[Haskell-beginners] Ignoring the result of a monadic computation

Magnus Therning magnus at therning.org
Fri Nov 19 10:37:10 EST 2010


On Fri, Nov 19, 2010 at 15:31, Brent Yorgey <byorgey at seas.upenn.edu> wrote:
> On Fri, Nov 19, 2010 at 03:26:04PM +0000, Magnus Therning wrote:
>> On Fri, Nov 19, 2010 at 15:21, Brent Yorgey <byorgey at seas.upenn.edu> wrote:
>> > On Fri, Nov 19, 2010 at 07:56:02AM +0100, Tim Baumgartner wrote:
>> >> Hi,
>> >>
>> >> while learning about monads, I had something like
>> >>
>> >> do
>> >>    line <- getLine
>> >>    something
>> >>    putStrLn line
>> >>
>> >> and I wondered if I could write it in one line, without naming of parameters.
>> >> I finally came up with
>> >>
>> >> getLine >>= ignore something >>= putStrLn
>> >>
>> >> using
>> >> ignore :: Monad m => m a -> b -> m b
>> >> ignore m a = m >> return a
>> >>
>> >> I'm satisfied with this solution but searching hoogle I didn't find
>> >> a standard function for my ignore. Am I missing something?
>> >
>> > Nope, there isn't such a function, but I like it.  It reminds me of
>> > (*>) and (<*) from Control.Applicative.  Note that you sometimes see
>> > the name 'ignore' used for a slightly different function, namely
>> >
>> >  ignore :: Monad m => m a -> m ()
>> >  ignore m = m >> return ()
>> >
>> > but yours is a bit more general.  Other names for your function might
>> > be 'passThrough' or something like that.
>>
>> I'm not sure I see any benefit of ': m a -> b -> m b' over 'm a -> m
>> ()'.  When would you want to use the former?
>
> >From the OP's message:
>
>  getLine >>= ignore something >>= putStrLn
>
> which executes 'something' for its side effect and passes the result
> of getLine through to putStrLn, without ever having to give a name to
> the result of getLine.  IIUC this was the whole point.

IMNSHO, not such a strong argument for such a function though. :-)

/M

-- 
Magnus Therning                        (OpenPGP: 0xAB4DFBA4)
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