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

Am I right in thinking that `catch` is equivalent to `catchJust fromException`? Perhaps the intention is that if you have an implementation of `fromException` but haven't defined an instance of `Exception` for whatever reason, you can use `catchJust` directly? Or if you have a slight variant of `fromException` that isn't quite worth its own typeclass instance? Analogously to `sortBy` vs. `sort`, for example. I have no particular vote on the proposal and do not intend the above as a defence of `catchJust`, just a possible perspective on it. On Wed, Jul 13, 2016 at 02:23:04PM -0700, Theodore Lief Gannon wrote:

+1 on catchMaybe from this corner of the peanut gallery, FWIW. It feels far more idiomatic, and provides the same power with fewer moving parts. On Jul 13, 2016 1:14 PM, "David Feuer" wrote:

...

I hate having to make arbitrary choices when writing code. With `catchJust`, I have to decide what to calculate in the selector and what to calculate in the handler. As far as I can tell, there's never any reason to leave any calculation for the handler.

I don't think the `Maybe (IO a)` type is nearly as hard to think about as exceptions themselves are. The handler either provides a recovery action or it doesn't. The catchMaybe signature strikes me, personally, as easier to understand, because I don't need to use parametricity to string the pieces together.

On Wed, Jul 13, 2016 at 3:36 PM, Andreas Abel wrote:

...

I can only guess why catchJust was designed like it is. A type like

b -> Maybe (IO a)

is not as intuitive as the types

e -> Maybe b -- ^ if you do not understand this, get back to Haskell school!

b -> IO a -- ^ a continuation, we know this from >>= and friends

A type like Maybe (IO a) is more unusual, requires more thinking.

+-0. I have no opinion on what is better.

On 12.07.2016 02:23, 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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-- Andreas Abel <>< Du bist der geliebte Mensch.

Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden

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