[Haskell-cafe] concatenate two Maybe String...

7 Jan 2019

      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.... :-)

[Haskell-cafe] concatenate two Maybe String...

Damien Mattei