HaskellでReplを作る

最小の実装

code:hs

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

参考

Read

code:hs

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

code:hs

a = do

_ <- print 1

print 2

a = print 1 >> print 2

こうなる

k >> f = k >>= \_ -> f

参考

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

参考