Extras.hs

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--Module       : Data.Typeable.Extras
--Author       : Daniel Schüssler
--License      : BSD3
--Copyright    : Daniel Schüssler
--
--Maintainer   : Daniel Schüssler
--Stability    : Experimental
--Portability  : Uses various GHC extensions
--
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--Description  : 
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module Data.Typeable.Extras where

import Control.Monad
import Data.Typeable
import System.IO.Unsafe -- this is just for the missing Ord instance for Data.Typeable.TypeRep 
import Data.Monoid
import Data.Function
import Control.Exception

dynEq :: (Typeable a, Typeable b, Eq b) => a -> b -> Bool
dynEq a b = case cast a of
              Just b' -> b' == b
              Nothing -> False

compareTypeReps x y = -- compare the string representations first
                  ((compare `on` show) x y)
                  `mappend`
                  -- not sure why 'typeRepKey' is in IO
                  (unsafePerformIO (liftM2 compare 
                                              (typeRepKey x) 
                                              (typeRepKey y)))

dynCompare :: (Typeable b, Typeable a, Ord b) => a -> b -> Ordering
dynCompare a b = case cast a of
                   Just b' -> compare b' b
                   Nothing -> let r = compareTypeReps (typeOf a) (typeOf b)
                             in assert (r/=EQ) r