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

[Simon Marlow]

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.

Cheers,
	Simon