Tc [typeOf ((undefined :: (a,b,c,d) -> a) tu),
 253                               typeOf ((undefined :: (a,b,c,d) -> b) tu),
 254                               typeOf ((undefined :: (a,b,c,d) -> c) tu),
 255                               typeOf ((undefined :: (a,b,c,d) -> d) tu)]
 256
 257 tup5Tc :: TyCon
 258 tup5Tc = mkTyCon ",,,,"
 259
 260 instance ( Typeable a
 261          , Typeable b
 262          , Typeable c
 263          , Typeable d
 264          , Typeable e) => Typeable (a,b,c,d,e) where
 265   typeOf tu = mkAppTy tup5Tc [typeOf ((undefined :: (a,b,c,d,e) -> a) tu),
 266                               typeOf ((undefined :: (a,b,c,d,e) -> b) tu),
 267                               typeOf ((undefined :: (a,b,c,d,e) -> c) tu),
 268                               typeOf ((undefined :: (a,b,c,d,e) -> d) tu),
 269                               typeOf ((undefined :: (a,b,c,d,e) -> e) tu)]
 270
 271 instance (Typeable a, Typeable b) => Typeable (a -> b) where
 272   typeOf f = mkFunTy (typeOf ((undefined :: (a -> b) -> a) f))
 273                      (typeOf ((undefined :: (a -> b) -> b) f))
 274
 275 INSTANCE_TYPEABLE0(Bool,boolTc,"Bool")
 276 INSTANCE_TYPEABLE0(Char,charTc,"Char")
 277 INSTANCE_TYPEABLE0(Float,floatTc,"Float")
 278 INSTANCE_TYPEABLE0(Double,doubleTc,"Double")
 279 INSTANCE_TYPEABLE0(Int,intTc,"Int")
 280 INSTANCE_TYPEABLE0(Integer,integerTc,"Integer")
 281 INSTANCE_TYPEABLE2(Either,eitherTc,"Either")
 282 INSTANCE_TYPEABLE1(IO,ioTc,"IO")
 283 INSTANCE_TYPEABLE1(Maybe,maybeTc,"Maybe")
 284 INSTANCE_TYPEABLE0(Ordering,orderingTc,"Ordering")
 285
 286 INSTANCE_TYPEABLE0(TyCon,tyconTc,"TyCon")
 287 INSTANCE_TYPEABLE0(TypeRep,typeRepTc,"TypeRep")
 288 INSTANCE_TYPEABLE0(Dynamic,dynamicTc,"Dynamic")