Re: [Haskell-cafe] Catch multiple exceptions using 'Control.Exception'
On 13-07-06 12:12 PM, Nikita Karetnikov wrote: Is there an open sum type or a closed existential type? OCaml has open sum types. I haven't really seen closed existential types. (I have seen some approximations, but each lacks one last bit.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Catch multiple exceptions using 'Control.Exception'
Here you go: import Control.Exception import Data.Typeable syncExceptions :: SomeException - Maybe SomeException syncExceptions e | Just _ - cast e :: Maybe AsyncException = Nothing | otherwise = Just e Thanks, but it doesn't work as expected: *Main syncExceptions $ toException StackOverflow Just stack overflow It should return 'Nothing' instead: *Main cast $ toException StackOverflow :: Maybe AsyncException Nothing I've received another solution, and it works fine. I also have other questions (see below). Start of forwarded message From: Albert Y. C. Lai tre...@vex.net [...] import Control.Exception syncExceptions :: SomeException - Maybe SomeException syncExceptions s = case fromException s :: Maybe AsyncException of Nothing - Just s Just _ - Nothing testsync = tryJust syncExceptions (throwIO DivideByZero) (is caught a Left) testasync = tryJust syncExceptions (throwIO StackOverflow) (is uncaught) The change from the old scheme to the new scheme is, conceptually, from a closed sum type to an open existential type (SomeException); programmatically, from pattern-matching to type-casing aka downcasting (using fromException). Therefore, changes to all call sites may be inevitable after all. For example: lr - tryJust syncExceptions action case lr of Right n - print n Left (IOException _) - putStrLn it's I/O Left (ArithException _) - putStrLn it's arith _ - putStrLn others must be changed to: lr - tryJust syncExceptions action case lr of Right n - print n Left s - case fromException s :: Maybe IOException of Just _ - putStrLn it's I/O Nothing - case fromException s :: Maybe ArithException of Just _ - putStrLn it's arith Nothing - putStrLn others at which point it is no longer clear that preserving syncExceptions implies minimum global change. It may be better off to use catches afterall. Or still use tryJust but every call site uses a tailored-made predicate for that site rather than thinking that one single syncExceptions fits all. End of forwarded message Could anyone elaborate on the following terms: a closed sum type and an open existential type? What is the meaning of the words open and closed? Is there an open sum type or a closed existential type? Also, I thought that a sum type [1] should only have two value constructors: data Add a b = AddL a | AddR b or data Either a b = Left a | Right b Is 'Exception' [2] a sum type? If so, is it because of the associative law? [1] http://chris-taylor.github.io/blog/2013/02/10/the-algebra-of-algebraic-data-types/ [2] http://hackage.haskell.org/packages/archive/base/4.1.0.0/doc/html/Control-OldException.html pgp3cyhQ1WilY.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Catch multiple exceptions using 'Control.Exception'
* Nikita Karetnikov nik...@karetnikov.org [2013-07-06 20:12:58+0400] Here you go: import Control.Exception import Data.Typeable syncExceptions :: SomeException - Maybe SomeException syncExceptions e | Just _ - cast e :: Maybe AsyncException = Nothing | otherwise = Just e Thanks, but it doesn't work as expected: *Main syncExceptions $ toException StackOverflow Just stack overflow It should return 'Nothing' instead: *Main cast $ toException StackOverflow :: Maybe AsyncException Nothing I've received another solution, and it works fine. I also have other questions (see below). Ah yes, fromException is the right function, of course. Sorry for confusion. Could anyone elaborate on the following terms: a closed sum type and an open existential type? What is the meaning of the words open and closed? Is there an open sum type or a closed existential type? Closed means that the set of possible constructors is, well, closed — you have to decide on it when you're defining your type and you can't later add anything to it. Open means the opposite. A classical example of an open type is any type in an OO language with subtyping, such as Java. You can extend the Object type without limitations, without modifying the Object type itself. Exceptions are very similar in this regard — you can add new classes of exceptions without a need to mess with the Control.Exception module, just by defining a couple of methods. For more details you may want to read An Extensible Dynamically-Typed Hierarchy of Exceptions. Also, I thought that a sum type [1] should only have two value constructors: ... [1] http://chris-taylor.github.io/blog/2013/02/10/the-algebra-of-algebraic-data-types/ That's just different terminology. When talking about Haskell, we usually call any algebraic type with multiple constructors a sum type. Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Catch multiple exceptions using 'Control.Exception'
I'm trying to update a package that uses 'Control.OldException' (works with GHC 6.10.4). Here is a relevant (and simplified) part: import Control.OldException -- | A predicate matching synchronous exceptions. syncExceptions :: Exception - Maybe Exception syncExceptions (AsyncException _) = Nothing syncExceptions e = Just e throwAsync :: IO a throwAsync = throwIO $ AsyncException StackOverflow throwArith :: IO a throwArith = throwIO $ ArithException DivideByZero 'syncExceptions' is usually used like this: *Main tryJust syncExceptions throwArith Left divide by zero *Main tryJust syncExceptions throwAsync -- pass through *** Exception: stack overflow The above doesn't work with GHC 7.6.3 because 'Control.OldException' [1] was removed. And 'Control.Exception' doesn't have the 'Exception' type. Is there a way to adapt 'syncExceptions' to work with 'Control.Exception'? [1] http://hackage.haskell.org/packages/archive/base/4.1.0.0/doc/html/Control-OldException.html pgpxks_kAP6YQ.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Catch multiple exceptions using 'Control.Exception'
Perhaps you can use `catches` [0]? Erik [0] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Exception.html#v:catches On Wed, Jul 3, 2013 at 12:47 PM, Nikita Karetnikov nik...@karetnikov.org wrote: I'm trying to update a package that uses 'Control.OldException' (works with GHC 6.10.4). Here is a relevant (and simplified) part: import Control.OldException -- | A predicate matching synchronous exceptions. syncExceptions :: Exception - Maybe Exception syncExceptions (AsyncException _) = Nothing syncExceptions e = Just e throwAsync :: IO a throwAsync = throwIO $ AsyncException StackOverflow throwArith :: IO a throwArith = throwIO $ ArithException DivideByZero 'syncExceptions' is usually used like this: *Main tryJust syncExceptions throwArith Left divide by zero *Main tryJust syncExceptions throwAsync -- pass through *** Exception: stack overflow The above doesn't work with GHC 7.6.3 because 'Control.OldException' [1] was removed. And 'Control.Exception' doesn't have the 'Exception' type. Is there a way to adapt 'syncExceptions' to work with 'Control.Exception'? [1] http://hackage.haskell.org/packages/archive/base/4.1.0.0/doc/html/Control-OldException.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Catch multiple exceptions using 'Control.Exception'
Perhaps you can use `catches` [0]? Maybe, but my idea is to replace 'syncExceptions' with a similar function. Otherwise, it'll be necessary to change (at least) all functions that use 'syncExceptions'. I'd like to avoid that. pgp3z2Qli9UKD.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Catch multiple exceptions using 'Control.Exception'
* Nikita Karetnikov nik...@karetnikov.org [2013-07-03 15:50:16+0400] Perhaps you can use `catches` [0]? Maybe, but my idea is to replace 'syncExceptions' with a similar function. Otherwise, it'll be necessary to change (at least) all functions that use 'syncExceptions'. I'd like to avoid that. Here you go: import Control.Exception import Data.Typeable syncExceptions :: SomeException - Maybe SomeException syncExceptions e | Just _ - cast e :: Maybe AsyncException = Nothing | otherwise = Just e Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
