[Haskell-cafe] Fwd: IO monad use
Tom Ellis
tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk
Sat Dec 29 14:02:34 UTC 2018
I suggest you look carefully at the types of the things you are combining.
Once you understand how the types fit together you will understand which
functions you should use.
On Sat, Dec 29, 2018 at 02:38:53PM +0100, Damien Mattei wrote:
> yes it works, i had not set the return
> in the mapM version, that's my problem with i too often throw dices, do not
> know why its mapM or map that works, i put a return in a deseperate
> moment.... :-)
> still learning monads, find this that explain the concept:
>
> http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html#translations
>
> and this but just at the beginning i am:
> https://wiki.haskell.org/All_About_Monads
>
> thanks again for your help
> Damien
>
> On Sat, Dec 29, 2018 at 11:51 AM Tom Ellis <
> tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
>
> > 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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