[project @ 2002-12-11 16:12:22 by ross]
[packages/random.git] / 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/base/LICENSE)
7 --
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : portable
11 --
12 -- The Dynamic interface provides basic support for dynamic types.
13 --
14 -- Operations for injecting values of arbitrary type into
15 -- a dynamically typed value, Dynamic, are provided, together
16 -- with operations for converting dynamic values into a concrete
17 -- (monomorphic) type.
18 --
19 -----------------------------------------------------------------------------
20
21 module Data.Dynamic
22 (
23 -- * The @Dynamic@ type
24 Dynamic, -- abstract, instance of: Show, Typeable
25
26 -- * Converting to and from @Dynamic@
27 toDyn, -- :: Typeable a => a -> Dynamic
28 fromDyn, -- :: Typeable a => Dynamic -> a -> a
29 fromDynamic, -- :: Typeable a => Dynamic -> Maybe a
30
31 -- * Applying functions of dynamic type
32 dynApply,
33 dynApp,
34
35 -- * Concrete Type Representations
36
37 -- | This section is useful if you need to define your own
38 -- instances of 'Typeable'.
39
40 Typeable(
41 typeOf), -- :: a -> TypeRep
42
43 -- ** Building concrete type representations
44 TypeRep, -- abstract, instance of: Eq, Show, Typeable
45 TyCon, -- abstract, instance of: Eq, Show, Typeable
46
47 mkTyCon, -- :: String -> TyCon
48 mkAppTy, -- :: TyCon -> [TypeRep] -> TypeRep
49 mkFunTy, -- :: TypeRep -> TypeRep -> TypeRep
50 applyTy, -- :: TypeRep -> TypeRep -> Maybe TypeRep
51
52 --
53 -- let fTy = mkTyCon "Foo" in show (mkAppTy (mkTyCon ",,")
54 -- [fTy,fTy,fTy])
55 --
56 -- returns "(Foo,Foo,Foo)"
57 --
58 -- The TypeRep Show instance promises to print tuple types
59 -- correctly. Tuple type constructors are specified by a
60 -- sequence of commas, e.g., (mkTyCon ",,,,") returns
61 -- the 5-tuple tycon.
62 ) where
63
64
65 import Data.Maybe
66 import Data.Either
67 import Data.Int
68 import Data.Word
69 import Foreign.Ptr
70 import Foreign.StablePtr
71
72 #ifdef __GLASGOW_HASKELL__
73 import GHC.Base
74 import GHC.Show
75 import GHC.Err
76 import GHC.Num
77 import GHC.Float
78 import GHC.IOBase
79 #endif
80
81 #ifdef __HUGS__
82 import Hugs.IO
83 import Hugs.IORef
84 import Hugs.IOExts
85 #endif
86
87 #ifdef __GLASGOW_HASKELL__
88 unsafeCoerce :: a -> b
89 unsafeCoerce = unsafeCoerce#
90 #endif
91
92 #include "Dynamic.h"
93
94 {-|
95 A value of type 'Dynamic' is an object encapsulated together with its type.
96
97 A 'Dynamic' may only represent a monomorphic value; an attempt to
98 create a value of type 'Dynamic' from a polymorphically-typed
99 expression will result in an ambiguity error (see 'toDyn').
100
101 'Show'ing a value of type 'Dynamic' returns a pretty-printed representation
102 of the object\'s type; useful for debugging.
103 -}
104 data Dynamic = Dynamic TypeRep Obj
105
106 instance Show Dynamic where
107 -- the instance just prints the type representation.
108 showsPrec _ (Dynamic t _) =
109 showString "<<" .
110 showsPrec 0 t .
111 showString ">>"
112
113 #ifdef __GLASGOW_HASKELL__
114 type Obj = forall a . a
115 -- Dummy type to hold the dynamically typed value.
116 --
117 -- In GHC's new eval/apply execution model this type must
118 -- be polymorphic. It can't be a constructor, because then
119 -- GHC will use the constructor convention when evaluating it,
120 -- and this will go wrong if the object is really a function. On
121 -- the other hand, if we use a polymorphic type, GHC will use
122 -- a fallback convention for evaluating it that works for all types.
