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

Doaitse Swierstra doaitse at swierstra.net
Tue Jan 8 13:42:00 UTC 2019


Something like:

resBDstrFloat = 
  case resDBWords of
    Nothing       -> trace "WARNING: BD contains no words”      Nothing
    Just []       -> trace "WARNING: BD contains no words”      Nothing
    Just [v]      -> trace "WARNING: BD contains only one word” (Just v)
    Just [v1,v2]) -> Just (v1 ++ “.” ++ v2)
    -             -> trace “unexpected garbage” Nothing 

seems optimal and easiest to understand to me,

 Doaitse

 

> Op 8 jan. 2019, om 11:40 heeft Damien Mattei <damien.mattei at gmail.com> het volgende geschreven:
> 
> i had this solution for now:
> 
> let resBDstrFloat = if lgBDwords == Just 0
>                            then trace "WARNING: BD contains no words" Nothing
>                            else
>                                if lgBDwords == Just 1
>                                   then trace "WARNING: BD contains only one word" fmap head resBDwords
>                                   else let f = fmap head resBDwords
>                                            s = fmap (head . tail) resBDwords
>                                            mp = Just "." :: Maybe String
>                                            (+++) = liftM2 (++)
>                                        in f +++ mp +++ s
> 
> still searching to express it with <*> ..., also there is the problem of if i define "." simply as it is not a Maybe String it fails, perheaps some viadic function that accept multi-type variable but this is complex to do.
> 
> On Mon, Jan 7, 2019 at 11:58 PM Jake <jake.waksbaum at gmail.com> wrote:
> 
> 
> On Mon, Jan 7, 2019 at 5:55 PM Jake <jake.waksbaum at gmail.com> wrote:
> 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 at 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 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.... :-)
> 
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.



More information about the Haskell-Cafe mailing list