[Haskell-cafe] Fwd: IO monad use
Tom Ellis
tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk
Sat Dec 29 10:51:11 UTC 2018
Why are you using map? You were right the first time with mapM:
lstNamesBD <- mapM (\(Only name) ->
do
let nameStr = Text.unpack name
bd <- getBD conn nameStr
return (nameStr,bd))
names
On Sat, Dec 29, 2018 at 11:42:05AM +0100, Damien Mattei wrote:
> 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 at 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
> >
> > On Sat, Dec 29, 2018 at 10:08:32AM +0100, Damien Mattei wrote:
> > > ---------- Forwarded message ---------
> > > From: Damien Mattei <damien.mattei at gmail.com>
> > > Date: Sat, Dec 29, 2018 at 9:46 AM
> > > Subject: IO monad use
> > > To: Damien MATTEI <damien.mattei at gmail.com>
> > >
> > >
> > > again an annoying error with my code, i want to apply sort of MAP on
> > 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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