How to Read from stdin in Haskell Posted in Polyglotbits on 02 of Dec, 2015

In haskell there are two ways to read the stdin stream. Which one to use depends on what you’re trying to accomplish I guess.

Either you can read each line in a loop.

import System.IO

main = myLoop

myLoop = do done <- isEOF
            if done
              then putStrLn ""
              else do inp <- getLine
                      putStrLn inp 

We call the myLoop function recursively until getLine returns EOF. Every time we get a string back, we print it to stdout and call ourselves again.

The other method involves reading the entire stream with getContents and then lazily read each line.

import Data.Char

main :: IO ()
main = do
   inp <- getContents
   let ls = lines inp
   mapM_ putStrLn ls

Here we get the stream into inp, split that into an array of lines, and map that array to the putStrLn function, which prints to stdout.