4945e13628f4a7d780c455be7b602dbf00891378
[packages/old-locale.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 __GLASGOW_HASKELL__
82 unsafeCoerce :: a -> b
83 unsafeCoerce = unsafeCoerce#
84 #endif
85
86 #include "Dynamic.h"
87
88 {-|
89 A value of type 'Dynamic' is an object encapsulated together with its type.
90
91 A 'Dynamic' may only represent a monomorphic value; an attempt to
92 create a value of type 'Dynamic' from a polymorphically-typed
93 expression will result in an ambiguity error (see 'toDyn').
94
95 'Show'ing a value of type 'Dynamic' returns a pretty-printed representation
96 of the object\'s type; useful for debugging.
97 -}
98 data Dynamic = Dynamic TypeRep Obj
99
100 instance Show Dynamic where
101 -- the instance just prints the type representation.
102 showsPrec _ (Dynamic t _) =
103 showString "<<" .
104 showsPrec 0 t .
105 showString ">>"
106
107 data Obj = Obj
108 -- dummy type to hold the dynamically typed value.
109
110 -- | A concrete representation of a (monomorphic) type. 'TypeRep'
111 -- supports reasonably efficient equality.
112 data TypeRep
113 = App TyCon [TypeRep]
114 | Fun TypeRep TypeRep
115 deriving ( Eq )
116
117 instance Show TypeRep where
118 showsPrec p (App tycon tys) =
119 case tys of
120 [] -> showsPrec p tycon
121 [x] | tycon == listTc -> showChar '[' . shows x . showChar ']'
122 xs
123 | isTupleTyCon tycon -> showTuple tycon xs
124 | otherwise ->
125 showParen (p > 9) $
126 showsPrec p tycon .
127 showChar ' ' .
128 showArgs tys
129
130 showsPrec p (Fun f a) =
131 showParen (p > 8) $
132 showsPrec 9 f . showString " -> " . showsPrec 8 a
133
134 -- | An abstract representation of a type constructor. 'TyCon' objects can
135 -- be built using 'mkTyCon'.
136 data TyCon = TyCon Int String
137
138 instance Eq TyCon where
139 (TyCon t1 _) == (TyCon t2 _) = t1 == t2
140
141 instance Show TyCon where
142 showsPrec _ (TyCon _ s) = showString s
143
144
145 -- | Converts an arbitrary value into an object of type 'Dynamic'.
146 --
147 -- The type of the object must be an instance of 'Typeable', which
148 -- ensures that only monomorphically-typed objects may be converted to
149 -- 'Dynamic'. To convert a polymorphic object into 'Dynamic', give it
150 -- a monomorphic type signature. For example:
151 --
152 -- > toDyn (id :: Int -> Int)
153 --
154 toDyn :: Typeable a => a -> Dynamic
155 toDyn v = Dynamic (typeOf v) (unsafeCoerce v)
156
157 -- | Converts a 'Dynamic' object back into an ordinary Haskell value of
158 -- the correct type. See also 'fromDynamic'.
159 fromDyn :: Typeable a
160 => Dynamic -- ^ the dynamically-typed object
161 -> a -- ^ a default value
162 -> a -- ^ returns: the value of the first argument, if
163 -- it has the correct type, otherwise the value of
164 -- the second argument.
165 fromDyn (Dynamic t v) def
166 | typeOf def == t = unsafeCoerce v
167 | otherwise = def
168
169 -- | Converts a 'Dynamic' object back into an ordinary Haskell value of
170 -- the correct type. See also 'fromDyn'.
171 fromDynamic
172 :: Typeable a
173 => Dynamic -- ^ the dynamically-typed object
174 -> Maybe a -- ^ returns: @'Just' a@, if the dyanmically-typed
175 -- object has the correct type (and @a@ is its value),
176 -- or 'Nothing' otherwise.
177 fromDynamic (Dynamic t v) =
178 case unsafeCoerce v of
179 r | t == typeOf r -> Just r
180 | otherwise -> Nothing
181
182 -- | The class 'Typeable' allows a concrete representation of a type to
183 -- be calculated.
184 class Typeable a where
185 typeOf :: a -> TypeRep
186 -- ^ Takes a value of type @a@ and returns a concrete representation
187 -- of that type. The /value/ of the argument should be ignored by
188 -- any instance of 'Typeable', so that it is safe to pass 'undefined' as
189 -- the argument.
