On Sat, May 23, 2009 at 9:09 AM, Ian Lynagh wrote:

On Sat, Apr 25, 2009 at 11:18:43AM -0700, Alexander Dunlap wrote:

...

In the extensible exceptions paper[1], which I believe is the guide behind the current Control.Exception in GHC 6.10, a SomeIOException type is discussed so that it would be possible to use the nice interface for IOExceptions. Is this implemented anywhere? In the GHC libraries, all I see is the old interface just plugged into a simple wrapper, so you have to use guard . isDoesNotExistError (for example) with catchJust to catch a certain type of exception. If not, are there any plans to implement it?

I'm not aware of anyone currently working on putting the various core libs' exceptions into a sensible hierarchy, but it would be great if someone was to do so!

I'd suggest that the best way to do this would be initially a combination of discussions on the mailing list (if anything is non-obvious) and building a design (and, if appropriate, rationale) on a wiki page, and ending with a library submissions proposal once you have a design worked out:

http://www.haskell.org/haskellwiki/Library_submissions

Thanks Ian

I'm moving this from the GHC users' mailing list to the haskell-cafe mailing list because I think the discussion probably involves the entire Haskell community. Hi all, We have this new exception hierarchy in GHC base-4 and a lot of new machinery for dealing with it. Other libraries also seem to be springing up to provide alternative methods of dealing with these new exceptions. However, I have found that in most of my programming, the new exception functionality cannot be used to its fullest extent because the exceptions in the core libraries are not part of any sensible "tree" of exceptions, even though having this giant exception tree seems to be one of the premises of the new exception library. For example, there is an ArithException type in Control.Exception which records various arithmetic exceptions. However, there are two problems: (1) in order to fit in with the extensible exceptions theme, the type ought to be called SomeArithException and all of the types of ArithExceptions ought to be sub-exceptions of it, and (2) to my knowledge, this exception type doesn't seem to be thrown by anything in the base libraries. I think the biggest culprit is the IOException type, which also happens to be probably the most commonly-used exception type, since exceptions from IOException cannot really be anticipated and prevented. The current design of IOException is as follows. There is an abstract type called IOException. This can be queried with functions like isDoesNotExistError (which determines if the exception is because a file did not exist). Furthermore, to make an IOException, one has to use the mkIOError function, which takes a value of type IOErrorType, which is again an abstract type. I believe the reason for the abstract types is to allow implementations to extend the range of exceptions. Unfortunately, this has even more problems. For example, the error returned by using readFile on a directory cannot be determined using any Haskell98 function, meaning that code that wants to detect this error cannot be portable to compilers that don't have the new InappropriateType IO exception. The biggest problem with the current design of IOException, however, is that it doesn't play very nicely with the main hierarchy. Like with ArithExceptions, there should be a SomeIOException type. http://www.haskell.org/~simonmar/papers/ext-exceptions.pdf, which was the original paper on extensible exceptions, recommends having a class for IOExceptions that would allow us to get standard information from any specific IOException. I asked the GHC-users mailing list about this and Ian Lynagh recommended starting a discussion about how to put all of the core library exceptions into a sensible hierarchy. I think the goals here ought to be to make our new hierarchy as backward-compatible as possible. In particular, since many programs rely on the isWhateverError functionality, it would be nice to keep these in place. For IOExceptions, I think it would be nice to have a more multi-layered hierarchy than just SomeIOException and its subtypes. For example, I could see a hierarchy for IOExceptions looking something like this: SomeIOException - SomeCouldNotOpenException - AlreadyExists, NoSuchThing, PermissionDenied, InappropriateType - SomeResourceException - ResourceBusy, ResourceExhausted, ResourceVanished etc. I'm not all that familiar with the various exceptions and how they are used, so I'm not really able to propose a complete hierarchy for all the exceptions. I guess the purpose of this email was just to start off a discussion about what our exception hierarchy ought to look like. Thanks for reading this (admittedly long) message. Alex