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