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

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" mailto:ollie@ocharles.org.uk> wrote:

On Tue, 12 Jul 2016, 1:23 a.m. David Feuer, mailto:david.feuer@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

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