[Haskell-beginners] database access,extracting result...

[Damien Mattei]

thank you David, i did something like that:

rows <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE
distance > 0.000278"

forM_ rows $ \(name,distance) ->
      putStrLn $  Text.unpack name ++ " " ++ show (distance :: Double)

output:

...


HJ 2900 3.333333333882682e-2
HJ 3340 1.6666666646205367e-2
HLD  73 1.666666666807325e-2
HLD 152 1.666666666807325e-2
HLD 158 1.666666666807325e-2
HO    6 0.19569724135990224
HO   15 1.666666666807325e-2

...

and it works

Damien


Le 03/12/2018 15:42, David McBride a écrit :
> Here's how it works, every time you supply a query, you supply the
> parameters to the query in a tuple.  Furthermore the results can be
> gotten back as a tuple by type hinting each value.  Warning:  I have not
> run this code, but it should be close.
> 
> query "select age, is_old from user where uid = ? and name = ? (uid ::
> Int, name :: String)  :: IO [(Integer, Bool)]
> 
> But what happens when you want to supply a single parameter (or receive
> a single value?)
> 
> query "select * from user where uid = ?" (uid)
> 
> The problem with that is (uid) is not a tuple.  It's just an integer in
> parenthesis.  There in fact is no way to specify a tuple of one length. 
> So mysql-simple (and other libraries) very often have a single type
> meant to be used as a single element tuple.  In mysql-simple's (and
> postgresql-simple) case that is the (Only a) type.  (Side note, I wish
> these were in the standard Tuple module, as this comes once in awhile).
> 
> query "select name from user where uid = ?" (Only uid) :: IO [Only String]
> 
> Remember that you can also make your own records implement the
> QueryParams and QueryResults classes so that you can write
> 
> On Mon, Dec 3, 2018 at 6:22 AM Damien Mattei <mattei at oca.eu
> <mailto:mattei at oca.eu>> wrote:
> 
>     {-# LANGUAGE OverloadedStrings #-}
> 
>     import Database.MySQL.Simple
>     import Control.Monad
>     import Data.Text as Text
>     import Data.Int
> 
> 
>     main :: IO ()
>     main = do
>       conn <- connect defaultConnectInfo
>         { connectHost = "moita",
>           connectUser = "mattei",
>           connectPassword = "******",
>           connectDatabase = "sidonie" }
> 
>       rows <- query_ conn "select `N° BD` from sidonie.Coordonnées where Nom
>     = 'A    20'"
> 
>       --putStrLn $ show rows
> 
>       forM_ rows $ \(fname, lname) ->
>          putStrLn $  fname ++ " " ++ Text.unpack lname ++ " "
> 
> 
>     here is the error:
> 
>     *Main> :load Toto
>     [1 of 1] Compiling Main             ( Toto.hs, interpreted )
>     Ok, one module loaded.
>     *Main> main
>     *** Exception: ConversionFailed {errSQLType = "1 values:
>     [(VarString,Just \"-04.3982\")]", errHaskellType = "2 slots in target
>     type", errFieldName = "[\"N\\194\\176 BD\"]", errMessage = "mismatch
>     between number of columns to convert and number in target type"}
> 
>     -04.3982 is the values i want to put in a variable,it's the N° BD
>     (Durchmusterung Number)
> 
> 
>     ???
> 
>     any help greatly appeciated ;-)
> 
> 
>     -- 
>     Damien.Mattei at unice.fr <mailto:Damien.Mattei at unice.fr>,
>     Damien.Mattei at oca.eu <mailto:Damien.Mattei at oca.eu>, UNS / OCA / CNRS
>     _______________________________________________
>     Beginners mailing list
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> 
> 
> 
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-- 
Damien.Mattei at unice.fr, Damien.Mattei at oca.eu, UNS / OCA / CNRS