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
{-# 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@unice.fr, Damien.Mattei@oca.eu, UNS / OCA / CNRS _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners