Haskell file reading

Not a bad start! The only thing to remember is that pure function application should use let instead of the binding <-.

import System.IO  
import Control.Monad

main = do  
        let list = []
        handle <- openFile "test.txt" ReadMode
        contents <- hGetContents handle
        let singlewords = words contents
            list = f singlewords
        print list
        hClose handle   

f :: [String] -> [Int]
f = map read

This is the minimal change needed to get the thing to compile and run. Stylistically, I have a few comments:

1. Binding list twice looks a bit shady. Note that this isn’t mutating the value list — it’s instead shadowing the old definition.
2. Inline pure functions a lot more!
3. When possible, using readFile is preferable to manually opening, reading, and closing a file.

Implementing these changes gives something like this:

main = do  
        contents <- readFile "test.txt"
        print . map readInt . words $ contents
-- alternately, main = print . map readInt . words =<< readFile "test.txt"

readInt :: String -> Int
readInt = read