e603933feee15aec5f6432f72875723c1e06f295
[packages/old-time.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 #ifdef __NHC__
93 import NonStdUnsafeCoerce (unsafeCoerce)
94 import NHC.IOExtras (IORef,newIORef,readIORef,writeIORef,unsafePerformIO)
95 #else
96 #include "Dynamic.h"
97 #endif
98
99 {-|
100 A value of type 'Dynamic' is an object encapsulated together with its type.
101
102 A 'Dynamic' may only represent a monomorphic value; an attempt to
103 create a value of type 'Dynamic' from a polymorphically-typed
104 expression will result in an ambiguity error (see 'toDyn').
105
106 'Show'ing a value of type 'Dynamic' returns a pretty-printed representation
107 of the object\'s type; useful for debugging.
108 -}
109 data Dynamic = Dynamic TypeRep Obj
110
111 instance Show Dynamic where
112 -- the instance just prints the type representation.
113 showsPrec _ (Dynamic t _) =
114 showString "<<" .
115 showsPrec 0 t .
116 showString ">>"
117
118 #ifdef __GLASGOW_HASKELL__
119 type Obj = forall a . a
120 -- Dummy type to hold the dynamically typed value.
121 --
122 -- In GHC's new eval/apply execution model this type must
123 -- be polymorphic. It can't be a constructor, because then
124 -- GHC will use the constructor convention when evaluating it,
125 -- and this will go wrong if the object is really a function. On
126 -- the other hand, if we use a polymorphic type, GHC will use
127 -- a fallback convention for evaluating it that works for all types.
128 -- (using a function type here would also work).
129 #else
130 data Obj = Obj
131 #endif
132
133 -- | A concrete representation of a (monomorphic) type. 'TypeRep'
134 -- supports reasonably efficient equality.
135 data TypeRep
136 = App TyCon [TypeRep]
137 | Fun TypeRep TypeRep
138 deriving ( Eq )
139
140 instance Show TypeRep where
141 showsPrec p (App tycon tys) =
142 case tys of
143 [] -> showsPrec p tycon
144 [x] | tycon == listTc -> showChar '[' . shows x . showChar ']'
145 xs
146 | isTupleTyCon tycon -> showTuple tycon xs
147 | otherwise ->
148 showParen (p > 9) $
149 showsPrec p tycon .
150 showChar ' ' .
151 showArgs tys
152
153 showsPrec p (Fun f a) =
154 showParen (p > 8) $
155 showsPrec 9 f . showString " -> " . showsPrec 8 a
156
157 -- | An abstract representation of a type constructor. 'TyCon' objects can
158 -- be built using 'mkTyCon'.
159 data TyCon = TyCon Int String
160
161 instance Eq TyCon where
162 (TyCon t1 _) == (TyCon t2 _) = t1 == t2
163
164 instance Show TyCon where
165 showsPrec _ (TyCon _ s) = showString s
166
167
168 -- | Converts an arbitrary value into an object of type 'Dynamic'.
169 --
170 -- The type of the object must be an instance of 'Typeable', which
171 -- ensures that only monomorphically-typed objects may be converted to
172 -- 'Dynamic'. To convert a polymorphic object into 'Dynamic', give it
173 -- a monomorphic type signature. For example:
174 --
175 -- > toDyn (id :: Int -> Int)
176 --
177 toDyn :: Typeable a => a -> Dynamic
178 toDyn v = Dynamic (typeOf v) (unsafeCoerce v)
179
180 -- | Converts a 'Dynamic' object back into an ordinary Haskell value of
181 -- the correct type. See also 'fromDynamic'.
182 fromDyn :: Typeable a
183 => Dynamic -- ^ the dynamically-typed object
184 -> a -- ^ a default value
185 -> a -- ^ returns: the value of the first argument, if
186 -- it has the correct type, otherwise the value of
187 -- the second argument.
188 fromDyn (Dynamic t v) def
189 | typeOf def == t = unsafeCoerce v
190 | otherwise = def
191
192 -- | Converts a 'Dynamic' object back into an ordinary Haskell value of
193 -- the correct type. See also 'fromDyn'.
194 fromDynamic
195 :: Typeable a
196 => Dynamic -- ^ the dynamically-typed object
197 -> Maybe a -- ^ returns: @'Just' a@, if the dyanmically-typed
198 -- object has the correct type (and @a@ is its value),
199 -- or 'Nothing' otherwise.
200 fromDynamic (Dynamic t v) =
201 case unsafeCoerce v of
202 r | t == typeOf r -> Just r
203 | otherwise -> Nothing
204
205 -- | The class 'Typeable' allows a concrete representation of a type to
206 -- be calculated.
