[Haskell-beginners] (SPAM 3)Re: monad and variable result

[Damien Mattei]

query returns a list i suppose because i can wirte this in my main code:
 bd_rows <- query conn qry_head (Only (name::String))

    putStrLn $ show bd_rows
    putStrLn $ show name

    let resLst = Prelude.map fromOnly bd_rows

i can o a map on bd_rows

i just want to convert this code in Main:

let qry_head = "select `N° BD` from sidonie.Coordonnées where Nom = ?"
:: Query

--    bd_rows <- query_ conn "select `N° BD` from sidonie.Coordonnées
where Nom = 'A    20'"

    bd_rows <- query conn qry_head (Only (name::String))

    putStrLn $ show bd_rows
    putStrLn $ show name

    let resLst = Prelude.map fromOnly bd_rows

    let noBDtxt = fromOnly (Prelude.head bd_rows) :: Text
--    let noBD2 =  _ (Prelude.head bd_rows)

    putStrLn $ show resLst

    putStrLn $ show noBDtxt

    forM_ bd_rows $ \(Only a) ->
      putStrLn $  Text.unpack a

    let noBDstr = Text.unpack noBDtxt :: String
    let noBDfp = read $ noBDstr :: Float

where i succeed in putting the Float extracted and converted from the
result of query in a variable of type Float,

i want to have this in  function getBD that take as arguments the
Connection and the String name ,i use name in the SQL query and get back
the noBD in text first ,string and finally float...

this should be done,i assume it is possible in haskell but i'm a
beginner....

i have problem finding good doc ,understanding Monads ans '<-' use and 'do'

but i assume the solution is perhaps trivial...

Le 10/12/2018 12:19, Francesco Ariis a écrit :
> Hy Damien,
> 
> On Mon, Dec 10, 2018 at 11:32:23AM +0100, Damien Mattei wrote:
>> i want  the variable local_bd_rows accessible in the 'where' clause
> 
> does `query` return IO ()? If so, no you can't*. Once the result
> is inside IO, it stays in IO. Move what you need inside out of where
> (and inside a `do`) and get ready to change the signature of
> `getBD` to
> 
>     getBD :: Connection -> String -> IO Float
> 
> -F
> 
> * (you can with unsafeSomething functions, but it is really really
> advisable not to do that).
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-- 
Damien.Mattei at unice.fr, Damien.Mattei at oca.eu, UNS / OCA / CNRS