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

On 19/11/10 19:20, Brent Yorgey wrote:

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

...

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

...

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

...

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

I can agree with that. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe