yes i use do notation, but for example i have code that works in main and not in a function! i print the code perheaps someone could help me: first the function, so you have the import list too: import Database.MySQL.Simple import Database.MySQL.Simple.QueryResults import Database.MySQL.Simple.Result import Database.MySQL.Simple.QueryParams import Database.MySQL.Simple.Param import Control.Monad import Data.Text as Text --import Data.Int as Int --import Data.List import Debug.Trace import Data.Maybe as Maybe -- this function will return th N°BD from Sidonie for a given name -- note: names have been standardized between Sidonie and WDS getBD3 :: Connection -> String -> Float getBD3 conn name = do let qry_head_BD_Sidonie = "select `N° BD` from Coordonnées where Nom = ?" :: Query (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name::String)) let noBDtxt = fromOnly (Prelude.head bd_rows) :: Text let noBDstr = Text.unpack noBDtxt :: String let noBDfp = read $ noBDstr :: Float return noBDfp with this function i have this error: Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:54:13: error: • Couldn't match expected type ‘Float’ with actual type ‘IO Float’ • In a stmt of a 'do' block: (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name :: String)) In the expression: do let qry_head_BD_Sidonie = ... (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name :: String)) let noBDtxt = ... let noBDstr = ... .... In an equation for ‘getBD3’: getBD3 conn name = do let qry_head_BD_Sidonie = ... (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name :: String)) let noBDtxt = ... .... | 54 | (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name::String)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. i do not understand the error complaining that i return an IO float,because i'm sure it's a float in noBDfp if i put the same lines of code in the main it works !!! : main :: IO () main = do conn <- connect defaultConnectInfo { connectHost = "moita", connectUser = "mattei", connectPassword = "sidonie2", connectDatabase = "sidonie" } let qry_head_BD_Sidonie = "select `N° BD` from Coordonnées where Nom = ?" :: Query (bd_rows :: [Only Text]) <- query conn qry_head_BD_Sidonie (Only (name::String)) putStr "bd_rows =" putStrLn $ show bd_rows let noBDtxt = fromOnly (Prelude.head bd_rows) :: Text let noBDstr = Text.unpack noBDtxt :: String let noBDfp = read $ noBDstr :: Float putStr "noBDfp =" (putStrLn (show noBDfp)) close conn it works i have output like this: *Main> main bd_rows =[Only {fromOnly = "-04.3982"}] noBDtxt ="-04.3982" noBDfp =-4.3982 noBDfp + 1 = -3.3982 i'm well getting a float in noBDfp , i even can add 1 to it :-) ( cool haskell...) but i'm just wanting to that in the function getDB3 but it does not compile... ?????? Damien On Sun, Dec 23, 2018 at 4:54 PM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
I think forgetting about monads and just using do-notation will help you.
On Sun, Dec 23, 2018 at 04:44:57PM +0100, Damien Mattei wrote:
i think learning Monads from scratch again will help me
On Sun, Dec 23, 2018 at 4:11 PM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
Yes, exactly!
On Sun, Dec 23, 2018 at 02:08:57PM +0100, Damien Mattei wrote:
lazyness....?
On Sun, Dec 23, 2018 at 8:40 AM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote:
i have inserted trace statement that output variable ... i have strange behavior of output:
Let's take a simpler example. Do you understand why the trace statments from this small program appear in the order that they do? (And for what it's worth I really think you'll be better off writing programs using do notation).
% cat test.hs import Debug.Trace
result = let a = trace "evaluating a" 2 b = trace "evaluating b" 10 c = trace "evaluating c" (a + b) in c ~% ghci -e result test.hs evaluating c evaluating b evaluating a 12
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