There's no good reason that mapException isn't there. Would someone like to add it to Control.Exception, and document it? Something like mapException f v = unsafePerformIO (catch (evaluate v) (\x -> throwIO (f x))) The unsafePerformIO is, of course, safe in this case. Simon | -----Original Message----- | From: Lauri Alanko [mailto:la@iki.fi] | Sent: 16 November 2002 09:18 | To: libraries@haskell.org | Subject: mapException | | On Mon, Nov 04, 2002 at 01:00:39PM -0000, Simon Marlow wrote: | > and the awkward squad paper will have to be updated to comply (although | > we just noticed it is already wrong about the behaviour of | > Prelude.catch). | | This reminds me: in the paper "A semantics for imprecise exceptions" a | primitive "mapException" is introduced: | | mapException :: (Exception -> Exception) -> a -> a | | However, I don't think GHC's exception library has ever had this. Is | this just a lapse, or is there some fundamental problem with a non-IO | mapException? The paper says that "its usefulness and desirability might | be debatable", but to me this seems very useful. Eg. in Java this is a | common idiom: | | try { | aux.library.Thing.doSomething(); | } catch (aux.library.ThingException e) { | throw new MyException(e.getProblem()); | } | | That is, one module uses another module internally to do a task, but of | course wishes to hide any implementation details from the caller. | Therefore we don't want to show the auxiliary library's exception | interface to the caller, but convert exceptions to match the publicly | defined exception interface. | | It'd be very inconvenient if doing this in Haskell would require making | a formerly pure function use the IO monad. | | | Lauri Alanko | la@iki.fi | | _______________________________________________ | Libraries mailing list | Libraries@haskell.org | http://www.haskell.org/mailman/listinfo/libraries