@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:fo...
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#...
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
This works:
main = do myargs <- getArgs mapM_ (\s -> putStrLn s ) myargs
and imagine that the "body" will be substantial rather than just a
On Sat, Apr 12, 2014 at 08:29:09AM -0500, John M. Dlugosz wrote: putStrLn.
My gut instinct is that the code ought to be arranged as:
<any needed keywords or punctuation> and <the collection of items>
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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