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{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Dynamic
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : portable
--
-- 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.
--
-----------------------------------------------------------------------------
module Data.Dynamic
(
-- Module Data.Typeable re-exported for convenience
module Data.Typeable,
-- * The @Dynamic@ type
Dynamic, -- abstract, instance of: Show, Typeable
-- * Converting to and from @Dynamic@
toDyn,
fromDyn,
fromDynamic,
-- * Applying functions of dynamic type
dynApply,
dynApp,
dynTypeRep
) where
import Data.Typeable
import Data.Maybe
import Unsafe.Coerce
import GHC.Base
import GHC.Show
import GHC.Exception
-------------------------------------------------------------
--
-- The type 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').
'Show'ing a value of type 'Dynamic' returns a pretty-printed representation
of the object\'s type; useful for debugging.
-}
data Dynamic = Dynamic TypeRep Obj
deriving Typeable
instance Show Dynamic where
-- the instance just prints the type representation.
showsPrec _ (Dynamic t _) =
showString "<<" .
showsPrec 0 t .
showString ">>"
-- here so that it isn't an orphan:
instance Exception Dynamic
type Obj = Any
-- Use GHC's primitive 'Any' type to hold the dynamically typed value.
--
-- In GHC's new eval/apply execution model this type must not look
-- like a data type. If it did, GHC would use the constructor convention
-- when evaluating it, and this will go wrong if the object is really a
-- function. Using Any forces GHC to use
-- a fallback convention for evaluating it that works for all types.
-- | 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)
--
toDyn :: Typeable a => a -> Dynamic
toDyn v = Dynamic (typeOf v) (unsafeCoerce v)
-- | Converts a 'Dynamic' object back into an ordinary Haskell value of
-- the correct type. See also 'fromDynamic'.
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.
fromDyn (Dynamic t v) def
| typeOf def == t = unsafeCoerce v
| otherwise = def
-- | Converts a 'Dynamic' object back into an ordinary Haskell value of
-- the correct type. See also 'fromDyn'.
fromDynamic
:: 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.
fromDynamic (Dynamic t v) =
case unsafeCoerce v of
r | t == typeOf r -> Just r
| otherwise -> Nothing
-- (f::(a->b)) `dynApply` (x::a) = (f a)::b
dynApply :: Dynamic -> Dynamic -> Maybe Dynamic
dynApply (Dynamic t1 f) (Dynamic t2 x) =
case funResultTy t1 t2 of
Just t3 -> Just (Dynamic t3 ((unsafeCoerce f) x))
Nothing -> Nothing
dynApp :: Dynamic -> Dynamic -> Dynamic
dynApp f x = case dynApply f x of
Just r -> r
Nothing -> error ("Type error in dynamic application.\n" ++
"Can't apply function " ++ show f ++
" to argument " ++ show x)
dynTypeRep :: Dynamic -> TypeRep
dynTypeRep (Dynamic tr _) = tr