Re: [Haskell-cafe] Correct way to "catch all exceptions"

I think 'shouldBeCaught' is more often than not the wrong thing. A whitelist of exceptions you're prepared to handle makes much more sense than excluding certain operations. Some common whitelists, e.g. filesystem exceptions or network exceptions, might be useful to have. I like Ertugrul's suggestion a lot, however it seems a bit more invasive. If we want to do that work, we could also restructure the wired-in exceptions so they're more hierarchical. There are some top-level exceptions we can get by type, such as ArithException, but sadly many interesting and useful exceptions are lumped together under IOException. On Wed, Jul 10, 2013 at 4:29 PM, Ertugrul Söylemez wrote:

Michael Snoyman wrote:

...

shouldBeCaught :: SomeException -> Bool

One first stab at such a function would be to return `False` for AsyncException and Timeout, and `True` for everything else, but I'm not convinced that this is sufficient. Are there any thoughts on the right approach to take here?

I think there is no one right approach. However, if you add such a function to the exception library, it really belongs into the Exception type class with the following type:

shouldBeCaught :: (Exception e) => e -> Bool

However, a better approach is to have exception tags. In most cases you don't want to catch killThread's or timeout's exception, but you do want to catch all error exceptions:

data Tag = Error | Abort | TryAgain | {- ... -} | Other String deriving (Data, Eq, Ord, Read, Show, Typeable)

instance IsString Tag where fromString t = Other t

This could then manifest in the following two functions in the Exception type class:

hasTag :: (Exception e) => Tag -> e -> Bool tagsOf :: (Exception e) => e -> [Tag]

Then exception catchers (functions that risk swallowing important exceptions) could filter by type and tag.

Greets, Ertugrul

-- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad.

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