The correct type annotation for getDB3 should be: getDB3 :: Connection -> String -> IO Float Note the IO at the end. Functions in Haskell are just pure computation; they can't have side effects -- so a function returning a Float can't possibly talk to a database. Instead, The type `IO a` represents a description of an action to perform. It's still just a value -- calling getDB3 doesn't *do* anything. You can stitch these together using do-notation or functions like >>=, and when the program is run the action defined by 'main' is performed. --- An analogy: you could imagine instead of IO we could give up, and write code that computes a program in some other (imperative) programming language) that we then hand off to an interpreter. For example, we could compute a python program that counts from 1 to 99 like so: printNum :: Int -> String printNum n = "print('" ++ show n ++ "')\n" pythonProgram = concatMap printNum [1..99] So we've defined a variable fullProgram that is a string with the source code to a python program like: print('1') print('2') print('3') ... print('99') ..but we haven't actually run it. To do that we'd have to pass the string off to the python interpreter. This is a good way to think about what IO is -- main is like our pythonProgram above, in that it is a description of actions to perform, but *evaluating* it doesn't have any side effects -- it just computes the description. When you run a Haskell program, this is like taking the description defined by Main and passing it off to an interpreter. --- So your definition of getDB3 is a description of actions to perform to get a float from the database, but your type declaration says it's a function that computes a float (without having to perform any "actions"). This is a critical distinction that exists in Haskell but not most other languages. Hope this helps, -Ian Quoting Damien Mattei (2018-12-25 15:07:35)
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 <[1]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 < > [2]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 < > > > [3]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 > > _______________________________________________ > > Haskell-Cafe mailing list > > To (un)subscribe, modify options or view archives go to: > > [4]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > Only members subscribed via the mailman list are allowed to post. > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > [5]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: [6]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
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