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

[Oleg Grenrus]

Maybe pointing out the obvious, but when used with lens, catchJust approach is handy:

— | here catchJust is from `exceptions` though
catching :: MonadCatch m => Getting (First a) SomeException a -> m r -> (a -> m r) -> m r
catching l = catchJust (preview l)

http://hackage.haskell.org/package/lens-4.14/docs/src/Control.Exception.Lens.html#catching

+/- 0 on catchMaybe

- Oleg

> On 12 Jul 2016, at 03:23, David Feuer <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)
> 
> This is exactly as powerful as catchJust:
> 
> catchMaybe m handler = catchJust handler m id
> catchJust p m handler = catchMaybe m $ fmap handler . p
> 
> But catchMaybe doesn't enforce the arbitrary separation between "selection" and "handling".
> 
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