Re: [Haskell-cafe] Fwd: IO monad use

29 Dec 2018


      thank you tom,
in fact reading doc as i saw i never used return read in monad doc i
already put it in my code this gives this that compiles:
 let lstNamesBD = Prelude.map (\(Only name) ->
                                         do
                                          let strName = Text.unpack name
                                          getBD conn strName >>= (\bd ->

return (strName,bd)))
                                 names

but sticked now for displaying it because it is now an IO tuple of 2 and i
can not display it with some functions like this , it fails to compile:

--    putStr "lstNamesBD ="
--    putStrLn $ show lstNamesBD

    forM_ lstNamesBD $ \t2 ->
          do
            n <- fst t2
            b <- snd t2
            putStrLn $  (show n) ++ " " ++ maybe "NULL" show b

or with that tested first:

 forM_ lstNamesBD $ \(name,bd) ->
            putStrLn $  (show name) ++ " " ++ maybe "NULL" show bd
Prelude> :load UpdateSidonie
[1 of 1] Compiling Main             ( UpdateSidonie.hs, interpreted )

UpdateSidonie.hs:207:22: error:
    • Couldn't match expected type ‘(IO a1, b0)’
                  with actual type ‘IO (String, Maybe Float)’
    • In the first argument of ‘fst’, namely ‘t2’
      In a stmt of a 'do' block: n <- fst t2
      In the expression:
        do n <- fst t2
           b <- snd t2
           putStrLn $ (show n) ++ " " ++ maybe "NULL" show b
    |
207 |             n <- fst t2
    |                      ^^

UpdateSidonie.hs:208:22: error:
    • Couldn't match expected type ‘(a0, IO (Maybe a2))’
                  with actual type ‘IO (String, Maybe Float)’
    • In the first argument of ‘snd’, namely ‘t2’
      In a stmt of a 'do' block: b <- snd t2
      In the expression:
        do n <- fst t2
           b <- snd t2
           putStrLn $ (show n) ++ " " ++ maybe "NULL" show b
    |
208 |             b <- snd t2
    |                      ^^

UpdateSidonie.hs:211:25: error:
    • Couldn't match expected type ‘IO (String, Maybe Float)’
                  with actual type ‘(a3, Maybe a4)’
    • In the pattern: (name, bd)
      In the second argument of ‘($)’, namely
        ‘\ (name, bd)
           -> putStrLn $ (show name) ++ " " ++ maybe "NULL" show bd’
      In a stmt of a 'do' block:
        forM_ lstNamesBD
          $ \ (name, bd)
              -> putStrLn $ (show name) ++ " " ++ maybe "NULL" show bd
    |
211 |     forM_ lstNamesBD $ \(name,bd) ->
    |                         ^^^^^^^^^
Failed, no modules loaded.
Prelude>


On Sat, Dec 29, 2018 at 11:30 AM Tom Ellis <
tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
...
I guess you want
return (nameStr, bd)
Every statement in a do-block must be of type `IO a` for some `a`.
`(nameStr, bd)` is a pure value of type `(String, Maybe Float)`.  You turn
it into a value of type `IO (String, Maybe Float)` using `return`.
Tom
...
---------- Forwarded message ---------
From: Damien Mattei 
Date: Sat, Dec 29, 2018 at 9:46 AM
Subject: IO monad use
To: Damien MATTEI 
again an annoying error with my code, i want to apply sort of MAP on
On Sat, Dec 29, 2018 at 10:08:32AM +0100, Damien Mattei wrote:
list
...
resut of my database IO accessed extracting info with another query,
queries works both but i can not MAP or if i can i do not know how to
SHOW
the result, here is the code:
lstNamesBD <- mapM (\(Only name) ->
                               do
                                let nameStr = Text.unpack name
                                bd <- getBD conn nameStr
                                (nameStr,bd))
                       names
it fails to compile with this error:
Prelude> :load UpdateSidonie
[1 of 1] Compiling Main             ( UpdateSidonie.hs, interpreted )
UpdateSidonie.hs:203:33: error:
    • Couldn't match type ‘(,) String’ with ‘IO’
      Expected type: IO (Maybe Float)
        Actual type: (String, Maybe Float)
    • In a stmt of a 'do' block: (nameStr, bd)
      In the expression:
        do let nameStr = unpack name
           bd <- getBD conn nameStr
           (nameStr, bd)
      In the first argument of ‘mapM’, namely
        ‘(\ (Only name)
            -> do let nameStr = ...
                  bd <- getBD conn nameStr
                  (nameStr, bd))’
    |
203 |                                 (nameStr,bd))
    |                                 ^^^^^^^^^^^^
Failed, no modules loaded.
if i code like this  show does not know how to display result:
let lstNamesBD = Prelude.map (\(Only name) ->
                                         do
                                          let nameStr = Text.unpack name
                                          bd <- getBD conn nameStr
                                          res <- (nameStr,bd)
                                          res)
                                 names
putStr "lstNamesBD ="
 putStrLn $ show lstNamesBD
*Main> :load UpdateSidonie
[1 of 1] Compiling Main             ( UpdateSidonie.hs, interpreted )
UpdateSidonie.hs:193:16: error:
    • No instance for (Show (IO (Maybe Float)))
        arising from a use of ‘show’
    • In the second argument of ‘($)’, namely ‘show lstNamesBD’
      In a stmt of a 'do' block: putStrLn $ show lstNamesBD
      In the expression:
        do conn <- connect
                     defaultConnectInfo
                       {connectHost = "moita", connectUser = "mattei",
                        connectPassword = "sidonie2", connectDatabase =
"sidonie"}
           (rows :: [(Text, Double)]) <- query_
                                           conn
                                           "SELECT Nom,distance FROM
AngularDistance WHERE distance > 0.000278"
           (names :: [Only Text]) <- query_
                                       conn
                                       "SELECT Nom FROM AngularDistance
WHERE distance > 0.000278"
           let resLstNames = Prelude.map fromOnly names
           ....
    |
193 |     putStrLn $ show lstNamesBD
    |                ^^^^^^^^^^^^^^^
Failed, no modules loaded.
help me please
...
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