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

[David McBride]

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> 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, Damien.Mattei at oca.eu, UNS / OCA / CNRS
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