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

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... +/- 0 on catchMaybe - Oleg

On 12 Jul 2016, at 03:23, David Feuer 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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