module Main where

import           System.IO
import           Control.Monad     (unless)

main :: IO ()
main = do
    input <- read_
    unless (input == ":quit") $ print_ (eval_ input) >> main

read_ :: IO String
read_ = putStr "hytl> " >> hFlush stdout >> getLine


eval_ :: String -> String
eval_ input = input

print_ :: String -> IO ()
print_ = putStrLn

import           System.IO

read_ :: IO String
read_ = putStr "hytl> " >> hFlush stdout >> getLine

main :: IO ()
main = do
    input <- read_
    print input

アクションから値を取り出して、それを捨てて次のアクションを実行する感じ

a = do
    _ <- print 1
    print 2

a = print 1 >> print 2

syntax - What does the >> symbol mean in Haskell - Stack Overflow

unless :: Applicative f => Bool -> f () -> f ()

Haskell アクションとラムダ 超入門 - Qiita