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

[☂Josh Chia (謝任中)]

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 at 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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