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Today's Topics:
1. monad and variable result (Damien Mattei)
2. Re: monad and variable result (Francesco Ariis)
----------------------------------------------------------------------
Message: 1
Date: Mon, 10 Dec 2018 11:32:23 +0100
From: Damien Mattei <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] monad and variable result
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
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?
note the goal is to do the same thing of my main function that works :
main :: IO ()
--main :: Int
main =
do
conn <- connect defaultConnectInfo
{ connectHost = "moita",
connectUser = "mattei",
connectPassword = "******",
connectDatabase = "sidonie" }
-- we get all the Double Stars that have angular distance superior to a
threshold of 1 second = 0.000278 degree
rows <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE
distance > 0.000278"
forM_ rows $ \(name,distance) ->
putStrLn $ Text.unpack name ++ " " ++ show (distance :: Double)
-- we will get the Durchmusterung Number BD from Sidonie and WDS and
compare them for a given name
-- Warning: there could be multiple result in WDS for a given entry name
(due to components)
-- first we get the N°BD from sidonie
let name = "A 20"
-- let qry = "select `N° BD` from Coordonnées where Nom = " ++ name
let qry_head = "select `N° BD` from sidonie.Coordonnées where Nom =
?" :: Query
-- bd_rows <- query_ conn "select `N° BD` from sidonie.Coordonnées
where Nom = 'A 20'"
bd_rows <- query conn qry_head (Only (name::String))
putStrLn $ show bd_rows
putStrLn $ show name
let resLst = Prelude.map fromOnly bd_rows
let noBDtxt = fromOnly (Prelude.head bd_rows) :: Text
-- let noBD2 = _ (Prelude.head bd_rows)
putStrLn $ show resLst
putStrLn $ show noBDtxt
forM_ bd_rows $ \(Only a) ->
putStrLn $ Text.unpack a
let noBDstr = Text.unpack noBDtxt :: String
let noBDfp = read $ noBDstr :: Float
putStr "noBDfp ="
(putStrLn (show noBDfp))
close conn
print "Exit."
for now i have errors in the function:
Prelude> :load UpdateSidonie
[1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted )
UpdateSidonie.hs:47:28: error:
• Ambiguous type variable ‘r0’ arising from a use of ‘query’
prevents the constraint ‘(QueryResults r0)’ from being solved.
Relevant bindings include
bd_rows :: IO [r0] (bound at UpdateSidonie.hs:46:9)
Probable fix: use a type annotation to specify what ‘r0’ should be.
These potential instances exist:
instance Result a => QueryResults (Only a)
-- Defined in ‘Database.MySQL.Simple.QueryResults’
instance (Result a, Result b) => QueryResults (a, b)
-- Defined in ‘Database.MySQL.Simple.QueryResults’
instance (Result a, Result b, Result c) => QueryResults (a, b, c)
-- Defined in ‘Database.MySQL.Simple.QueryResults’
...plus 21 others
(use -fprint-potential-instances to see them all)
• In a stmt of a 'do' block:
local_bd_rows <- query conn qry_head (Only (name :: String))
In the expression:
do local_bd_rows <- query conn qry_head (Only (name :: String))
return local_bd_rows
In an equation for ‘bd_rows’:
bd_rows
= do local_bd_rows <- query conn qry_head (Only (name ::
String))
return local_bd_rows
|
47 | local_bd_rows <- query conn qry_head (Only (name::String))
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, no modules loaded.
--
[email protected], [email protected], UNS / OCA / CNRS
------------------------------
Message: 2
Date: Mon, 10 Dec 2018 12:19:43 +0100
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] monad and variable result
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Hy Damien,
On Mon, Dec 10, 2018 at 11:32:23AM +0100, Damien Mattei wrote:
> i want the variable local_bd_rows accessible in the 'where' clause
does `query` return IO ()? If so, no you can't*. Once the result
is inside IO, it stays in IO. Move what you need inside out of where
(and inside a `do`) and get ready to change the signature of
`getBD` to
getBD :: Connection -> String -> IO Float
-F
* (you can with unsafeSomething functions, but it is really really
advisable not to do that).
------------------------------
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