[Haskell-cafe] Fwd: [Haskell-beginners] monad and variable result

[Damien Mattei]

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

but it fails due to the IO :

Prelude> :load UpdateSidonie
[1 of 1] Compiling Main             ( UpdateSidonie.hs, interpreted )

UpdateSidonie.hs:53:42: error:
    • Couldn't match expected type ‘[Only [Text]]’
                  with actual type ‘IO [Only Text]’
    • In the first argument of ‘Prelude.head’, namely ‘bd_rows’
      In the first argument of ‘fromOnly’, namely
        ‘(Prelude.head bd_rows)’
      In the expression: fromOnly (Prelude.head bd_rows)
   |
53 |         noBDtxt = fromOnly (Prelude.head bd_rows)
   |                                          ^^^^^^^

UpdateSidonie.hs:55:31: error:
    • Couldn't match expected type ‘Text’ with actual type ‘[Text]’
    • In the first argument of ‘unpack’, namely ‘noBDtxt’
      In the expression: unpack noBDtxt :: String
      In an equation for ‘noBDstr’: noBDstr = unpack noBDtxt :: String
   |
55 |         noBDstr = Text.unpack noBDtxt :: String
   |                               ^^^^^^^
Failed, no modules loaded.




Le 10/12/2018 17:19, Seph Shewell Brockway a écrit :
> Hi Damien,
> 
> On Mon, Dec 10, 2018 at 03:18:48PM +0100, Damien Mattei wrote:
>> have some code that works but want to put it in a simple function
>> without sucess:
>>
>> 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
>>
>>
>> i want  the variable local_bd_rows accessible in the 'where' clause
>>
>> how can i do that?
> 
> You don’t seem to be using the function bd_rows anywhere in the main
> body of the definition. You would need to do something like
> 
>          getBD :: Connection -> String -> IO Float
>          getBD = do rows <- bd_rows
>             {- code that does something with the returned data -}
> 
> Note the change to the type signature—querying the database is an IO
> action, and therefore takes place in the IO monad.
> 
> Incidently, your use of do notation in the definition of bd_rows is
> unnecessary:
> 
>    do x <- doSomething
>       return x
> 
> is actually syntactic sugar for
> 
>    doSomething >>= \x -> return x
> 
> which the monad laws state is equivalent to just doSomething. This is a
> common misapprehension among Haskell novices: the do notation is just a
> syntactic convenience, and it is perfectly possible to write monadic
> functions, including in the IO monad, without it.
>             
> Hope at least some of this helps.
> 
> Seph
> 

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