gistfile1.hs

{-# LANGUAGE GADTs, MultiParamTypeClasses, ScopedTypeVariables #-}
module Main where
import System.IO
import Control.Monad.Operational.Simple

data FileIO a where
    Open :: FilePath -> FileIO Handle
    Get :: Handle -> FileIO String
    Echo :: String -> FileIO ()
    Close :: Handle -> FileIO String

instance Operational FileIO IO where
--    singleton :: FileIO a -> IO a
    singleton (Open path) = openFile path ReadMode
    singleton (Get hdl) = hGetLine hdl
    singleton (Close hdl) = hClose hdl >> return "closed"
    singleton (Echo str) = putStrLn str

open :: FilePath -> Program FileIO Handle
open = singleton . Open
get :: Handle -> Program FileIO String
get = singleton . Get
close :: Handle -> Program FileIO String
close = singleton . Close
echo :: String -> Program FileIO ()
echo = singleton . Echo

main :: IO ()
main = do
    val <- fromProgram $ do
        hdl <- open "./hoho"
        line <- get hdl
        echo line
        close hdl
        return "FileIO計算結果"
    putStrLn val