207 class Typeable a where
208 typeOf :: a -> TypeRep
209 -- ^ Takes a value of type @a@ and returns a concrete representation
210 -- of that type. The /value/ of the argument should be ignored by
211 -- any instance of 'Typeable', so that it is safe to pass 'undefined' as
212 -- the argument.
213
214 isTupleTyCon :: TyCon -> Bool
215 isTupleTyCon (TyCon _ (',':_)) = True
216 isTupleTyCon _ = False
217
218 -- If we enforce the restriction that there is only one
219 -- @TyCon@ for a type & it is shared among all its uses,
220 -- we can map them onto Ints very simply. The benefit is,
221 -- of course, that @TyCon@s can then be compared efficiently.
222
223 -- Provided the implementor of other @Typeable@ instances
224 -- takes care of making all the @TyCon@s CAFs (toplevel constants),
225 -- this will work.
226
227 -- If this constraint does turn out to be a sore thumb, changing
228 -- the Eq instance for TyCons is trivial.
229
230 -- | Builds a 'TyCon' object representing a type constructor. An
231 -- implementation of "Data.Dynamic" should ensure that the following holds:
232 --
233 -- > mkTyCon "a" == mkTyCon "a"
234 --
235 -- NOTE: GHC\'s implementation is quite hacky, and the above equation
236 -- does not necessarily hold. For defining your own instances of
237 -- 'Typeable', try to ensure that only one call to 'mkTyCon' exists
238 -- for each type constructor (put it at the top level, and annotate the
239 -- corresponding definition with a @NOINLINE@ pragma).
240 mkTyCon
241 :: String -- ^ the name of the type constructor (should be unique
242 -- in the program, so it might be wise to use the
243 -- fully qualified name).
244 -> TyCon -- ^ A unique 'TyCon' object
245 mkTyCon str = unsafePerformIO $ do
246 v <- readIORef uni
247 writeIORef uni (v+1)
248 return (TyCon v str)
249
250 {-# NOINLINE uni #-}
251 uni :: IORef Int
252 uni = unsafePerformIO ( newIORef 0 )
253
254 -- Some (Show.TypeRep) helpers:
255
256 showArgs :: Show a => [a] -> ShowS
257 showArgs [] = id
258 showArgs [a] = showsPrec 10 a
259 showArgs (a:as) = showsPrec 10 a . showString " " . showArgs as
260
261 showTuple :: TyCon -> [TypeRep] -> ShowS
262 showTuple (TyCon _ str) args = showChar '(' . go str args
263 where
264 go [] [a] = showsPrec 10 a . showChar ')'
265 go _ [] = showChar ')' -- a failure condition, really.
266 go (',':xs) (a:as) = showsPrec 10 a . showChar ',' . go xs as
267 go _ _ = showChar ')'
268
269
270 -- | Applies a type constructor to a sequence of types
271 mkAppTy :: TyCon -> [TypeRep] -> TypeRep
272 mkAppTy tyc args = App tyc args
273
274 -- | A special case of 'mkAppTy', which applies the function type constructor to
275 -- a pair of types.
276 mkFunTy :: TypeRep -> TypeRep -> TypeRep
277 mkFunTy f a = Fun f a
278
279 -- Auxillary functions
280
281 -- (f::(a->b)) `dynApply` (x::a) = (f a)::b
282 dynApply :: Dynamic -> Dynamic -> Maybe Dynamic
283 dynApply (Dynamic t1 f) (Dynamic t2 x) =
284 case applyTy t1 t2 of
285 Just t3 -> Just (Dynamic t3 ((unsafeCoerce f) x))
286 Nothing -> Nothing
287
288 dynApp :: Dynamic -> Dynamic -> Dynamic
289 dynApp f x = case dynApply f x of
290 Just r -> r
291 Nothing -> error ("Type error in dynamic application.\n" ++
292 "Can't apply function " ++ show f ++
293 " to argument " ++ show x)
294
295 -- | Applies a type to a function type. Returns: @'Just' u@ if the
296 -- first argument represents a function of type @t -> u@ and the
297 -- second argument represents a function of type @t@. Otherwise,
298 -- returns 'Nothing'.
