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| Data.Dynamic | | Portability | portable | | Stability | experimental | | Maintainer | libraries@haskell.org |
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| Contents |
- The Dynamic type
- Converting to and from Dynamic
- Applying functions of dynamic type
- Concrete Type Representations
- Building concrete type representations
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| Description |
The Dynamic interface provides basic support for dynamic types. Operations for injecting values of arbitrary type into
a dynamically typed value, Dynamic, are provided, together
with operations for converting dynamic values into a concrete
(monomorphic) type.
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| Synopsis |
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| The Dynamic type |
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| data Dynamic |
A value of type Dynamic is an object encapsulated together with its type. A Dynamic may only represent a monomorphic value; an attempt to
create a value of type Dynamic from a polymorphically-typed
expression will result in an ambiguity error (see toDyn). Showing a value of type Dynamic returns a pretty-printed representation
of the object's type; useful for debugging.
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| Converting to and from Dynamic |
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| toDyn :: (Typeable a) => a -> Dynamic |
Converts an arbitrary value into an object of type Dynamic. The type of the object must be an instance of Typeable, which
ensures that only monomorphically-typed objects may be converted to
Dynamic. To convert a polymorphic object into Dynamic, give it
a monomorphic type signature. For example: toDyn (id :: Int -> Int)
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| fromDyn |
| :: (Typeable a) | | | => Dynamic | the dynamically-typed object | | -> a | a default value | | -> a | returns: the value of the first argument, if
it has the correct type, otherwise the value of
the second argument. | | Converts a Dynamic object back into an ordinary Haskell value of
the correct type. See also fromDynamic. |
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| fromDynamic |
| :: (Typeable a) | | | => Dynamic | the dynamically-typed object | | -> Maybe a | returns: Just a, if the dyanmically-typed
object has the correct type (and a is its value),
or Nothing otherwise. | | Converts a Dynamic object back into an ordinary Haskell value of
the correct type. See also fromDyn. |
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| Applying functions of dynamic type |
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| dynApply :: Dynamic -> Dynamic -> Maybe Dynamic |
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| dynApp :: Dynamic -> Dynamic -> Dynamic |
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| Concrete Type Representations |
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| This section is useful if you need to define your own
instances of Typeable. |
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| class Typeable a where |
| The class Typeable allows a concrete representation of a type to
be calculated. | | | Methods | | typeOf :: a -> TypeRep | | Takes a value of type a and returns a concrete representation
of that type. The value of the argument should be ignored by
any instance of Typeable, so that it is safe to pass undefined as
the argument. |
| | | Instances | | Typeable Exception | | Typeable IOException | | Typeable ArithException | | Typeable ArrayException | | Typeable AsyncException | | (Typeable a, Typeable b) => Typeable (ST a b) | | (Typeable a, Typeable b) => Typeable (Array a b) | | (Typeable a, Typeable b) => Typeable (UArray a b) | | (Typeable a, Typeable b, Typeable c) => Typeable (STArray a b c) | | (Typeable a, Typeable b, Typeable c) => Typeable (STUArray a b c) | | (Typeable a, Typeable b) => Typeable (IOArray a b) | | (Typeable a, Typeable b) => Typeable (IOUArray a b) | | (Typeable a) => Typeable (Complex a) | | (Typeable a) => Typeable [a] | | Typeable () | | (Typeable a, Typeable b) => Typeable (a, b) | | (Typeable a, Typeable b, Typeable c) => Typeable (a, b, c) | | (Typeable a, Typeable b, Typeable c, Typeable d) => Typeable (a, b, c, d) | | (Typeable a, Typeable b, Typeable c, Typeable d, Typeable e) => Typeable (a, b, c, d, e) | | (Typeable a, Typeable b) => Typeable (a -> b) | | Typeable Bool | | Typeable Char | | Typeable Float | | Typeable Double | | Typeable Int | | Typeable Integer | | (Typeable a, Typeable b) => Typeable (Either a b) | | (Typeable a) => Typeable (IO a) | | (Typeable a) => Typeable (Maybe a) | | Typeable Ordering | | Typeable Handle | | (Typeable a) => Typeable (Ptr a) | | (Typeable a) => Typeable (StablePtr a) | | Typeable Int8 | | Typeable Int16 | | Typeable Int32 | | Typeable Int64 | | Typeable Word8 | | Typeable Word16 | | Typeable Word32 | | Typeable Word64 | | Typeable TyCon | | Typeable TypeRep | | Typeable Dynamic | | (Typeable a) => Typeable (IORef a) | | Typeable PackedString | | (Typeable a, Typeable b) => Typeable (STRef a b) | | Typeable CChar | | Typeable CSChar | | Typeable CUChar | | Typeable CShort | | Typeable CUShort | | Typeable CInt | | Typeable CUInt | | Typeable CLong | | Typeable CULong | | Typeable CLLong | | Typeable CULLong | | Typeable CFloat | | Typeable CDouble | | Typeable CLDouble | | Typeable CPtrdiff | | Typeable CSize | | Typeable CWchar | | Typeable CSigAtomic | | Typeable CClock | | Typeable CTime | | (Typeable a) => Typeable (ForeignPtr a) | | (Typeable a) => Typeable (StableName a) | | (Typeable a) => Typeable (Weak a) | | Typeable CDev | | Typeable CIno | | Typeable CMode | | Typeable COff | | Typeable CPid | | Typeable CSsize | | Typeable CGid | | Typeable CNlink | | Typeable CUid | | Typeable CCc | | Typeable CSpeed | | Typeable CTcflag | | Typeable CRLim | | Typeable Fd |
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| cast :: (Typeable a, Typeable b) => a -> Maybe b |
| The type-safe cast operation |
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| Building concrete type representations |
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| data TypeRep |
| A concrete representation of a (monomorphic) type. TypeRep
supports reasonably efficient equality. | | Instances | |
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| data TyCon |
| An abstract representation of a type constructor. TyCon objects can
be built using mkTyCon. | | Instances | |
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| mkTyCon |
| :: String | the name of the type constructor (should be unique
in the program, so it might be wise to use the
fully qualified name). | | -> TyCon | A unique TyCon object | Builds a TyCon object representing a type constructor. An
implementation of Data.Dynamic should ensure that the following holds: mkTyCon "a" == mkTyCon "a" NOTE: GHC's implementation is quite hacky, and the above equation
does not necessarily hold. For defining your own instances of
Typeable, try to ensure that only one call to mkTyCon exists
for each type constructor (put it at the top level, and annotate the
corresponding definition with a NOINLINE pragma). |
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| mkAppTy :: TyCon -> [TypeRep] -> TypeRep |
| Applies a type constructor to a sequence of types |
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| mkFunTy :: TypeRep -> TypeRep -> TypeRep |
| A special case of mkAppTy, which applies the function
type constructor to a pair of types. |
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| applyTy :: TypeRep -> TypeRep -> Maybe TypeRep |
| Applies a type to a function type. Returns: Just u if the
first argument represents a function of type t -> u and the
second argument represents a function of type t. Otherwise,
returns Nothing. |
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| Produced by Haddock version 0.4 |
