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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