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

[Brent Yorgey]

On Fri, Nov 19, 2010 at 03:37:10PM +0000, Magnus Therning wrote:
> 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. :-)

It depends what the "argument" is about.  I agree there is not a
strong argument for including such a function in the standard
libraries.  But it's a perfectly nice function to define and use in
one's own code.  Haskell makes this sort of abstraction very cheap.

-Brent