Why is this variable out of scope? (Haskell)

Question

I'll get right to it (I removed a bunch of extraneous code, in case this looks a little funny -- trying to make this a MCVE):

import System.IO
import Control.Monad
import Data.IORef

main :: IO ()
main = do
    hSetBuffering stdout NoBuffering 

    input_line <- getLine
    let input = words input_line
    let x0 = read (input!!0) :: Int
    x <- newIORef x0
    loop

loop :: IO ()
loop = do
    input_line <- getLine
    let the_dir = input_line :: String 

    putStrLn x

    loop

Just to test it I tried outputting x and it's saying it's out of scope. Why? How do I fix it so it is in scope? I need to access the variables but I also need to have them initialized before I enter the loop for the first time.

Can you make loop accept an input parameter and pass x to it? — shree.pat18
– shree.pat18, Commented Nov 13, 2017 at 7:53
I don't know how, but I would assume that is a valid way to do it — user8930358
– user8930358, Commented Nov 13, 2017 at 7:55
More important, what are you trying to do that needs IORef and such? — shree.pat18
– shree.pat18, Commented Nov 13, 2017 at 7:56
I have a game loop that runs over and over again and I need to read input and update values / output accordingly, I have no idea how you're supposed to do that if everything is immutable — user8930358
– user8930358, Commented Nov 13, 2017 at 7:59
I see. Perhaps you can update your question with the above, and provide some sample data so that we could see another solution to this? — shree.pat18
– shree.pat18, Commented Nov 13, 2017 at 8:15

ice1000 · Accepted Answer · 2017-11-13 09:23:54Z

1

You can make x a parameter.

import System.IO
import Control.Monad
import Data.IORef

main :: IO ()
main = do
    hSetBuffering stdout NoBuffering 

    input_line <- getLine
    let input = words input_line
    let x0 = read (input!!0) :: Int
    x <- newIORef x0
    loop x

loop :: Int -> IO ()
loop x = do
    input_line <- getLine
    let the_dir = input_line :: String 

    putStrLn $ show x

    loop

Or simply use forever.

import Control.Monad

...

    x <- newIORef x0
    forever $ do
        input_line <- getLine
        let the_dir = input_line :: String 

        putStrLn $ show x

edited Nov 13, 2017 at 9:23

answered Nov 13, 2017 at 8:59

ice1000

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6 Comments

chi Over a year ago

This won't type check. Above, x is an IORef Int, then is used as a String. With slightly more work it should work, though.

user8930358 Over a year ago

@chi I changed the parameter to IORef Int -- how do I get it to print? I tried print $ x and print (readIORef x) etc, nothing is working

chi Over a year ago

@user8930358 Try print =<< readIORef x or split the read and the print: as in do .... ; xVal <- readIORef x ; print xVal

user8930358 Over a year ago

@chi I have to print two of them on the same line with a space separator, to be specific, will print =<< readIORef x syntax let me do that or do I have to do some string conversion thing using your second method?

user8930358 Over a year ago

Looks like printf "%d %d" x_out y_out works after doing x_out <- readIORef x and importing Text.Printf although I don't know if this is nonstandard

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