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

On Mon, Jan 7, 2019 at 5:55 PM Jake wrote:

Like Josh mentioned, Applicative Functors http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.ht... 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 (++) <$> s1 <*> s2. In fact, liftA2 must satisfy the equation liftA2 http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.ht... f x y = f <$> x <*> http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.ht... 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 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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