Lazy streams and unsafeInterleaveIO

Remi Turk
Tue, 24 Dec 2002 19:09:53 +0100

On Mon, Dec 23, 2002 at 09:05:00AM +0000, Glynn Clements wrote:
> Jyrinx wrote:
> > So is this lazy-stream-via-unsafeInterleaveIO not so nasty, then, so 
> > long as a few precautions (not reading too far into the stream, 
> > accounting for buffering, etc.) are taken? I like the idiom Hudak uses 
> > (passing a stream of I/O results to the purely functional part of the 
> > program), so if it's kosher enough I'd like to get hacking elsewhere ...
> It depends upon the amount and the complexity of the program's I/O,
> and the degree of control which you require. For a simple stream
> filter (read stdin, write stdout), lazy I/O is fine; for a program
> which has more complex I/O behaviour, lazy I/O may become a nuisance
> as the program grows more complex or as you need finer control.

just for fun I wrote a slightly-enhanced version of my previous one-liner ;o)
It needs to be compiled with GHC's "-package util" as it uses GNU Readline.
I guess it demonstrates why lazy io may not always be a good idea when doing
more complex things with IO.

Happy hacking,

P.S.   Have fun with forward-references as program-input ;-D
P.P.S. GNU Readline implements history-functions itself of
       course. Who talked about reinventing the wheel? :D

module Main where

import Monad (liftM, zipWithM_)
import Maybe (catMaybes, isJust)
import Readline (readline)
import System.IO.Unsafe (unsafeInterleaveIO)

-- Like the prelude-function sequence, but lazy
lazySequenceIO          :: [IO a] -> IO [a]
lazySequenceIO []       = return []
lazySequenceIO (p:ps)   = do
                            x <- unsafeInterleaveIO p
                            unsafeInterleaveIO $ liftM (x:) (lazySequenceIO ps)

{- Given a list of prompts, read lines with GNU Readline until
   either we've had all prompts or the users presses ^D -}
readLines   :: [String] -> IO [String]
readLines   = liftM (catMaybes . takeWhile isJust)
            . lazySequenceIO . map (unsafeInterleaveIO . readline)

main        = do
                putStrLn "N        Add the number N"
                putStrLn "<enter>  Again"
                putStrLn "!N       Repeat input N"
                putStrLn "?N       Enter result N as input"
                input <- readLines $ map (\n -> show n ++ "> ") [0..]
                let output = scanl1 (+) $ zipWith (parse input output)
                                                [0..] input
                zipWithM_ printRes [0..] output
        printResult         :: Integer -> Integer -> IO ()
        printResult nr res  = putStrLn $ show nr ++ ": " ++ show res

        parse   :: [String] -> [Integer] -> Int -> String -> Integer
        parse input output nr s
                = let p nr s
                        -- last number again
                        | null s        = p (nr-1) (input !! nr)
                        -- repeat input N
                        | head s == '!' = let index = read (tail s)
                                          in  p index (input !! index)
                        -- enter result N
                        | head s == '?' = let index = read (tail s)
                                          in  output !! index
                        -- just a number
                        | otherwise     = read s
                  in  p nr s

Diese Augen haben es gesehen
Doch diese Augen schliessen sich
Und ungehindert fliesst das Blut
Und das Schweigen wird unerträglich laut