Re: [Haskell-cafe] Absolutely confused with error/exception handling

Hi Saurabh, I've found that the `safe-exceptions` package [1] and related articles [2] to be some of the best reading around exceptions in Haskell. To answer your third question, the `IO` instance for `MonadThrow`/`MonadCatch`/etc. do use Haskell's extensible exception machinery. The monad-ste [3] package uses Haskell's exception hierarchy in a controlled manner where evaluating `Either` ends up being expensive. I've also found exceptions to be one of the hardest parts about Haskell. Interestingly, they're one of the few dynamically typed bits with nearly OO-style inheritance. [1]: https://hackage.haskell.org/package/safe-exceptions [2]: https://haskell-lang.org/tutorial/exception-safety [3]: https://hackage.haskell.org/package/monad-ste Matt Parsons On Thu, Dec 15, 2016 at 10:30 PM, Saurabh Nanda wrote:

Thanks. The air is clearing up a bit.

Any reason for "error" and "catch" to be in different packages? Or, why isn't "catch" in prelude?

"catch" needs a action in IO. What if I have an action in MonadIO?

-- Saurabh.

On 16 Dec 2016 8:00 am, "Brandon Allbery" wrote:

...

whoops, pressed wrong key...

No guarantees are made as to when the exception thrown by `throw` occurs; this allows it to be thrown from pure code. `throwIO` is sequenced as any other I/O operation, so you have some guarantee as to I/O operations before the `throwIO` being performed.

On Thu, Dec 15, 2016 at 9:28 PM, Brandon Allbery wrote:

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On Thu, Dec 15, 2016 at 8:42 PM, Saurabh Nanda wrote:

...

* What exactly is done by the "error" function? How does one "trap" those errors and react to it? Can "catch" trap those errors? What is "e" in the case of errors raised by "error"?

`error` specifically raises a UserError exception, with a user-specified payload (message). `catch` can catch them as such.

...

* How is "error" different from "throw" and "throwIO" (in the Control.Exception package, I believe)

`throw` (for pure code / asynchronous) and `throwIO` (for code in IO) can throw any exception. No guarantees are made as to when the exception thrown by `throw`

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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