W3cubDocs

/Haskell 8

Data.Dynamic

Copyright	(c) The University of Glasgow 2001
License	BSD-style (see the file libraries/base/LICENSE)
Maintainer	[email protected]
Stability	experimental
Portability	portable
Safe Haskell	Trustworthy
Language	Haskell2010

The Dynamic type
Converting to and from Dynamic
Applying functions of dynamic type
Convenience re-exports

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.

The Dynamic type

data Dynamic where Source

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.

Constructors

Dynamic :: forall a. TypeRep a -> a -> Dynamic

Instances

Instances details

Show Dynamic	Since: base-2.1
Instance details Defined in Data.Dynamic Methods showsPrec :: Int -> Dynamic -> ShowS Source show :: Dynamic -> String Source showList :: [Dynamic] -> ShowS Source
Exception Dynamic	Since: base-4.0.0.0
Instance details Defined in Data.Dynamic Methods toException :: Dynamic -> SomeException Source fromException :: SomeException -> Maybe Dynamic Source displayException :: Dynamic -> String Source

Converting to and from Dynamic

toDyn :: Typeable a => a -> Dynamic Source

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)

fromDyn Source

Arguments

:: 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.

fromDynamic Source

Arguments

:: forall a. Typeable a
=> Dynamic	the dynamically-typed object
-> Maybe a	returns: `Just a`, if the dynamically-typed object has the correct type (and `a` is its value), or `Nothing` otherwise.