[Haskell-cafe] concatenate two Maybe String...
Damien Mattei
damien.mattei at gmail.com
Tue Jan 8 10:40:33 UTC 2019
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
>> <http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.html#v:liftA2>
>> 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 (++) <$> s1 <*> s2. In fact, liftA2
>> must satisfy the equation liftA2
>> <http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.html#v:liftA2>
>> f x y = f <$> x <*>
>> <http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.html#v:-60--42--62->
>> 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.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20190108/458615fa/attachment-0001.html>
More information about the Haskell-Cafe
mailing list