---------- 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 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
