1 {-# LANGUAGE CPP, NoImplicitPrelude #-}
2 #ifdef __GLASGOW_HASKELL__
3 {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-}
6 -----------------------------------------------------------------------------
8 -- Module : Data.Dynamic
9 -- Copyright : (c) The University of Glasgow 2001
10 -- License : BSD-style (see the file libraries/base/LICENSE)
12 -- Maintainer : libraries@haskell.org
13 -- Stability : experimental
14 -- Portability : portable
16 -- The Dynamic interface provides basic support for dynamic types.
18 -- Operations for injecting values of arbitrary type into
19 -- a dynamically typed value, Dynamic, are provided, together
20 -- with operations for converting dynamic values into a concrete
21 -- (monomorphic) type.
23 -----------------------------------------------------------------------------
28 -- Module Data.Typeable re-exported for convenience
31 -- * The @Dynamic@ type
32 Dynamic, -- abstract, instance of: Show, Typeable
34 -- * Converting to and from @Dynamic@
35 toDyn, -- :: Typeable a => a -> Dynamic
36 fromDyn, -- :: Typeable a => Dynamic -> a -> a
37 fromDynamic, -- :: Typeable a => Dynamic -> Maybe a
39 -- * Applying functions of dynamic type
51 #ifdef __GLASGOW_HASKELL__
65 import NHC.IOExtras (IORef,newIORef,readIORef,writeIORef,unsafePerformIO)
70 -------------------------------------------------------------
74 -------------------------------------------------------------
77 A value of type 'Dynamic' is an object encapsulated together with its type.
79 A 'Dynamic' may only represent a monomorphic value; an attempt to
80 create a value of type 'Dynamic' from a polymorphically-typed
81 expression will result in an ambiguity error (see 'toDyn').
83 'Show'ing a value of type 'Dynamic' returns a pretty-printed representation
84 of the object\'s type; useful for debugging.
87 data Dynamic = Dynamic TypeRep Obj
90 INSTANCE_TYPEABLE0(Dynamic,dynamicTc,"Dynamic")
92 instance Show Dynamic where
93 -- the instance just prints the type representation.
94 showsPrec _ (Dynamic t _) =
99 #ifdef __GLASGOW_HASKELL__
100 -- here so that it isn't an orphan:
101 instance Exception Dynamic
104 #ifdef __GLASGOW_HASKELL__
106 -- Use GHC's primitive 'Any' type to hold the dynamically typed value.
108 -- In GHC's new eval/apply execution model this type must not look
109 -- like a data type. If it did, GHC would use the constructor convention
110 -- when evaluating it, and this will go wrong if the object is really a
111 -- function. Using Any forces GHC to use
112 -- a fallback convention for evaluating it that works for all types.
113 #elif !defined(__HUGS__)
117 -- | Converts an arbitrary value into an object of type 'Dynamic'.
119 -- The type of the object must be an instance of 'Typeable', which
120 -- ensures that only monomorphically-typed objects may be converted to
121 -- 'Dynamic'. To convert a polymorphic object into 'Dynamic', give it
122 -- a monomorphic type signature. For example:
124 -- > toDyn (id :: Int -> Int)
126 toDyn :: Typeable a => a -> Dynamic
127 toDyn v = Dynamic (typeOf v) (unsafeCoerce v)
129 -- | Converts a 'Dynamic' object back into an ordinary Haskell value of
130 -- the correct type. See also 'fromDynamic'.
131 fromDyn :: Typeable a
132 => Dynamic -- ^ the dynamically-typed object
133 -> a -- ^ a default value
134 -> a -- ^ returns: the value of the first argument, if
135 -- it has the correct type, otherwise the value of
136 -- the second argument.
137 fromDyn (Dynamic t v) def
138 | typeOf def == t = unsafeCoerce v
141 -- | Converts a 'Dynamic' object back into an ordinary Haskell value of
142 -- the correct type. See also 'fromDyn'.
145 => Dynamic -- ^ the dynamically-typed object
146 -> Maybe a -- ^ returns: @'Just' a@, if the dynamically-typed
147 -- object has the correct type (and @a@ is its value),
148 -- or 'Nothing' otherwise.
149 fromDynamic (Dynamic t v) =
150 case unsafeCoerce v of
151 r | t == typeOf r -> Just r
152 | otherwise -> Nothing
154 -- (f::(a->b)) `dynApply` (x::a) = (f a)::b
155 dynApply :: Dynamic -> Dynamic -> Maybe Dynamic
156 dynApply (Dynamic t1 f) (Dynamic t2 x) =
157 case funResultTy t1 t2 of
158 Just t3 -> Just (Dynamic t3 ((unsafeCoerce f) x))
161 dynApp :: Dynamic -> Dynamic -> Dynamic
162 dynApp f x = case dynApply f x of
164 Nothing -> error ("Type error in dynamic application.\n" ++
165 "Can't apply function " ++ show f ++
166 " to argument " ++ show x)
168 dynTypeRep :: Dynamic -> TypeRep
169 dynTypeRep (Dynamic tr _) = tr