123 -- (using a function type here would also work).
124 #else
125 data Obj = Obj
126 #endif
127
128 -- | A concrete representation of a (monomorphic) type. 'TypeRep'
129 -- supports reasonably efficient equality.
130 data TypeRep
131 = App TyCon [TypeRep]
132 | Fun TypeRep TypeRep
133 deriving ( Eq )
134
135 instance Show TypeRep where
136 showsPrec p (App tycon tys) =
137 case tys of
138 [] -> showsPrec p tycon
139 [x] | tycon == listTc -> showChar '[' . shows x . showChar ']'
140 xs
141 | isTupleTyCon tycon -> showTuple tycon xs
142 | otherwise ->
143 showParen (p > 9) $
144 showsPrec p tycon .
145 showChar ' ' .
146 showArgs tys
147
148 showsPrec p (Fun f a) =
149 showParen (p > 8) $
150 showsPrec 9 f . showString " -> " . showsPrec 8 a
151
152 -- | An abstract representation of a type constructor. 'TyCon' objects can
153 -- be built using 'mkTyCon'.
154 data TyCon = TyCon Int String
155
156 instance Eq TyCon where
157 (TyCon t1 _) == (TyCon t2 _) = t1 == t2
158
159 instance Show TyCon where
160 showsPrec _ (TyCon _ s) = showString s
161
162
163 -- | Converts an arbitrary value into an object of type 'Dynamic'.
164 --
165 -- The type of the object must be an instance of 'Typeable', which
166 -- ensures that only monomorphically-typed objects may be converted to
167 -- 'Dynamic'. To convert a polymorphic object into 'Dynamic', give it
168 -- a monomorphic type signature. For example:
169 --
170 -- > toDyn (id :: Int -> Int)
171 --
172 toDyn :: Typeable a => a -> Dynamic
173 toDyn v = Dynamic (typeOf v) (unsafeCoerce v)
174
175 -- | Converts a 'Dynamic' object back into an ordinary Haskell value of
176 -- the correct type. See also 'fromDynamic'.
177 fromDyn :: Typeable a
178 => Dynamic -- ^ the dynamically-typed object
179 -> a -- ^ a default value
180 -> a -- ^ returns: the value of the first argument, if
181 -- it has the correct type, otherwise the value of
182 -- the second argument.
183 fromDyn (Dynamic t v) def
184 | typeOf def == t = unsafeCoerce v
185 | otherwise = def
186
187 -- | Converts a 'Dynamic' object back into an ordinary Haskell value of
188 -- the correct type. See also 'fromDyn'.
189 fromDynamic
190 :: Typeable a
191 => Dynamic -- ^ the dynamically-typed object
192 -> Maybe a -- ^ returns: @'Just' a@, if the dyanmically-typed
193 -- object has the correct type (and @a@ is its value),
194 -- or 'Nothing' otherwise.
195 fromDynamic (Dynamic t v) =
196 case unsafeCoerce v of
197 r | t == typeOf r -> Just r
198 | otherwise -> Nothing
199
200 -- | The class 'Typeable' allows a concrete representation of a type to
201 -- be calculated.
202 class Typeable a where
203 typeOf :: a -> TypeRep
204 -- ^ Takes a value of type @a@ and returns a concrete representation
205 -- of that type. The /value/ of the argument should be ignored by
206 -- any instance of 'Typeable', so that it is safe to pass 'undefined' as
207 -- the argument.
208
209 isTupleTyCon :: TyCon -> Bool
210 isTupleTyCon (TyCon _ (',':_)) = True
211 isTupleTyCon _ = False
212
213 -- If we enforce the restriction that there is only one
214 -- @TyCon@ for a type & it is shared among all its uses,
215 -- we can map them onto Ints very simply. The benefit is,
216 -- of course, that @TyCon@s can then be compared efficiently.
217
218 -- Provided the implementor of other @Typeable@ instances
219 -- takes care of making all the @TyCon@s CAFs (toplevel constants),
220 -- this will work.
221
222 -- If this constraint does turn out to be a sore thumb, changing
223 -- the Eq instance for TyCons is trivial.
