[Haskell-cafe] trace output statements

[Damien Mattei]

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 at 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 at 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 at 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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