[Haskell-cafe] Error handling package

Nicolas Pouillard nicolas.pouillard at gmail.com
Tue Oct 20 11:17:57 EDT 2009

On Tue, Oct 20, 2009 at 3:12 PM, Michael Snoyman <michael at snoyman.com> wrote:
> On Tue, Oct 20, 2009 at 11:04 AM, Nicolas Pouillard
> <nicolas.pouillard at gmail.com> wrote:
>> On Sun, Oct 18, 2009 at 9:45 PM, Michael Snoyman <michael at snoyman.com>
>> wrote:
>> > While working on the next release of data-object, I wanted to represent


>> About the name Attempt, I think that 'Outcome' would be a better name.
> The problem with a name like "Outcome" is that it doesn't really imply the
> possibility of failure, simply that we are wrapping of the result for some
> reason.

I don't see why, the outcome of an action could be either a success or
a failure.
Even if the only *desired* outcome is success.

>> > 2) Are there any other FromAttempt instances I should provide out of the
>> > box?
>> None that I see.
>> > 3) I was considering adding specialized versions of the fromAttempt
>> > function, ie ioFromAttempt, maybeFromAttempt. Thoughts?
>> It is a bit long to spell.
> True, but it can be more convenient than giving explicit type signatures.
> The main reason this might come up would be regarding the (Either String)
> and (Either SomeException) instances.
>> > 4) Should I follow the naming scheme attemptJoin, attemptLookup, etc, or
>> > just call the functions join, lookup and force people to import the
>> > module
>> > qualified?
>> A nice alternative would be to make a Data.Attempt.Extra (or a better
>> name) which
>> would be imported qualified.
> I like. The only other name I can think of right now is Data.Attempt.Helper,
> but that's just painting the bike shed.

I also prefer Helper.

>> > 5) Any other suggestions for attempt functions? I've considered
>> > head/tail/etc.
>> Why not, maybe the 'safe' package should be of some inspiration.
> Once they're all in their own module, I don't see a problem adding as many
> as possible.


>> attempt :: (forall e. Exception e -> b) -> (a -> b) -> Attempt a -> b
> I was trying to more model it after fromMaybe, but instead just ended up
> with something ugly. I'm going to rewrite attempt using this type signature
> and then add fromAttempt as well.

Fine, fromAttempt should follow fromMaybe

fromAttempt :: (forall e. Exception e -> a) -> Attempt a -> a
fromAttempt f a = attempt f id a


Nicolas Pouillard

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