[Haskell-cafe] Fwd: IO monad use
Damien Mattei
damien.mattei at gmail.com
Sat Dec 29 13:38:53 UTC 2018
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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