On Sun, May 15, 2011 at 1:23 PM, Daniel Fischer
On Sunday 15 May 2011 18:53:32, Christopher Howard wrote:
I have one spot in some code where I just want to raise an exception that contains the message "Database is formatted incorrectly." Is there some kind of generic way to throw that (like, e.g., in Ruby you just call 'raise "exception message"'. In this case, I don't particularly want to take the time to think about creating an Exception type or finding a suitable one.
Well, the simple thing would be calling
error "Database is formatted incorrectly."
which raises an exception of type ErrorCall. If you don't know what type of exception you should throw, ErrorCall might fit.
(BTW: Is there a hierarchical list of standard Haskell Exception instances somewhere?)
There's no hierarchy, either a type is an instance of Exception or not, since there's no inheritance, there's no hierarchy (unless you count the universal Exception-wrapper SomeException, in which case we'd have a two- level hierarchy with that on top).
The haddock docs for the Exception class in Control.Exception [to be found locally, on hackage, or on the ghc page] list the known instances.
The documentation for Control.Exception lists an example of how to create an exception hierarchy: http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Exc... You can also create somehting like a hierarchy with ADTs:
data MyException = IOEx IOException | Arith ArithException | Assert AssertionFailed deriving (Show, Typeable)
instance Exception MyException where toException (IOEx e) = toException e toException (Arith e) = toException e toException (Assert e) = toException e
fromException (fromException -> Just e) = Just (IOEx e) fromException (fromException -> Just e) = Just (Arith e) fromException (fromException -> Just e) = Just (Assert e)
You can even catch something as an 'IOException' even if it was thrown as a 'MyException', or the other way around (it is the same with the method described in the above link). (Note that my example above uses ViewPatterns, a GHC extension. It can be done without, but I am lazy.) I'm not sure how useful the ADT approach is, but I think it is neat what can be done with the extensible exceptions. Antoine
_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners