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_.