How to do something with data from stdin, line by line, a maximum number of times and printing the number of line in Haskell

Question

This code reads the number of lines to process from the first line of stdin, then it loops number_of_lines_to_process times doing some calculations and prints the result. I want it to print the line number in "Line #" after "#" but I don't know how to obtain it

import IO
import Control.Monad (replicateM)

main :: IO ()

main = do
    hSetBuffering stdin LineBuffering
    s <- getLine
    let number_of_lines_to_process = read s :: Integer
    lines <- replicateM (fromIntegral(number_of_lines_to_process)) $ do
        line <- getLine
        let number = read line :: Integer
            result = number*2 --example
        putStrLn ("Line #"++": "++(show result)) --I want to print the number of the iteration and the result
    return ()

I guess that the solution to this problem is really easy, but I'm not familiar with Haskell (coding in it for the first time) and I didn't find any way of doing this. Can anyone help?

Answer 1

You could use forM_ instead of replicateM:

import IO
import Control.Monad

main :: IO ()
main = do
    hSetBuffering stdin LineBuffering
    s <- getLine
    let number_of_lines_to_process = read s :: Integer

    forM_ [1..number_of_lines_to_process] (\i -> do
        line <- getLine
        let number = read line :: Integer
            result = number * 2
        putStrLn $ "Line #" ++ show i ++ ": " ++ show result)

Note that because you use forM_ (which discards the results of each iteration) you don't need the additional return () at the end - the do block returns the value of the last statement, which in this case is the () which is returned by forM_.

Answer 2

The trick is to first create a list of all the line numbers you want to print, and to then loop through that list, printing each number in turn. So, like this:

import Control.Monad

import System.IO

main :: IO ()
main = do
  hSetBuffering stdin LineBuffering
  s <- getLine
  let lineCount = read s :: Int
      -- Create a list of the line numbers
      lineNumbers = [1..lineCount]

  -- `forM_` is like a "for-loop"; it takes each element in a list and performs
  -- an action function that takes the element as a parameter
  forM_ lineNumbers $ \ lineNumber -> do
    line <- getLine
    let number = read line :: Integer
        result = number*2 --example
    putStrLn $ "Line #" ++ show lineNumber ++ ": " ++ show result
  return ()

Read the definition of forM_.

By the way, I wouldn't recommend using the old Haskell98 IO library. Use System.IO instead.