Network.firstSuccesful: 'throw' vs 'throwIO' usage

[Roman Cheplyaka]

* Simon Marlow <marlowsd at gmail.com> [2012-09-06 12:35:52+0100]
> On 06/09/2012 11:05, Roman Cheplyaka wrote:
> >* Herbert Valerio Riedel <hvr at gnu.org> [2012-09-06 11:40:23+0200]
> >>Hello,
> >>
> >>while reading over the source code of network[1], I noticed a use of 'throw' where I'd
> >>expect 'throwIO':
> >>
> >>     import qualified Control.Exception as Exception
> >>
> >>     catchIO :: IO a -> (Exception.IOException -> IO a) -> IO a
> >>     catchIO = Exception.catch
> >>
> >>     -- Returns the first action from a list which does not throw an exception.
> >>     -- If all the actions throw exceptions (and the list of actions is not empty),
> >>     -- the last exception is thrown.
> >>     firstSuccessful :: [IO a] -> IO a
> >>     firstSuccessful [] = error "firstSuccessful: empty list"
> >>     firstSuccessful (p:ps) = catchIO p $ \e ->
> >>         case ps of
> >>             [] -> Exception.throw e
> >>             _  -> firstSuccessful ps
> >>
> >>
> >>
> >>...so, is `throw` used properly in the code above, or should it rather
> >>be `throwIO`?
> >>
> >>
> >>  [1]: http://hackage.haskell.org/packages/archive/network/2.3.1.0/doc/html/src/Network.html#firstSuccessful
> >
> >In this particular situation it doesn't matter.
> >
> >If you use throwIO, then, if all actions fail, firstSuccesful will
> >return a proper IO action which, when sequenced, throws an exception.
> >
> >If you use throw, then in the same situation the result of
> >firstSuccessful will throw an exception before yielding a proper IO
> >value.
> >
> >However, I agree with you that throwIO would be somewhat more idiomatic
> >here. (And IIRC I wrote this code, so you can blame me.)
> 
> Here is some background reading:
> 
> http://hackage.haskell.org/trac/ghc/ticket/1171
> 
> The bottom line is that it's hard to tell what will happen if you use
> throw here.  Always use throwIO if you can.

So, regarding this example, does it mean that, under some circumstances,
`firstSuccessful [a]` can throw `error "firstSuccessful: empty list"`?

Would you also advise changing `error` to `throwIO . ErrorCall` here?

-- 
Roman I. Cheplyaka :: http://ro-che.info/