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

On Fri, Nov 19, 2010 at 15:31, Brent Yorgey wrote:

On Fri, Nov 19, 2010 at 03:26:04PM +0000, Magnus Therning wrote:

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On Fri, Nov 19, 2010 at 15:21, Brent Yorgey wrote:

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On Fri, Nov 19, 2010 at 07:56:02AM +0100, Tim Baumgartner wrote:

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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?

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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) magnus＠therning．org          Jabber: magnus＠therning．org http://therning.org/magnus         identi.ca|twitter: magthe