Like Josh mentioned, Applicative Functors are what you want. There are two idiomatic ways to do it:

• You can just use liftA2, which has type (a -> b -> c) -> f a -> f b -> f c. That means it lifts a binary function to some applicative functor like maybe, so liftA2 (++) :: Maybe String -> Maybe String -> Maybe String
  
• In general, for any arity f that you want to lift to an applicative functor. So you have a function g that takes a bunch of arguments of types a, b, c, ... and gives you back an r and you want to get a function that takes an f a, f b, f c, ... and gives you back an f r, you can write f <$> a <*> b <*> c ... This works because <$> let's you apply g to type f a and gives you an f (b -> c ..) -> f r, and <*> basically let's you take the function back out of the f and apply it to the b to get a f (c ..) -> f r and so on. tl;dr you can write (++) <$> s1 <*> s2. In fact, liftA2 must satisfy the equation liftA2 f x y = f <$> x <*> y so these are the same thing.

בתאריך יום ב׳, 7 בינו׳ 2019, 17:41, מאת ☂Josh Chia (謝任中) <joshchia@gmail.com>:

Firstly, because "resBDwords :: Maybe String", not "resBDwords :: String", "lgBDwords = length resBDwords" probably is not what you want -- it does not give you the number of words in the String that may be in there.

Second, for the problem you asked about, you could just use a function that takes a String and do it "the hard way" like you said, using case outside before calling the function. Another way is to use an applicative functor to allow you to have a "Maybe String -> Maybe String -> Maybe String". This is used once for each "++" that you want to do.

I don't know exactly what you need to accomplish but I would just write a function "f :: String -> Maybe String" implementing the logic you listed in the second code snippet but operating on String instead of "Maybe String" and do "join . fmap f $ resBDwords".

On Tue, Jan 8, 2019 at 12:13 AM Damien Mattei <mattei@oca.eu> wrote:

hello,

i have a variable resBDwords of type ( i expect) Maybe [String], for
info it is integer and fractional part of a number

example looks like this :
resBDwords =Just ["-04","3982"]

i want to concatanate "-04" and "3982" in the example, i begin to
understand fmap to use the functor hidden in the Maybe ,it worked
previously:

let resBDstr = fmap Tx.unpack resBDtxt
    putStr "resBDstr ="
    putStrLn (show resBDtxt)

    let resBDwords = fmap words resBDstr
    putStr "resBDwords ="
    putStrLn (show resBDwords)

which gives:

resBDtxt ="-04 3982"
resBDstr =Just "-04 3982"


just after in my code i have this to concatanate the two strings f and s
that are the first and second element of the array:


putStr "resBDwords ="
    putStrLn (show resBDwords)

    let lgBDwords = length resBDwords

    let resBDstrFloat = if lgBDwords == 0
                           then trace "WARNING: BD contains no words"
Nothing
                           else
                               if lgBDwords == 1
                                  then trace "WARNING: BD contains only
one word" fmap head resBDwords
                                  else let f = fmap head resBDwords
                                           s = fmap (head . tail) resBDwords
                                       in f ++ "." ++ S

but i do not know how to concatanate the Maybe String in an elegant way,
using somethin like fmap variable which have handled Nothing (from
Maybe) automatically i need the counter part for multipe variable

i do not want to do it using the hard way with case... of Just x ->
nothing .........

i got this error :
*Main> :load UpdateSidonie
[1 of 1] Compiling Main             ( UpdateSidonie.hs, interpreted )

UpdateSidonie.hs:339:43: error:
    • Couldn't match expected type ‘[Char]’
                  with actual type ‘Maybe String’
    • In the first argument of ‘(++)’, namely ‘f’
      In the expression: f ++ "." ++ s
      In the expression:
        let
          f = fmap head resBDwords
          s = fmap (head . tail) resBDwords
        in f ++ "." ++ s
    |
339 |                                        in f ++ "." ++ s
    |                                           ^

UpdateSidonie.hs:339:43: error:
    • Couldn't match expected type ‘Maybe String’
                  with actual type ‘[Char]’
    • In the expression: f ++ "." ++ s
      In the expression:
        let
          f = fmap head resBDwords
          s = fmap (head . tail) resBDwords
        in f ++ "." ++ s
      In the expression:
        if lgBDwords == 1 then
            trace "WARNING: BD contains only one word" fmap head resBDwords
        else
            let
              f = fmap head resBDwords
              s = fmap (head . tail) resBDwords
            in f ++ "." ++ s
    |
339 |                                        in f ++ "." ++ s
    |                                           ^^^^^^^^^^^^^

UpdateSidonie.hs:339:55: error:
    • Couldn't match expected type ‘[Char]’
                  with actual type ‘Maybe String’
    • In the second argument of ‘(++)’, namely ‘s’
      In the second argument of ‘(++)’, namely ‘"." ++ s’
      In the expression: f ++ "." ++ s
    |
339 |                                        in f ++ "." ++ s
    |                                                       ^
Failed, no modules loaded.

for now this page has been of valuable help:

https://pbrisbin.com/posts/maybe_is_just_awesome/

i'm sure it's an obvious question but.... :-)

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