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

[Magnus Therning]

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?

/M

-- 
Magnus Therning                        (OpenPGP: 0xAB4DFBA4)
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