[Haskell-cafe] Error handling package

[Michael Snoyman]

On Tue, Oct 20, 2009 at 5:17 PM, Nicolas Pouillard <
nicolas.pouillard at gmail.com> wrote:

> 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.
>

I think I'm going to stick with Attempt. The only other point that came to
me is it's nicer to have a function called "attempt" than "outcome" IMO.

>
> >> > 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.
>

Done. Not full-featured yet, but does provide a nice core.

>
> >> > 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
>

And of course I now realize this name will conflict with the FromAttempt
typeclass ;). I think we'll be safe leaving this function out, considering
how trivial it is to implement in terms of attempt.

Also, I've added a monad transformer to the github repo. I've tried it out
in some local code I have, and it *really* cleans up the error handling.

Michael
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