[Haskell-cafe] Fwd: IO monad use
Damien Mattei
damien.mattei at gmail.com
Sat Dec 29 10:42:05 UTC 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 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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