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

[Brent Yorgey]

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.

-Brent