190
191 isTupleTyCon :: TyCon -> Bool
192 isTupleTyCon (TyCon _ (',':_)) = True
193 isTupleTyCon _ = False
194
195 -- If we enforce the restriction that there is only one
196 -- @TyCon@ for a type & it is shared among all its uses,
197 -- we can map them onto Ints very simply. The benefit is,
198 -- of course, that @TyCon@s can then be compared efficiently.
199
200 -- Provided the implementor of other @Typeable@ instances
201 -- takes care of making all the @TyCon@s CAFs (toplevel constants),
202 -- this will work.
203
204 -- If this constraint does turn out to be a sore thumb, changing
205 -- the Eq instance for TyCons is trivial.
206
207 -- | Builds a 'TyCon' object representing a type constructor. An
208 -- implementation of "Data.Dynamic" should ensure that the following holds:
209 --
210 -- > mkTyCon "a" == mkTyCon "a"
211 --
212 -- NOTE: GHC\'s implementation is quite hacky, and the above equation
213 -- does not necessarily hold. For defining your own instances of
214 -- 'Typeable', try to ensure that only one call to 'mkTyCon' exists
215 -- for each type constructor (put it at the top level, and annotate the
216 -- corresponding definition with a @NOINLINE@ pragma).
217 mkTyCon
218 :: String -- ^ the name of the type constructor (should be unique
219 -- in the program, so it might be wise to use the
220 -- fully qualified name).
221 -> TyCon -- ^ A unique 'TyCon' object
222 mkTyCon str = unsafePerformIO $ do
223 v <- readIORef uni
224 writeIORef uni (v+1)
225 return (TyCon v str)
226
227 {-# NOINLINE uni #-}
228 uni :: IORef Int
229 uni = unsafePerformIO ( newIORef 0 )
230
231 -- Some (Show.TypeRep) helpers:
232
233 showArgs :: Show a => [a] -> ShowS
234 showArgs [] = id
235 showArgs [a] = showsPrec 10 a
236 showArgs (a:as) = showsPrec 10 a . showString " " . showArgs as
237
238 showTuple :: TyCon -> [TypeRep] -> ShowS
239 showTuple (TyCon _ str) args = showChar '(' . go str args
240 where
241 go [] [a] = showsPrec 10 a . showChar ')'
242 go _ [] = showChar ')' -- a failure condition, really.
243 go (',':xs) (a:as) = showsPrec 10 a . showChar ',' . go xs as
244 go _ _ = showChar ')'
245
246
247 -- | Applies a type constructor to a sequence of types
248 mkAppTy :: TyCon -> [TypeRep] -> TypeRep
249 mkAppTy tyc args = App tyc args
250
251 -- | A special case of 'mkAppTy', which applies the function type constructor to
252 -- a pair of types.
253 mkFunTy :: TypeRep -> TypeRep -> TypeRep
254 mkFunTy f a = Fun f a
255
256 -- Auxillary functions
257
258 -- (f::(a->b)) `dynApply` (x::a) = (f a)::b
259 dynApply :: Dynamic -> Dynamic -> Maybe Dynamic
260 dynApply (Dynamic t1 f) (Dynamic t2 x) =
261 case applyTy t1 t2 of
262 Just t3 -> Just (Dynamic t3 ((unsafeCoerce f) x))
263 Nothing -> Nothing
264
265 dynApp :: Dynamic -> Dynamic -> Dynamic
266 dynApp f x = case dynApply f x of
267 Just r -> r
268 Nothing -> error ("Type error in dynamic application.\n" ++
269 "Can't apply function " ++ show f ++
270 " to argument " ++ show x)
271
272 -- | Applies a type to a function type. Returns: @'Just' u@ if the
273 -- first argument represents a function of type @t -> u@ and the
274 -- second argument represents a function of type @t@. Otherwise,
275 -- returns 'Nothing'.
