Can not print to file using IO Monad - haskell

Question

Hello i have done my JSon type and i am trying to it to a file.
I can do this from the prelude but i can't do it when using the IO Monad.I get the following error:
Main.hs:13:24: error:
* Couldn't match type `Char' with `[Char]'
Expected type: String
Actual type: Char
* In the second argument of `writeFile', namely `val'
In a stmt of a 'do' block: writeFile out val
In the expression:
do val <- renderJValue sample
writeFile out val
|
13 | writeFile out val
| ^^^
Main
module Main where
import Jlib
import Put
import Data.Typeable
import System.Environment
out="data.txt"
main::IO()
main=do
val<-renderJValue sample
writeFile out val
Why would this not work in the IO Monad since renderJValue sample in the prelude works ok .
Jlib.hs
data JValue=JString String
|JNumber Double
|JBool Bool
|JNull
|JObject [(String,JValue)]
|JArray [JValue]
deriving (Eq,Ord,Show)
Put.hs
sample=JArray[
JObject [("name",JString "adita"),("age",JNumber 13)],
JObject [("name",JString "dan"),("kids",JNumber 3)] ,
JNumber 3,
JBool False,
JString "Howdy"
]
P.S renderJValue returns a string
P.S: if i start the prelude i load the module and i render the value it works:
Prelude System.Environment Put> :load Put
Ok, two modules loaded.
Prelude System.Environment Put> renderJValue sample
"[{name:adita,age:13.0},{name:dan,kids:3.0},3.0,False,Howdy]"

Answer 1

You here use renderJValue sample as if it is an IO String:
main :: IO()
main=do
val <- renderJValue sample
writeFile out val
But it is in fact (given it is a function similar to this one) a function with signature renderJValue :: JValue -> String. So no IO is involved. In that case we do not use the arrow notation.
We can call the function "inline":
main :: IO()
main = do
writeFile out (renderJValue sample)
or even shorter:
main :: IO()
main = writeFile out (renderJValue sample)
But in case the expression is rather long, this can become quite ugly. We can decide to use a let statement in that case.
You can solve this by either removing the putStrLn:
main :: IO()
main = do
let val = renderJValue sample
writeFile out val