Join two IOs with - in haskell

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I need to join two IO Strings with a - in between. Here's what I came up with, which works - what's the right way?

import System.Environment
f :: String -> String -> IO String
f x y = (foldl1 (++)) <$> sequence [(getEnv x),(return "-"),(getEnv y)]

You could here use an applicative style function:

f :: String -> String -> IO String
f x y = withHyp <$> getEnv x <*> getEnv y
    where withHyp ex ey = ex ++ '-' : ey

So here we join the two Strings that are then joined with a hypen in the middle through the withHyp function.

Or for a list of environment variables that we need to fetch, we can use mapM and perform an intercalate:

import Data.List(intercalate)

f :: [String] -> IO String
f xs = intercalate "-" <$> mapM getEnv xs

How to combine two strings i Haskell and return a new string, combine = zipWith (++). zipWith takes two lists, and applies the function given to the first item of both lists, then the second, etc. If one list is longer than the other,  Actions are inert until run. Only one IO action in a Haskell program is run (main). Do-blocks combine multiple actions together into a single action. Combined IO actions are executed sequentially with observable side-effects. Arrows are used to bind action results in a do-block. Return is a function that builds actions. It is not a form of

I'll be honest, the idea behind your approach actually looks pretty sane to me. To start with, I'd probably use concat intsead of foldl1 (++), and drop some parens, getting us to:

f x y = concat <$> sequence [getEnv x, return "-", getEnv y]

This really doesn't seem that bad to me. But if I really wanted to push farther, here's some thoughts I would have. First, I'd recall the intercalate function.

f x y = intercalate "-" <$> sequence [getEnv x, getEnv y]

There's a handy shorthand for applying a function to each element of a list, too; mapM f = sequence . map f. So:

f x y = intercalate "-" <$> mapM getEnv [x,y]

I would stop there; it looks quite clean and maintainable to me.

Merging streams of IO actions? : haskell, When you have multiple producers and want to consume them in a single consumer you usually do Haskell Game Enpuzzled Released for Android and iOS. haskell documentation: Merge Sort. Example. Ordered merging of two ordered lists. Preserving the duplicates: merge :: Ord a => [a] -> [a] -> [a] merge xs [] = xs

One way of joining two IO Strings would be:

dash :: IO String -> IO String -> IO String
dash x y = do
    s1 <- x
    s2 <- y
    return $ s1 <> "-" <> s2

We "unbox" each of x and y to get the contained Strings, then "rebox` them with a hyphen (using the analogy for Functors).

It can be shortened to:

dash = liftA2 (\s1 s2 -> s1 <> "-" <> s2)

Where liftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c takes a binary function and "lifts" it into a binary function on Applicatives, which are a superset of Monads.

Your f can then be implemented as f x y = dash (getEnv x) (getEnv y).

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  • Are you sure that works the way you want? Looks essentially equivalent to f x y = getEnv x (up to some nonsense with yours throwing extra exceptions, perhaps) to me.
  • no it doesn't. fixed.
  • even uglier now!
  • Just a note for OP about concat: foldl1 (++) for a 3 item list is evaluated as (a ++ b) ++ c, which is slightly slower than a ++ (b ++ c) due to traversing a twice. So concat is actually implemented as something more like foldr (++) [] (give or take some differences in syntax)
  • change signature to match String -> String -> IO String please