Data/Dynamic.hs

    {-# LANGUAGE GADTs #-}
    {-# LANGUAGE NoImplicitPrelude #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE TypeApplications #-}

    -----------------------------------------------------------------------------
    -- |
    -- 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
      (

            -- * The @Dynamic@ type
            Dynamic(..),

            -- * Converting to and from @Dynamic@
            toDyn,
            fromDyn,
            fromDynamic,

            -- * Applying functions of dynamic type
            dynApply,
            dynApp,
            dynTypeRep,

            -- * Convenience re-exports
            Typeable

      ) where

    import Data.Type.Equality
    import Type.Reflection
    import Data.Maybe

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

    -- | @since 2.01
    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:
    -- | @since 4.0.0.0
    instance Exception Dynamic

     -- 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 typeRep 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
      | Just HRefl <- t `eqTypeRep` typeOf def = v
      | otherwise                              = def

    -- | Converts a 'Dynamic' object back into an ordinary Haskell value of
    -- the correct type.  See also 'fromDyn'.
    fromDynamic
            :: 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.
    fromDynamic (Dynamic t v)
      | Just HRefl <- t `eqTypeRep` rep = Just v
      | otherwise                       = Nothing
      where rep = typeRep :: TypeRep a

    -- (f::(a->b)) `dynApply` (x::a) = (f a)::b
    dynApply :: Dynamic -> Dynamic -> Maybe Dynamic
    dynApply (Dynamic (Fun ta tr) f) (Dynamic ta' x)
      | Just HRefl <- ta `eqTypeRep` ta'
      , Just HRefl <- typeRep @Type `eqTypeRep` typeRepKind tr
      = Just (Dynamic tr (f x))
    dynApply _ _
      = Nothing

    dynApp :: Dynamic -> Dynamic -> Dynamic
    dynApp f x = case dynApply f x of
                 Just r -> r
                 Nothing -> errorWithoutStackTrace ("Type error in dynamic application.\n" ++
                                   "Can't apply function " ++ show f ++
                                   " to argument " ++ show x)

    dynTypeRep :: Dynamic -> SomeTypeRep
    dynTypeRep (Dynamic tr _) = SomeTypeRep tr