[Haskell-cafe] Fwd: [Haskell-beginners] monad and variable result

[Damien Mattei]

Thank you Seph, with your help and reading some pages i can get to this:

getBD :: Connection -> String -> IO Float
getBD conn name = noBDfp
  where qry_head = "select `N° BD` from sidonie.Coordonnées where Nom =
?" :: Query
        bd_rows :: IO [Only Text]
        bd_rows = query conn qry_head (Only (name::String))
--        noBDtxt :: [Text]
--        noBDtxt = fromOnly (Prelude.head bd_rows)
--        noBDtxt :: IO [Text]
        noBDtxt :: IO Text
        noBDtxt = fmap (fromOnly . Prelude.head) bd_rows
--        noBDstr :: String
--        noBDstr = Text.unpack noBDtxt
        noBDstr :: IO String
        noBDstr = fmap Text.unpack noBDtxt
--        noBDfp = read $ noBDstr :: Float
        noBDfp = fmap read noBDstr :: IO Float


which compile

but the question rest entire :how can i get the loat number from all this?

i have a Main that looks like this:


main :: IO ()
--main :: Int
main =

  do
    conn <- connect defaultConnectInfo
      { connectHost = "moita",
        connectUser = "mattei",
        connectPassword = "sidonie2",
        connectDatabase = "sidonie" }



-- first we get the N°BD from sidonie

    let name = "A    20"

let noBD_IO = getBD conn name

--    putStrLn $ show $ read $ Text.unpack noBD_IO

    close conn

    print "Exit."

how can i get the float number from noBD_IO ?

Regards,

Damien
Le 10/12/2018 19:56, Seph Shewell Brockway a écrit :
> On Mon, Dec 10, 2018 at 06:06:30PM +0100, Damien Mattei wrote:
>> for now i'm here:
>>
>> getBD :: Connection -> String -> Float
>> getBD conn name = noBDfp
>>   where qry_head = "select `N° BD` from sidonie.Coordonnées where Nom =
>> ?" :: Query
>>         -- bd_rows =
>>         --   do
>>         --     local_bd_rows <- query conn qry_head (Only (name::String))
>>         --     return local_bd_rows
>>         bd_rows :: IO [Only Text]
>>         bd_rows = query conn qry_head (Only (name::String))
>>         noBDtxt :: [Text]
>>         noBDtxt = fromOnly (Prelude.head bd_rows)
>>         noBDstr :: String
>>         noBDstr = Text.unpack noBDtxt :: String
>>         noBDfp = read $ noBDstr :: Float
> 
> Okay, I think I understand how your code is structured now. The point to
> recognize is that bd_rows has type IO [Only Text], which is not the same
> type as [Only Text]. Prelude.head has the type [a] -> a, and so it can’t
> be used on bd_rows as-is. Fortunately, being a monad, IO has an instance
> of Functor for free, and we can go from
>    
>    head :: [Only Text] -> Only Text
> 
>    to
> 
>    fmap head :: IO [Only Text] -> IO (Only Text)
> 
> which takes an IO operation returning [Only Text] and applies head to
> its result, giving an IO operation returning an Only Text.
> 
> Can you see how the IO monad follows you through everything that uses
> its result? If you rewrite noBDtxt as
> 
>    noBDtxt :: IO [Text]
>    noBDtxt = fmap (fromOnly . Prelude.head) bd_rows
> 
> then noBDstr has to be rewritten in a similar way, and so on through to
> the final result, giving the main function a type of
> Connection -> String -> IO Float.
> 
> A lot of the functions in your where clause can be amalgamated, for
> example by combining noBDtxt and noBDstr as
> 
>    noBDstr = fmap (Text.unpack . fromOnly . Prelude.head)
> 
> Similarly, getBD takes the result of noBDfp and returns it unaltered, so
> why not just write
>    
>    getBD = fmap read noBDstr
> 
> ?
> 
> Let me know if you would like me to explain anything in this message in more
> detail.
> 
> Regards,
> 
> Seph
> 

-- 
Damien.Mattei at unice.fr, Damien.Mattei at oca.eu, UNS / OCA / CNRS