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

[Oliver Charles]

On Tue, 12 Jul 2016, 1:23 a.m. 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)
>
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20160714/95a2b921/attachment.html>