299 applyTy :: TypeRep -> TypeRep -> Maybe TypeRep
300 applyTy (Fun t1 t2) t3
301 | t1 == t3 = Just t2
302 applyTy _ _ = Nothing
303
304 -- Prelude types
305
306 listTc :: TyCon
307 listTc = mkTyCon "[]"
308
309 instance Typeable a => Typeable [a] where
310 typeOf ls = mkAppTy listTc [typeOf ((undefined:: [a] -> a) ls)]
311
312 unitTc :: TyCon
313 unitTc = mkTyCon "()"
314
315 instance Typeable () where
316 typeOf _ = mkAppTy unitTc []
317
318 tup2Tc :: TyCon
319 tup2Tc = mkTyCon ","
320
321 instance (Typeable a, Typeable b) => Typeable (a,b) where
322 typeOf tu = mkAppTy tup2Tc [typeOf ((undefined :: (a,b) -> a) tu),
323 typeOf ((undefined :: (a,b) -> b) tu)]
324
325 tup3Tc :: TyCon
326 tup3Tc = mkTyCon ",,"
327
328 instance ( Typeable a , Typeable b , Typeable c) => Typeable (a,b,c) where
329 typeOf tu = mkAppTy tup3Tc [typeOf ((undefined :: (a,b,c) -> a) tu),
330 typeOf ((undefined :: (a,b,c) -> b) tu),
331 typeOf ((undefined :: (a,b,c) -> c) tu)]
332
333 tup4Tc :: TyCon
334 tup4Tc = mkTyCon ",,,"
335
336 instance ( Typeable a
337 , Typeable b
338 , Typeable c
339 , Typeable d) => Typeable (a,b,c,d) where
340 typeOf tu = mkAppTy tup4Tc [typeOf ((undefined :: (a,b,c,d) -> a) tu),
341 typeOf ((undefined :: (a,b,c,d) -> b) tu),
342 typeOf ((undefined :: (a,b,c,d) -> c) tu),
343 typeOf ((undefined :: (a,b,c,d) -> d) tu)]
344
345 tup5Tc :: TyCon
346 tup5Tc = mkTyCon ",,,,"
347
348 instance ( Typeable a
349 , Typeable b
350 , Typeable c
351 , Typeable d
352 , Typeable e) => Typeable (a,b,c,d,e) where
353 typeOf tu = mkAppTy tup5Tc [typeOf ((undefined :: (a,b,c,d,e) -> a) tu),
354 typeOf ((undefined :: (a,b,c,d,e) -> b) tu),
355 typeOf ((undefined :: (a,b,c,d,e) -> c) tu),
356 typeOf ((undefined :: (a,b,c,d,e) -> d) tu),
357 typeOf ((undefined :: (a,b,c,d,e) -> e) tu)]
358
359 instance (Typeable a, Typeable b) => Typeable (a -> b) where
360 typeOf f = mkFunTy (typeOf ((undefined :: (a -> b) -> a) f))
361 (typeOf ((undefined :: (a -> b) -> b) f))
362
363 #ifndef __NHC__
364 INSTANCE_TYPEABLE0(Bool,boolTc,"Bool")
365 INSTANCE_TYPEABLE0(Char,charTc,"Char")
366 INSTANCE_TYPEABLE0(Float,floatTc,"Float")
367 INSTANCE_TYPEABLE0(Double,doubleTc,"Double")
368 INSTANCE_TYPEABLE0(Int,intTc,"Int")
369 INSTANCE_TYPEABLE0(Integer,integerTc,"Integer")
370 INSTANCE_TYPEABLE2(Either,eitherTc,"Either")
371 INSTANCE_TYPEABLE1(IO,ioTc,"IO")
372 INSTANCE_TYPEABLE1(Maybe,maybeTc,"Maybe")
373 INSTANCE_TYPEABLE0(Ordering,orderingTc,"Ordering")
374 INSTANCE_TYPEABLE0(Handle,handleTc,"Handle")
375 INSTANCE_TYPEABLE1(Ptr,ptrTc,"Ptr")
376 INSTANCE_TYPEABLE1(StablePtr,stablePtrTc,"StablePtr")
377
378 INSTANCE_TYPEABLE0(Int8,int8Tc, "Int8")
379 INSTANCE_TYPEABLE0(Int16,int16Tc,"Int16")
380 INSTANCE_TYPEABLE0(Int32,int32Tc,"Int32")
381 INSTANCE_TYPEABLE0(Int64,int64Tc,"Int64")
382
383 INSTANCE_TYPEABLE0(Word8,word8Tc, "Word8" )
384 INSTANCE_TYPEABLE0(Word16,word16Tc,"Word16")
385 INSTANCE_TYPEABLE0(Word32,word32Tc,"Word32")
386 INSTANCE_TYPEABLE0(Word64,word64Tc,"Word64")
387
388 INSTANCE_TYPEABLE0(TyCon,tyconTc,"TyCon")
389 INSTANCE_TYPEABLE0(TypeRep,typeRepTc,"TypeRep")
390 INSTANCE_TYPEABLE0(Dynamic,dynamicTc,"Dynamic")
391
392 #include "Dynamic.h"
393 INSTANCE_TYPEABLE1(IORef,ioRefTc,"IORef")
394 #endif