276 applyTy :: TypeRep -> TypeRep -> Maybe TypeRep
277 applyTy (Fun t1 t2) t3
278 | t1 == t3 = Just t2
279 applyTy _ _ = Nothing
280
281 -- Prelude types
282
283 listTc :: TyCon
284 listTc = mkTyCon "[]"
285
286 instance Typeable a => Typeable [a] where
287 typeOf ls = mkAppTy listTc [typeOf ((undefined:: [a] -> a) ls)]
288
289 unitTc :: TyCon
290 unitTc = mkTyCon "()"
291
292 instance Typeable () where
293 typeOf _ = mkAppTy unitTc []
294
295 tup2Tc :: TyCon
296 tup2Tc = mkTyCon ","
297
298 instance (Typeable a, Typeable b) => Typeable (a,b) where
299 typeOf tu = mkAppTy tup2Tc [typeOf ((undefined :: (a,b) -> a) tu),
300 typeOf ((undefined :: (a,b) -> b) tu)]
301
302 tup3Tc :: TyCon
303 tup3Tc = mkTyCon ",,"
304
305 instance ( Typeable a , Typeable b , Typeable c) => Typeable (a,b,c) where
306 typeOf tu = mkAppTy tup3Tc [typeOf ((undefined :: (a,b,c) -> a) tu),
307 typeOf ((undefined :: (a,b,c) -> b) tu),
308 typeOf ((undefined :: (a,b,c) -> c) tu)]
309
310 tup4Tc :: TyCon
311 tup4Tc = mkTyCon ",,,"
312
313 instance ( Typeable a
314 , Typeable b
315 , Typeable c
316 , Typeable d) => Typeable (a,b,c,d) where
317 typeOf tu = mkAppTy tup4Tc [typeOf ((undefined :: (a,b,c,d) -> a) tu),
318 typeOf ((undefined :: (a,b,c,d) -> b) tu),
319 typeOf ((undefined :: (a,b,c,d) -> c) tu),
320 typeOf ((undefined :: (a,b,c,d) -> d) tu)]
321
322 tup5Tc :: TyCon
323 tup5Tc = mkTyCon ",,,,"
324
325 instance ( Typeable a
326 , Typeable b
327 , Typeable c
328 , Typeable d
329 , Typeable e) => Typeable (a,b,c,d,e) where
330 typeOf tu = mkAppTy tup5Tc [typeOf ((undefined :: (a,b,c,d,e) -> a) tu),
331 typeOf ((undefined :: (a,b,c,d,e) -> b) tu),
332 typeOf ((undefined :: (a,b,c,d,e) -> c) tu),
333 typeOf ((undefined :: (a,b,c,d,e) -> d) tu),
334 typeOf ((undefined :: (a,b,c,d,e) -> e) tu)]
335
336 instance (Typeable a, Typeable b) => Typeable (a -> b) where
337 typeOf f = mkFunTy (typeOf ((undefined :: (a -> b) -> a) f))
338 (typeOf ((undefined :: (a -> b) -> b) f))
339
340 INSTANCE_TYPEABLE0(Bool,boolTc,"Bool")
341 INSTANCE_TYPEABLE0(Char,charTc,"Char")
342 INSTANCE_TYPEABLE0(Float,floatTc,"Float")
343 INSTANCE_TYPEABLE0(Double,doubleTc,"Double")
344 INSTANCE_TYPEABLE0(Int,intTc,"Int")
345 INSTANCE_TYPEABLE0(Integer,integerTc,"Integer")
346 INSTANCE_TYPEABLE2(Either,eitherTc,"Either")
347 INSTANCE_TYPEABLE1(IO,ioTc,"IO")
348 INSTANCE_TYPEABLE1(Maybe,maybeTc,"Maybe")
349 INSTANCE_TYPEABLE0(Ordering,orderingTc,"Ordering")
350 INSTANCE_TYPEABLE0(Handle,handleTc,"Handle")
351 INSTANCE_TYPEABLE1(Ptr,ptrTc,"Ptr")
352 INSTANCE_TYPEABLE1(StablePtr,stablePtrTc,"StablePtr")
353
354 INSTANCE_TYPEABLE0(Int8,int8Tc, "Int8")
355 INSTANCE_TYPEABLE0(Int16,int16Tc,"Int16")
356 INSTANCE_TYPEABLE0(Int32,int32Tc,"Int32")
357 INSTANCE_TYPEABLE0(Int64,int64Tc,"Int64")
358
359 INSTANCE_TYPEABLE0(Word8,word8Tc, "Word8" )
360 INSTANCE_TYPEABLE0(Word16,word16Tc,"Word16")
361 INSTANCE_TYPEABLE0(Word32,word32Tc,"Word32")
362 INSTANCE_TYPEABLE0(Word64,word64Tc,"Word64")
363
364 INSTANCE_TYPEABLE0(TyCon,tyconTc,"TyCon")
365 INSTANCE_TYPEABLE0(TypeRep,typeRepTc,"TypeRep")
366 INSTANCE_TYPEABLE0(Dynamic,dynamicTc,"Dynamic")