[Haskell-beginners] general structuring of "foreach" logic in IO
John M. Dlugosz
ngnr63q02 at sneakemail.com
Sat Apr 12 13:29:09 UTC 2014
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
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
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