224
225 -- | Builds a 'TyCon' object representing a type constructor. An
226 -- implementation of "Data.Dynamic" should ensure that the following holds:
227 --
228 -- > mkTyCon "a" == mkTyCon "a"
229 --
230 -- NOTE: GHC\'s implementation is quite hacky, and the above equation
231 -- does not necessarily hold. For defining your own instances of
232 -- 'Typeable', try to ensure that only one call to 'mkTyCon' exists
233 -- for each type constructor (put it at the top level, and annotate the
234 -- corresponding definition with a @NOINLINE@ pragma).
235 mkTyCon
236 :: String -- ^ the name of the type constructor (should be unique
237 -- in the program, so it might be wise to use the
238 -- fully qualified name).
239 -> TyCon -- ^ A unique 'TyCon' object
240 mkTyCon str = unsafePerformIO $ do
241 v <- readIORef uni
242 writeIORef uni (v+1)
243 return (TyCon v str)
244
245 {-# NOINLINE uni #-}
246 uni :: IORef Int
247 uni = unsafePerformIO ( newIORef 0 )
248
249 -- Some (Show.TypeRep) helpers:
250
251 showArgs :: Show a => [a] -> ShowS
252 showArgs [] = id
253 showArgs [a] = showsPrec 10 a
254 showArgs (a:as) = showsPrec 10 a . showString " " . showArgs as
255
256 showTuple :: TyCon -> [TypeRep] -> ShowS
257 showTuple (TyCon _ str) args = showChar '(' . go str args
258 where
259 go [] [a] = showsPrec 10 a . showChar ')'
260 go _ [] = showChar ')' -- a failure condition, really.
261 go (',':xs) (a:as) = showsPrec 10 a . showChar ',' . go xs as
262 go _ _ = showChar ')'
263
264
265 -- | Applies a type constructor to a sequence of types
266 mkAppTy :: TyCon -> [TypeRep] -> TypeRep
267 mkAppTy tyc args = App tyc args
268
269 -- | A special case of 'mkAppTy', which applies the function type constructor to
270 -- a pair of types.
271 mkFunTy :: TypeRep -> TypeRep -> TypeRep
272 mkFunTy f a = Fun f a
273
274 -- Auxillary functions
275
276 -- (f::(a->b)) `dynApply` (x::a) = (f a)::b
277 dynApply :: Dynamic -> Dynamic -> Maybe Dynamic
278 dynApply (Dynamic t1 f) (Dynamic t2 x) =
279 case applyTy t1 t2 of
280 Just t3 -> Just (Dynamic t3 ((unsafeCoerce f) x))
281 Nothing -> Nothing
282
283 dynApp :: Dynamic -> Dynamic -> Dynamic
284 dynApp f x = case dynApply f x of
285 Just r -> r
286 Nothing -> error ("Type error in dynamic application.\n" ++
287 "Can't apply function " ++ show f ++
288 " to argument " ++ show x)
289
290 -- | Applies a type to a function type. Returns: @'Just' u@ if the
291 -- first argument represents a function of type @t -> u@ and the
292 -- second argument represents a function of type @t@. Otherwise,
293 -- returns 'Nothing'.
