IO Monad for Nim

main.nim

# IO Monad for Nim!

type IO*[T] = proc (): T

type EmptyTuple* = tuple[]

proc callIO*[T](f: IO[T]): auto {.inline.} = f()

proc `>>`*[A, B](a: IO[A], b: IO[B]): IO[B] = 
  discard callIO(a)
  b

proc `>>=`*[A, B](a: IO[A], f: proc (x: A): IO[B]): IO[B] =
 f(callIO(a))

# hFlush :: Handle -> IO ()
let hFlush*: proc (a: File): IO[EmptyTuple]  = proc (a: File): IO[EmptyTuple] =
  return proc (): EmptyTuple =
    a.flushFile()
    return ()

# hPutChar :: Handle -> Char -> IO ()
let hPutChar*: (proc (a: File): proc (c: char): IO[EmptyTuple]) = proc (a: File): (proc (c: char): IO[EmptyTuple]) =
  return proc (c: char): IO[EmptyTuple] = 
    return proc (): EmptyTuple = 
      a.write(c)
      return ()

# getChar :: IO Char
let getChar*: IO[char] = proc (): char = stdin.readChar()

# getLine :: IO String
let getLine*: IO[string] = proc (): string = stdin.readLine()

# putStrLn :: String -> IO ()
let putStrLn*: proc (a: string): IO[EmptyTuple] = proc (a: string): IO[EmptyTuple] =
  return proc (): EmptyTuple = 
    stdout.writeLine(a)
    return ()

# readFile :: FilePath -> IO String (FilePath=String)
let readFile*: proc (a: string): IO[string] = proc (a: string): IO[string] =
  return proc (): string = 
    return system.readFile(a) # readFile is defined at system.io, not our function

let putC: IO[EmptyTuple] = (hPutChar stdout)('C') 

# main :: IO ()
let main: IO[EmptyTuple] = 
  (getLine >>= readFile >>= putStrLn) >> 
  putC >> 
  (hFlush stdout)

discard callIO(main)