@John

It looks like you are looking for `forM_` from Control.Monad:
http://hackage.haskell.org/package/base-4.7.0.0/docs/Control-Monad.html#v:forM_

That way you can write code that looks like:

main = do
    myargs <- getArgs
    forM_ myargs $ \s -> do
        putStrLn s
        putStrLn $ "Second string" ++ s

This is just `mapM_` with the its two parameters fliped. `forM_` is defined in a an idiomatic fashion, with the `flip`(prelude function) function.

Definition of forM_
http://hackage.haskell.org/package/base-4.7.0.0/docs/src/Control-Monad.html#forM_

Magnus solution has been used so often by some people that some have created idioms around it.

om f m x = m >>= flip f x 


main = do
    om forM_ getArgs $ \s -> do
        putStrLn s
        putStrLn $ "Second string: " ++ s

You can read the discussion around om:
http://www.reddit.com/r/haskell/comments/1i2zmq/a_useful_function_om/

Hope that helps.

Patrick


On Sun, Apr 13, 2014 at 1:48 AM, Magnus Therning <magnus@therning.org> wrote:
On Sat, Apr 12, 2014 at 08:29:09AM -0500, John M. Dlugosz wrote:
> This works:
>
> main = do
>     myargs <- getArgs
>     mapM_ (\s -> putStrLn s ) myargs
>
> and imagine that the "body" will be substantial rather than just a putStrLn.
> My gut instinct is that the code ought to be arranged as:
>
>       <any needed keywords or punctuation> and <the collection of items>
>       <body to perform for every element
>               ...
>               ...
>               >
>
> Meanwhile, there is no need to name the result of getArgs into myargs.
>
> So, getArgs is of type IO [String], and I want to apply that in the
> manner of a list. Without the Monad wrappers, plain
>       map ( blah ) strings
> could be ( blah ) <$> strings, and in this particular case I don't
> see a reversed-arg version, although there is one for <*> (as <**>).
> But, for monad stuff in general there are reversed arrows for
> (most?) everything, and that's where I'm heading.
>
> So the first question is, how do I do the equivalent
> map-as-nondeterministic-apply when the strings is further wrapped in
> IO, as is the function being applied.
>
>       getArgs >>= mapM_ (\s -> putStrLn s )
>
> does double-duty of moving the argument from last place to the left,
> as it makes use of eta reduction.  Because I have two things going
> on (list applicative and IO monad) I'm losing the slickness of using
> applicative syntax.  Is there a nice way to make these work
> together?
>
> And more generally, how would you write such a construct?  I'm
> naturally biased with my knowledge in other languages, so maybe
> there's a completely different "normal" way of approaching this?

I'm not entirely sure I get what you are asking for, but I'll take a
stab and just let me know if I'm completely off the mark.

If all you want is keep the 'string generator' on the right-hand side,
then you have (=<<):

    mapM_ putStrLn =<< getArgs

Personally I often like keeping the 'string generator' on the left
(i.e. using (>>=) because when the expression to be mapped grows it
allows this structuring of the code:

    getArgs >>= mapM_ $ \ s -> do
        ...

/M

--
Magnus Therning                      OpenPGP: 0xAB4DFBA4
email: magnus@therning.org   jabber: magnus@therning.org
twitter: magthe               http://therning.org/magnus

Perl is another example of filling a tiny, short-term need, and then
being a real problem in the longer term.
     -- Alan Kay

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Patrick Wheeler
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