294 applyTy :: TypeRep -> TypeRep -> Maybe TypeRep
295 applyTy (Fun t1 t2) t3
296 | t1 == t3 = Just t2
297 applyTy _ _ = Nothing
298
299 -- Prelude types
300
301 listTc :: TyCon
302 listTc = mkTyCon "[]"
303
304 instance Typeable a => Typeable [a] where
305 typeOf ls = mkAppTy listTc [typeOf ((undefined:: [a] -> a) ls)]
306
307 unitTc :: TyCon
308 unitTc = mkTyCon "()"
309
310 instance Typeable () where
311 typeOf _ = mkAppTy unitTc []
312
313 tup2Tc :: TyCon
314 tup2Tc = mkTyCon ","
315
316 instance (Typeable a, Typeable b) => Typeable (a,b) where
317 typeOf tu = mkAppTy tup2Tc [typeOf ((undefined :: (a,b) -> a) tu),
318 typeOf ((undefined :: (a,b) -> b) tu)]
319
320 tup3Tc :: TyCon
321 tup3Tc = mkTyCon ",,"
322
323 instance ( Typeable a , Typeable b , Typeable c) => Typeable (a,b,c) where
324 typeOf tu = mkAppTy tup3Tc [typeOf ((undefined :: (a,b,c) -> a) tu),
325 typeOf ((undefined :: (a,b,c) -> b) tu),
326 typeOf ((undefined :: (a,b,c) -> c) tu)]
327
328 tup4Tc :: TyCon
329 tup4Tc = mkTyCon ",,,"
330
331 instance ( Typeable a
332 , Typeable b
333 , Typeable c
334 , Typeable d) => Typeable (a,b,c,d) where
335 typeOf tu = mkAppTy tup4Tc [typeOf ((undefined :: (a,b,c,d) -> a) tu),
336 typeOf ((undefined :: (a,b,c,d) -> b) tu),
337 typeOf ((undefined :: (a,b,c,d) -> c) tu),
338 typeOf ((undefined :: (a,b,c,d) -> d) tu)]
339
340 tup5Tc :: TyCon
341 tup5Tc = mkTyCon ",,,,"
342
343 instance ( Typeable a
344 , Typeable b
345 , Typeable c
346 , Typeable d
347 , Typeable e) => Typeable (a,b,c,d,e) where
348 typeOf tu = mkAppTy tup5Tc [typeOf ((undefined :: (a,b,c,d,e) -> a) tu),
349 typeOf ((undefined :: (a,b,c,d,e) -> b) tu),
350 typeOf ((undefined :: (a,b,c,d,e) -> c) tu),
351 typeOf ((undefined :: (a,b,c,d,e) -> d) tu),
352 typeOf ((undefined :: (a,b,c,d,e) -> e) tu)]
353
354 instance (Typeable a, Typeable b) => Typeable (a -> b) where
355 typeOf f = mkFunTy (typeOf ((undefined :: (a -> b) -> a) f))
356 (typeOf ((undefined :: (a -> b) -> b) f))
357
358 INSTANCE_TYPEABLE0(Bool,boolTc,"Bool")
359 INSTANCE_TYPEABLE0(Char,charTc,"Char")
360 INSTANCE_TYPEABLE0(Float,floatTc,"Float")
361 INSTANCE_TYPEABLE0(Double,doubleTc,"Double")
362 INSTANCE_TYPEABLE0(Int,intTc,"Int")
363 INSTANCE_TYPEABLE0(Integer,integerTc,"Integer")
364 INSTANCE_TYPEABLE2(Either,eitherTc,"Either")
365 INSTANCE_TYPEABLE1(IO,ioTc,"IO")
366 INSTANCE_TYPEABLE1(Maybe,maybeTc,"Maybe")
367 INSTANCE_TYPEABLE0(Ordering,orderingTc,"Ordering")
368 INSTANCE_TYPEABLE0(Handle,handleTc,"Handle")
369 INSTANCE_TYPEABLE1(Ptr,ptrTc,"Ptr")
370 INSTANCE_TYPEABLE1(StablePtr,stablePtrTc,"StablePtr")
371
372 INSTANCE_TYPEABLE0(Int8,int8Tc, "Int8")
373 INSTANCE_TYPEABLE0(Int16,int16Tc,"Int16")
374 INSTANCE_TYPEABLE0(Int32,int32Tc,"Int32")
375 INSTANCE_TYPEABLE0(Int64,int64Tc,"Int64")
376
377 INSTANCE_TYPEABLE0(Word8,word8Tc, "Word8" )
378 INSTANCE_TYPEABLE0(Word16,word16Tc,"Word16")
379 INSTANCE_TYPEABLE0(Word32,word32Tc,"Word32")
380 INSTANCE_TYPEABLE0(Word64,word64Tc,"Word64")
381
382 INSTANCE_TYPEABLE0(TyCon,tyconTc,"TyCon")
383 INSTANCE_TYPEABLE0(TypeRep,typeRepTc,"TypeRep")
384 INSTANCE_TYPEABLE0(Dynamic,dynamicTc,"Dynamic")
385
386 #ifndef __NHC__
387 #include "Dynamic.h"
388 INSTANCE_TYPEABLE1(IORef,ioRefTc,"IORef")
389 #endif