[Haskell-cafe] background question about IO monad

[Uwe Hollerbach]

Thanks, I'm going to have to study this a bit...

Uwe

On 2/7/08, Ryan Ingram <ryani.spam at gmail.com> wrote:
> On 2/6/08, Uwe Hollerbach <uhollerbach at gmail.com> wrote:
> > And, coming back to my scheme interpreter, this is at least somewhat
> > irrelevant, because, since I am in a REPL of my own devising, I'm
> > firmly in IO-monad-land, now and forever.
>
> This is not entirely true; a REPL can be pure.
>
> Consider the following simple stack-based-calculator; all the IO
> happens within "interact", the REPL itself is pure:
>
> import System.IO
>
> main = hSetBuffering stdout NoBuffering >> interact replMain
>
> replMain s = "Stack calculator\n> " ++ repl [] s
>
> repl :: [Int] -> String -> String
> repl _ [] = ""
> repl _ ('q':_) = ""
> repl s ('\n':xs) = show s ++ "\n> " ++ repl s xs
> repl s xs@(x:_) | x >= '0' && x <= '9' =
>     let (v, xs') = head $ reads xs in repl (v:s) xs'
> repl s (c:xs) | c `elem` validCommands = case command c s of
>     Just s' -> repl s' xs
>     Nothing -> "<stack underflow>\n" ++ repl s xs
> repl s (_:xs) = repl s xs -- ignore unrecognized characters
>
> validCommands = ".d+c"
> command :: Char -> [Int] -> Maybe [Int]
> command '.' (x:xs) = Just xs
> command 'd' (x:xs) = Just $ x:x:xs
> command '+' (x:y:xs) = Just $ (x+y):xs
> command 'c' _ = Just []
> command _ _ = Nothing
>
> You can go further than "interact" if you want to abstract away the
> impurity in your system and take input from some outside process which
> has a limited set of impure operations.  Take a look here for an
> example using "Prompt" (which has seen some discussion here on
> haskell-cafe): http://paste.lisp.org/display/53766
>
> In that example, "guess n" is an action in the pure Prompt monad;
> different interpretation functions allow this monad to interact with
> an AI (in a semi-pure setting; it outputs strings), or with a real
> player via the full IO interface.  A similar mechanism could be used
> for the scheme REPL to make it as "pure" as possible, with
> "getClockTime" being replaced by "prompt GetClockTime" to interact
> with the outside world.
>