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

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_pictur...

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@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@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

...

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

...

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.