[project @ 2002-11-22 10:52:23 by stolz]

Add 'mapException' as proposed in "A semantics for imprecise exceptions"

Reminded by: Lauri Alanko

diff --git a/Control/Exception.hs b/Control/Exception.hs
index 5b653e2..b0a76b8 100644 (file)
--- a/Control/Exception.hs
+++ b/Control/Exception.hs
@@ -54,6 +54,9 @@ module Control.Exception (
        -- ** The @evaluate@ function
        evaluate,  -- :: a -> IO a
 
+       -- ** The @mapException@ function
+       mapException,           -- :: (Exception -> Exception) -> a -> a
+
        -- ** Exception predicates
        
        -- $preds
@@ -123,6 +126,7 @@ import Hugs.Exception       as ExceptionBase
 
 import Prelude                 hiding ( catch )
 import System.IO.Error
+import System.IO.Unsafe (unsafePerformIO)
 import Data.Dynamic
 
 #include "Dynamic.h"
@@ -244,6 +248,18 @@ evaluate a = a `seq` return a      -- dummy implementation: to be fixed
 #endif
 
 -----------------------------------------------------------------------------
+-- 'mapException'
+
+-- | This function maps one exception into another as proposed in the
+-- paper "A semantics for imprecise exceptions".
+
+-- Notice that the usage of 'unsafePerformIO' is safe here.
+
+mapException :: (Exception -> Exception) -> a -> a
+mapException f v = unsafePerformIO (catch (evaluate v)
+                                          (\x -> throw (f x)))
+
+-----------------------------------------------------------------------------
 -- 'try' and variations.
 
 -- | Similar to 'catch', but returns an 'Either' result which is