Discussion: add more idiomatic versions of catchJust and/or handleJust

[David Feuer]

You are very welcome to do so if you don't think further discussion will be
useful. Those who feel otherwise may continue to discuss it.

On Jul 15, 2016 9:23 AM, "Carter Schonwald" <carter.schonwald at gmail.com>
wrote:

> Again, at this juncture it sounds like effort would be better spent first
> writing out an alternative userland lib to understand these design ideas /
> articulate their impact better
>
> On Friday, July 15, 2016, David Thomas <davidleothomas at gmail.com> wrote:
>
>> I kinda like "ExceptionDisposition" for the type, no particular ideas
>> on the constructors.  Offered only in the spirit of tossing out ideas
>> - feel free to ignore me if it's not helpful :)
>>
>> On Thu, Jul 14, 2016 at 3:20 PM, Carter Schonwald
>> <carter.schonwald at gmail.com> wrote:
>> > At this point maybe it'd be better to write out these ideas as a
>> library and
>> > implement a bunch of example codes for different approaches.  Changing
>> the
>> > interfaces for exception handling code is subtle stuff!
>> >
>> >
>> > On Jul 14, 2016 5:42 PM, "Oliver Charles" <ollie at ocharles.org.uk>
>> wrote:
>> >>
>> >> data ExceptionHandler a = Rethrow | HandleException (IO a)
>> >>
>> >> would be my offering of paint.
>> >>
>> >> On Thu, Jul 14, 2016 at 9:52 PM David Feuer <david.feuer at gmail.com>
>> wrote:
>> >>>
>> >>> Handle is no good because it clashes with file handles. How about
>> >>> Reaction?
>> >>>
>> >>>
>> >>> On Jul 14, 2016 1:53 PM, "Andreas Abel" <andreas.abel at ifi.lmu.de>
>> wrote:
>> >>>>
>> >>>> Maybe rather
>> >>>>
>> >>>>   data Handle a = Rethrow | Handle (IO a)
>> >>>>
>> >>>> !?
>> >>>> On 14.07.2016 19:12, David Feuer wrote:
>> >>>>>
>> >>>>> That makes sense, I guess. Something like
>> >>>>>
>> >>>>> data Handle a = Rethrow | Catch (IO a)
>> >>>>>
>> >>>>> I suppose? My names are awful though.
>> >>>>>
>> >>>>>
>> >>>>> On Jul 14, 2016 12:56 PM, "Oliver Charles" <ollie at ocharles.org.uk
>> >>>>> <mailto:ollie at ocharles.org.uk>> wrote:
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>     On Tue, 12 Jul 2016, 1:23 a.m. David Feuer, <
>> david.feuer at gmail.com
>> >>>>>     <mailto:david.feuer at gmail.com>> wrote:
>> >>>>>
>> >>>>>         The catchJust and handleJust functions seem a bit weird and
>> >>>>>         unidiomatic.
>> >>>>>
>> >>>>>         catchJust
>> >>>>>                  :: Exception e
>> >>>>>                  => (e -> Maybe b)         -- ^ Predicate to select
>> >>>>>         exceptions
>> >>>>>                  -> IO a                   -- ^ Computation to run
>> >>>>>                  -> (b -> IO a)            -- ^ Handler
>> >>>>>                  -> IO a
>> >>>>>         catchJust p a handler = catch a handler'
>> >>>>>            where handler' e = case p e of
>> >>>>>                                  Nothing -> throwIO e
>> >>>>>                                  Just b  -> handler b
>> >>>>>
>> >>>>>         This takes two functions and then puts them together. I
>> would
>> >>>>>         think the more natural API would be
>> >>>>>
>> >>>>>         catchMaybe :: Exception e => IO a -> (e -> Maybe (IO a)) ->
>> IO
>> >>>>> a
>> >>>>>         catchMaybe m handler = catch m handler' where
>> >>>>>            handler' e = fromMaybe (throwIO e) (handler e)
>> >>>>>
>> >>>>>     A point don't feel super strongly about, but feel I should
>> raise,
>> >>>>> is
>> >>>>>     that Nothing seems to be somewhat confusing. catchJust
>> >>>>>     (\FileNotFound -> Nothing) seems to suggest that if FileNotFound
>> >>>>>     occurs then nothing will happen, that is - the exception is
>> >>>>> ignored.
>> >>>>>     However, that is not the case, rather than Nothing happening
>> >>>>>     something certainly happens - an exception is (re)thrown!
>> >>>>>
>> >>>>>     Whether or not this confusion is likely to happen in practice I
>> >>>>>     don't know, but it suggests a type isomorphic to Maybe is a
>> better
>> >>>>> fit.
>> >>>>>
>> >>>>>     Ollie
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>> _______________________________________________
>> >>>>> Libraries mailing list
>> >>>>> Libraries at haskell.org
>> >>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>> >>>>>
>> >>>>
>> >>>>
>> >>>> --
>> >>>> Andreas Abel  <><      Du bist der geliebte Mensch.
>> >>>>
>> >>>> Department of Computer Science and Engineering
>> >>>> Chalmers and Gothenburg University, Sweden
>> >>>>
>> >>>> andreas.abel at gu.se
>> >>>> http://www2.tcs.ifi.lmu.de/~abel/
>> >>
>> >>
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>> >>
>> >
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>>
>
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